Acoustic Treatment Do It Yourself

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Acoustic Treatment and Design for Recording Studios and Listening Rooms

by Ethan Winer

Prepared by: Deady on da Beatz

http://wwwmyspacecom/deadyondabeatz

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Acoustic Treatment and Design for Recording Studios and

Listening Rooms

by Ethan Winer

This page was last updated on December 2, 2008.

!"TR#D$%T!#"

I've been very pleased to see the currentgrowing interest in acoustic treatment. Evenas recently as five years ago, it was rare to

read a magazine article or newsgroupposting about acoustics, bass traps,diffusors, room modes, and so forth. Todaysuch discussions are common. And wellthey should be - the acoustics of arecording or listening room are arguablymore important than almost anything else

These days, all gear is acceptably flat overthe most important parts of the audio range.!istortion, aside from loudspea"ers andmicrophones, is low enough to be inconse#uential. And noise - a big problemwith analog tape recorders - is now pretty much irrelevant with modern digitalrecording. Indeed, given the current high #uality of even semi-pro audio gear, thereal issue these days is your s"ill as a recording engineer and the #uality of therooms in which you record and ma"e mi$ing decisions. Top

%hat's the point in buying a microphone preamp that is ruler flat from !& tomicrowaves when the acoustics in your control room create pea"s and dips aslarge as ( d) throughout the entire bass range* +ow important really are itterartifacts ( d) below the music when standing waves in your studio cause ahuge hole at ( +z e$actly where you placed a mi"e for the acoustic bass*&learly, fre#uency response errors of this magnitude are an enormous problem,yet most studios and control rooms suffer from this defect. %orse, many studioowners have no idea their rooms have such a s"ewed response %ithout"nowing what your music really sounds li"e, it is difficult to produce a #ualityproduct, and even more difficult to create mi$es that sound the same outsideyour control room.

TA)/E 01 &02TE2T3

IntroductionPart & ' Acoustic Treatment

!iffusors and Absorbers4idrange and +igh 1re#uency Absorbers

5igid 1iberglass)ass Traps 0verview1iberglass )ass Traps0ptimizing the Air 6ap

)etter )ass TrapsPart ( ' Room Design and Layout

5oom 3izes and 3hapes5oom 3ymmetry

/ive or !ead*2oise &ontrol

4ore 5esources

Part ) ' Sidebar artic*es3idebar - 3tanding %aves

3idebar - 1ine Tuning the &ontrol 5oom3idebar - 4easuring Absorption3idebar - The 2umbers 6ame

3idebar - )ig %aves, 3mall 5ooms3idebar - +ard 1loor, 3oft &eiling

3idebar - 5oom 4odes and 4ode&alc3idebar - &reating an 517

5evisions About Ethan %iner

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This article e$plains the basic principles of acoustic treatment. 3ome of thematerial is ta"en from my bass traps plans published in Electronic 4usicianmagazine, some is from my company's web site, and some is from my postingsin audio newsgroups. +owever the vast maority is new content that does notappear anywhere else. I have consolidated this information here to provide a

single comprehensive source that is free of commercial references. 4y goal is tooffer advice that is complete and accurate, yet easy to understand using commonsense e$planations instead of math and formulas. Although many boo"s aboutrecording studio and listening room acoustics are available, most of the betterones are too technical for the average audio enthusiast to understand withouteffort. And you'd need to purchase and read many boo"s to learn a few relevantitems from each. Top

8lease understand that I am not a degreed acoustician or studio architect.+owever, I am very e$perienced in acoustic treatment, and particularly in basstraps. The recommendations described here are the result of my own personal

e$perience and should not be ta"en as the final word. Although this article isintended mainly for audio engineers and home recordists, all of the informationapplies e#ually to home theaters, small churches and auditoriums, and otherrooms where high #uality reproduction of audio and music is re#uired.

This te$t will surely e$pand as I learn more. And as people as" me #uestions orre#uest elaboration, I will incorporate the answers and additions here. Also, thereis a growing list of newer acoustics articles on the Articles page of my company'sweb site you can browse through. If you have #uestions or comments aboutanything related to acoustics, please do not send me email . 5ather, I prefer thatyou post publicly in my E9 4agazine Acoustics forum. This way the effort I put

into answering can help others, and you can benefit from the answers of otherstoo. Top

PART &: A%#$ST!% TR+AT,+"T

There are four primary goals of acoustic treatment: ; To prevent standing wavesand acoustic interference from affecting the fre#uency response of recordingstudios and listening rooms< ; to reduce modal ringing in small rooms and lowerthe reverb time in larger studios, churches, and auditoriums< =; to absorb ordiffuse sound in the room to avoid ringing and flutter echoes, and improve stereoimaging< and >; to "eep sound from lea"ing into or out of a room. That is, to

prevent your music from disturbing the neighbors, and to "eep the sound ofpassing truc"s from getting into your microphones.

8lease understand that acoustic treatment as described here is designed tocontrol the sound #uality within a room. It is not intended to prevent soundpropagation between rooms. 3ound transmission and lea"age are reduced viaconstruction - using thic" massive walls, and isolating the building structures -generally by floating the walls and floors, and hanging the ceilings with shoc"

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mounts. 3ound isolation issues are beyond the scope of this article. 1or learningmore about isolation and the types of construction needed I recommend HomeRecording tudio! "uild it #i$e the %ros by 5od 6ervais.

8roper acoustic treatment can transform a muddy sounding room, having poor

midrange definition and erratic bass response, into one that sounds clear andtight, and is a pleasure to wor" and listen in. %ithout effective acoustic treatment,it is difficult to hear what you're doing, ma"ing you wor" much harder to create agood mi$. In a home theater, poor acoustics can ma"e the sound less clear,harder to localize, and with an uneven fre#uency response. Even if you spentmany thousands of dollars on the most accurate loudspea"ers and othere#uipment available, the fre#uency response you actually realize in an untreatedroom is li"ely to vary by =( d) or even more. Top

There are two basic types of acoustic treatment - absorbers and diffusors. Thereare also two types of absorbers. 0ne type controls midrange and high fre#uency

reflections< the other, a bass trap, is mainly for low fre#uencies. All three types oftreatment are usually re#uired before a room is suitable for ma"ing mi$ingdecisions and for serious listening.

4any studio owners and audiophiles install acoustic foam all over their walls,mista"enly believing that is sufficient. After all, if you clap your hands in a roomtreated with foam ?or fiberglass, blan"ets, or egg crates;, you won't hear anyreverb or echoes. )ut thin treatments do nothing to control low fre#uency reverbor reflections, and hand claps won't reveal that. )asement studios and livingrooms having walls made of bric" or concrete are especially prone to thisproblem - the more rigid the walls, the more reflective they are at low

fre#uencies. Indeed, simply building a new sheet roc" wall a few inches inside anouter cement wall helps to reduce reflections at the lowest fre#uencies becausea sheet roc" wall that fle$es also absorbs a little.

@ou may as" why you need acoustic treatment at all, since few people listeningto your music will be in a room that is acoustically treated. The reason is simple:

All rooms sound differently, both in their amount of liveness and their fre#uencyresponse. If you create a mi$ that sounds good in your room, which has its ownparticular fre#uency response, it is li"ely to sound very different in other rooms.1or e$ample, if your room has a severe lac" of deep bass, your mi$es willprobably contain too much bass as you incorrectly compensate based on whatyou are hearing. And if someone else plays your music in a room that has toomuch deep bass, the error will be e$aggerated, and they will hear way too muchdeep bass. Therefore, the only practical solution is to ma"e your room asaccurate as possible so any variation others e$perience is due solely to theresponse of their room. Top

D!--$S#RS A"D ABS#RB+RS

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!iffusors are used to reduce or eliminate repetitive echoes that occur in roomshaving parallel walls and a flat ceiling. Although there are different philosophiesabout how much natural reverberation recording studios and listening roomsshould have, all professional studio designers agree that periodic reflectionscaused by parallel walls are best avoided. Therefore, diffusion is often used in

addition to absorption to tame these reflections. 3uch treatment is universallyaccepted as better than ma"ing the room completely dead by covering all of thewalls with absorbent material. 1or me, the ideal listening room has a mi$ ofreflective and absorptive surfaces, with no one large area all live or all deadsounding. nderstand that BliveB and BdeadB as described here concern only themid and upper fre#uencies. /ow fre#uency treatment is another matter entirely,and will be described separately.

The simplest type of diffusor is one or more sheets of plywood attached to a wallat a slight angle, to prevent sound from bouncing repeatedly between the sametwo walls. Alternatively, the plywood can be bent into a curved shape, though

that is more difficult to install. In truth, this is really a deflector, not a diffuser, asdescribed in more detail below. +owever, a deflector is sufficient to avoid flutterechoes between parallel surfaces.

The photo below shows a curved deflector a friend and I built for the control roomin his home recording studio. It is placed opposite the control room window and ise$actly the same size as the window ?si$ by three feet; to maintain symmetry inthe room. If you build a deflector li"e this, be sure to pac" fluffy fiberglass in theair space behind the wood to "eep the cavity from resonating. Ideally, the amountof curve should be greater than shown here, with the center of the panel fartherfrom the wall. 4y friend already had some =C-inch plywood so we used that, but

it was very difficult to bend. +ad we used C>-inch plywood it would have bentmore easily, letting us increase the amount of curving. Top

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8hoto courtesy of Avid 5ecorders.

5eal diffusor designs use an irregular surface having a comple$ pattern to scatterthe sound waves even more thoroughly. @et another type, shown below, useschambers having different depths. 2ote that for diffusion to be effective, youneed to treat more than ust a few small areas. %hen walls are parallel, adding

diffusion to only a small percentage of the surface area will not reduceobectionable echoes nearly as well as treating one or both walls morecompletely. Top



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8hoto courtesy of5ealTraps.

Again, the angled and curved walls described earlier are deflectors, not diffusors. A true diffusor scatters sound waves in different directions based on theirfre#uency, rather than merely redirecting all waves in the same direction. This isan important distinction because a flat surface that is angled or curved stillfosters the bo$y sounding response pea"s and dips "nown as comb &iltering . Areal diffusor avoids direct reflections altogether, and thus has a much more open,transparent, and natural sound than a simple flat or curved surface. )esidessounding less colored than an angled or curved wall in a control room, diffusorsserve another useful purpose in recording rooms: they can reduce lea"agebetween instruments being recorded at the same time. %here an angled wallsimply deflects a sound - possibly toward a microphone meant to pic" up anotherinstrument - a diffusor scatters the sound over a much wider range. 3o whateverarrives at the wrong microphone is greatly reduced in level because only a smallpart of the original sound arrived there. The rest was scattered to other parts ofthe room.

nfortunately the better commercial diffusors are not cheap. 3o what are somealternatives for the rest of us* Aside from the s"yline type diffusors, which aresometimes made of plastic and sold for not too much money, you can ma"e awall either totally dead or partially dead. 1or someone with a very small budget,ma"ing the rear wall of a control room totally dead may be the only solution. Atleast that gets rid of flutter echoes between the front and rear wall, though at thee$pense of sounding stuffy and unnatural. )ut it's better than the hollow bo$ysound you get from a plain flat reflective surface. Another option is to ma"e therear wall of a control room partially reflective and partially absorbent. @ou can dothis by ma"ing the wall totally dead, and then covering it with thin vertical strips ofwood to reflect some of the sound bac" into the room. If you vary the spacing


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from strip to strip a little, you'll reduce the coherence of the reflections a littlewhich further improves the sound.

1ast repetitive echoes - also called &lutter echoes - can color the sound in theroom and cause an emphasis at fre#uencies whose wavelengths correspond to

the distance between the walls, and between the floor and ceiling. 1lutter echoesare often identified as a BboingB sound that has a specific pitch. If you clap yourhands in a live room or an empty stairwell or tunnel, you can easily hear the tone.If the room is large, you'll probably notice more of a rapid echo rat-a-tat-tat effect- the Bflutter.B 3maller rooms resonate at higher fre#uencies, so there you aremore li"ely to hear a specific tone that continues even after the original soundhas stopped. This effect is called ringing . )esides the obvious ill effects causedby the echoes, ringing creates an unpleasant sonic signature that can permeaterecordings made in that room and negatively affect the sound of everythingplayed through loudspea"ers in that room. Top

2ote that echo, flutter echo, and ringing are intimately related, so the delay timeand pitch always depends on the distances between opposing surfaces. %ithsmall spacings the flutter echo's pitch is directly related to the distance. I have along stairwell in my home with a spacing of =D. inches between walls. %hen Iclap my hands loudly I hear a distinct tone at the 1F whose pitch is about D +z,and the half wavelength for D +z is =D. inches. )ut with larger distances youmay hear a higher fre#uency than the spacing would indicate, depending on whatsound source e$cites the echoes. 1or e$ample, when you clap your hands orotherwise e$cite a room with only midrange fre#uencies, the only resonancesthat can respond are also at midChigh fre#uencies. 3o if the distance betweenparallel walls fosters a resonance at, say, ( +z, you might hear (( +z, or =(

+z, when you clap your hands.

/i"e diffusion, midrange and high fre#uency absorption helps minimize echoesand ringing. )ut unli"e diffusion, absorption also reduces a room's reverb time.This ma"es the sound clearer and lets you hear better what is in the recording byminimizing the room's contribution. 1or e$ample, if you ma"e mi$ing decisions ina room that is too reverberant, you will probably add too little reverb electronicallybecause what you hear includes the room's inherent reverb. /i"ewise, if the roomis overly bright sounding due to insufficient absorption, your mi$es will tend tosound muffled when played on other systems because the treble adustmentsyou ma"e will be incorrect. Therefore, diffusion is used to avoid flutter echo,ringing, and comb filtering, but without reducing the room's natural ambience.

/ow fre#uency absorbers - bass traps - can be used to reduce the low fre#uencyreverb time in a large space, but they are more commonly used in recordingstudios and listening rooms to reduce modal ringing and flatten the fre#uencyresponse in the bass range. This is especially true in smaller rooms where a poorlow fre#uency response is the main problem. In fact, small rooms don't reallyhave reverb at all at low fre#uencies. 5ather, ringing at the room's individual



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mode fre#uencies dominates. )ut in large recording studios, churches, andauditoriums, reducing low fre#uency reverb is an important reason for addingbass traps. Top

,!DRA".+ A"D !. -R+0$+"%1 ABS#RB+RS

%ithout #uestion, the most effective absorber for midrange and high fre#uenciesis rigid fiberglass. 0wens-&orning G(= and G(, or e#uivalents from othermanufacturers, are the standard absorbing materials used by professional studiodesigners. )esides being e$tremely absorbent they are also fireproof and, whenapplied to a wall, can even retard the spread of heat. 5igid fiberglass is availablein panels by > feet and in thic"nesses ranging from to > inches. /arger sizesare available, but by > is more convenient for most studio applications, and canbe shipped more economically. As with all absorbent materials, the thic"er it is,the lower in fre#uency it will absorb to. That is, G(= fiberglass one inch thic"absorbs reasonably well down to (( +z. %hen two inches thic", the same

material is e#ually absorbent down to ( +z. 3ee the sidebar 'easuring (bsorption for more information about how these measurements are made.

1or a given thic"ness, G(= is about twice as absorbent as acoustic foam at thelower fre#uencies, and it generally costs much less. Even better for lowfre#uencies is G(-15H, which is much more absorbent than G(= at +z andbelow. 15H stands for 1oil 5einforced Hraft paper. This is similar to the paperthat grocery bags are made of, but with a thin layer of metal foil bonded to oneside. The 15H paper was not intended for acoustic purposes, but to serve as avapor barrier in homes. It ust happens to be good acoustically too. )e aware thatthe paper reflects mid and high fre#uencies when installed with that side facing

the room< this may or may not be desirable for a given application. G( is alsoavailable without a paper bac"ing. Top

Although G(= and G( fiberglass panels are more effective than foam of thesame thic"ness, they are usually covered with fabric for appearance, and toprevent the glass fibers from escaping into the air. This adds to the e$pense anddifficulty of building and installing them. ?In practice, fiberglass particles are notli"ely to escape into the air unless the material is disturbed.; A comparison ofG(=, G(-15H with the reflective paper e$posed, and typical foam is shown inTable below. 2ote that foam panels sold as acoustic treatment are oftensculpted for appearance, and to better absorb sound arriving at an angle.5emoving some of the material reduces foam's effectiveness at low fre#uencies.If rigid fiberglass was compared to solid foam panels of the same thic"ness, thedisparity in low fre#uency performance would li"ely be less. +owever, not havinga sculpted surface would then reduce foam's absorption at higher fre#uencies.

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,ateria*

&(2 z(23z

233z

&333z

(333z

4333z

"R%

0wens-&orning G(= (.G (.D .> .(G .( (. .((

0wens-&orning G(-15H (.D( (.( (.D= (. (.> (.=> (.D(

Typical sculpted acousticfoam

(. (.=( (. .( (. .(( (.(

Tab*e &: (bsorption coe&&icients o& )0*, )0+-R, and a popular brand o& sculpted acoustic &oamat di&&erent &re/uencies. (ll material is two inches thic$ and applied directly to a wall. This datawas obtained &rom the respectie manu&acturer1s published literature.

It's not difficult to understand why G( fiberglass is so much more absorbent thantypical sculpted foam at low fre#uencies. )esides the fact that sculpted foam hasabout half the mass of solid foam due to material being removed to create theirregular surface, another consideration is density. According to test datapublished by several manufacturers of rigid fiberglass and roc" wool, the densertypes absorb more at low fre#uencies. The data published by Johns-4anville fortheir line of rigid fiberglass shown below is one e$ample. Acoustic foam has adensity of less than pounds per cubic foot ?pcf; compared to G( fiberglasswhich has a density of D pcf.

4y own tests in a certified acoustics lab confirm this, showing denser types ofrigid fiberglass absorb as much as >( percent more than less dense types at +z and below. 4ore recently I performed T+I3 series of measurements in mycompany's test lab, which shows the relationship between density and low

fre#uency performance even more conclusively. 5egardless of the reason, thereis no disputing that for a given panel size and thic"ness, G(-15H is substantiallymore effective at low fre#uencies than the same thic"ness of typical acousticfoam. +owever, it is important to understand that a material's density is but onecontributor to its effectiveness as an absorber. 0bviously, if the density is madetoo high the material will reflect more than it absorbs, so it's a mista"e toconclude that higher densities are always better. 1or this reason, test data mustbe the final arbiter of a product's effectiveness. Top

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As the data above clearly shows, D pcf rigid fiberglass panels absorb substantially more at low fre#uenciesthan less dense = pcf material.

0ne important way to improve the low fre#uency performance of any absorbentmaterial - besides ma"ing it thic"er - is to space it away from the wall or ceiling.1or a given material thic"ness, increasing the depth of the air gap lowers thefre#uency range it absorbs. 1or e$ample, G(= that is two inches thic" andmounted directly against a wall has an absorption coefficient of (.G at +z.3pacing the same material D inches away from the wall increases that to (.>( -a nearly three-fold improvement. 0f course, few people are willing to give up thatmuch space in their rooms And even very thic" ?four inch; G(-15H with a one-foot gap will not absorb the lowest fre#uencies as well as a purpose-built basstrap which is optimized for that purpose. )ass traps, absorption coefficients, and

spacing of absorbent material will be described separately and in more detaillater. Top

!S 5R!.!D -!B+R.LASS5 A" #61,#R#"7

There is some confusion about the term Brigid fiberglassB because it is not reallyrigid li"e a piece of wood or hard plastic. 5ather, the term rigid is used todifferentiate products such as G(= from the fluffy fiberglass commonly used for



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home insulation. 5igid fiberglass is made of the same material as regularfiberglass, but it is woven and compressed to reduce its size and increase itsdensity. 5igid fiberglass that is one inch thic" contains about the same amount ofraw material as = to D inches of regular fiberglass. The photo below shows apiece of G(= one inch thic" folded slightly. As you can see, it is rigid enough that

it doesn't flop over when not supported ?right side of photo;, but not so rigid that itcan't be bent or s#ueezed.

G(= BrigidB fiberglass is not really very rigid.

2ow that you "now what rigid fiberglass is, where the hec" do you buy it* @ouprobably won't find it at your local hardware store or lumber yard, but many

insulation suppliers stoc" it or can order it. 3tart by loo"ing in your telephonedirectory under nsulation and also Heating 3 (ir 4onditioning uppliers. @ou canfind the name of an 0wens-&orning dealer near you by calling ((-6ET-8I2H?((->=-G>D; or from the /ocator page on the 0wens-&orning web site. 0thercompanies, such as Hnauf , Armstrong, and !elta, ma"e similar products, andthey often cost less than fiberglass from 0wens-&orning. @ou can contact themdirectly to find a distributor near you. In the interest of completeness, here aresome other manufacturers that ma"e similar products: Johns-4anville,&ertainTeed, 5o$ul, 0ttawa 1ibre, and 1ibre$. Top

%hen assessing rigid fiberglass, it is important to "now its density so you can

compare e#uivalent products. 0wens-&orning G(= has a density of about threepounds per cubic foot ?> "ilograms per cubic meter;, and G( is about si$pounds per cubic foot ?( "ilograms per cubic meter;. Therefore, products fromother companies that have a similar density will have similar absorptioncharacteristics at the same fre#uencies. 2ote that some companies call theirproducts mineral wool , mineral &iber , or roc$ wool , but acoustically they aree#uivalent to fiberglass.

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5igid fiberglass is great stuff, and you can cut it fairly easily with a razor "nife, butit's not very pleasant to wor" with because the fibers can ma"e your s"in itch.%hile handling it you should wear wor" gloves, and you won't be too cautious ifyou also wear a dust mas". The usual way to mount rigid fiberglass to a wall iswith sheet roc" screws and large diameter washers with a small hole, often

called &ender washers. These washers are needed to prevent the screw headsfrom pulling through the fiberglass. 1ender washers are available at +ome !epotand other hardware stores. If your wall is made of cement or bric", you caninstead use construction glue li"e /i#uid 2ails to attach small strips of wood tothe wall, and then screw the fiberglass to the strips. 3ince fiberglass wor"s betterwhen spaced away from a wall or ceiling, wood strips ma"e sense even whenyou are able to screw directly into the wall. Top

0nce the fiberglass is attached to the wall, you can build a wooden framecovered with fabric and place the frame over the fiberglass for appearance. Ifthat's too much wor", you can cut pieces of fabric and staple them to the edges

of the wood strips. 2early any porous fabric is appropriate, and one popularbrand is 6uilford type 15G(. nfortunately, it's very e$pensive. 0ne "ey featureof 15G( is that it's made of polyester so it won't shrin" or loosen with changes inhumidity when stretched on a frame. )ut polyester is a common materialavailable in many styles and patterns at any local fabric store. Another feature of15G( is that it's one of the few commercial fabrics rated to be acousticallytransparent. )ut since you're not using it as spea"er grill cloth to place in front ofa tweeter, that feature too is not necessary.

3hiny fabrics having a tight weave should be avoided because they reflect higherfre#uencies. The standard test for acoustic fabric is to hold it to your mouth and

try to blow air through it. If you can blow through it easily, it will pass sound intothe fiberglass. )urlap and 4uslin are two ine$pensive options, but nearly any softfabric will wor" and also "eep the glass fibers safely in place. Top

BASS TRAPS ' #8+R8!+9

The most common application of bass traps in recording studios and controlrooms is to minimize standing waves and acoustic interference which s"ew theroom's low fre#uency response. ?3ee the sidebar, Why They1re 4alled tandingWaes.; As you can see in 1igure below, acoustic interference occurs inside aroom when sound waves bounce off the floor, walls, and ceiling, and collide witheach other and with waves still coming from the loudspea"er or other soundsource. /eft untreated, this creates severe pea"s and dips in the fre#uencyresponse that change as you move around in the room. At the listening position,there might be near-total cancellation centered at, say, (( +z, while in the bac"of the room, (( +z is boosted by d) but G( +z is partially canceled. Top

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-igure &: (coustic inter&erence causes direct and re&lected waes to combine inthe air, creating pea$s and dips in the &re/uency response.

+ere, a positive wave front from the loudspea"er ?left; is reflected off the rearwall on the right, and the reflection collides with other waves that continue toemanate from the loudspea"er. !epending on the room dimensions and thewavelength ?fre#uency; of the tones, the air pressure of the reflected waveseither adds to or subtracts from the pressure of the waves still coming from thespea"er. %orse, different locations in the room respond differently, with a boostat some fre#uencies and a reduction at others. %hen waves combine in phaseand reinforce each other, the increase in level can be as much as D d). )ut whenthey combine destructively, the dip in response can be much more severe. /evelreductions of d) or more are typical in untreated rooms, and near-totalcancellation at some fre#uencies and locations is not uncommon. 1urther, most

rooms have many pea"s and dips throughout the entire bass range, not ust atone or two fre#uencies. 1igure below shows the fre#uency response of the (-by D-foot untreated control room at a friend's studio. 2ote the large number ofripples, and their magnitude, all within ust one octave Top



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-igure (: 5our worst nightmare6 5es, this response really is typical &or an untreatedsmall room7

The action of sound waves colliding and combining in the air is called acousticinter&erence, and this occurs in all rooms at all low fre#uencies - not ust thoserelated to the room's dimensions. The only thing that changes with fre#uency iswhere in the room the pea"s and nulls occur. The principle is identical to howphaser and flanger effects wor", e$cept the comb filtering happens acoustically inthe air. Top

The only way to get rid of these pea"s and dips is to avoid, or at least reduce, the

reflections that cause them. This is done by applying treatment that absorbs lowfre#uencies to the corners, walls, and other surfaces so the surfaces do notreflect the waves bac" into the room. A device that absorbs low fre#uencies iscalled a bass trap. Although it may seem counter-intuitive, adding bass traps to aroom usually increases the amount of bass produced by loudspea"ers andmusical instruments. %hen the cancellations caused by reflections are reduced,the most noticeable effect is increasing the bass level and ma"ing the lowfre#uency response more uniform. As with listening rooms, bass traps are alsouseful in studio recording rooms for the same reasons - to flatten the response ofinstruments captured by microphones and, with large studios, to improve theacoustics by reducing the low fre#uency reverb decay time which ma"es the

music sound more clear.

1or recording engineers, problems caused by standing waves and acousticinterference are often first noticed when you realize your mi$es are notBportable,B or do not BtranslateB well. That is, songs you have e#ualized andbalanced to sound good in your control room do not sound the same in otherrooms. 0f course, variations from different loudspea"ers are a factor too. )utbass fre#uencies are the most difficult to udge when mi$ing because acoustic



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interference affects them more than higher fre#uencies. Another problem is thatthe level and tone #uality of bass instruments vary as you wal" around the room.The sound is thin here, too bassy over there, yet not accurate anywhere. Indeed,even if you own all the latest and most e$pensive recording gear, your mi$es willstill suffer if you can't hear what's really happening in the low end. Aside from

portability concerns, it's very difficult to get the bass instrument and "ic" drumbalance right when acoustic interference and modal ringing combine to reduceclarity. And when every location in the room has a different low-end response,there's no way to "now how the music really sounds. Top

4any people wrongly believe that using near-field monitor spea"ers avoids theneed for acoustic treatment. In truth, even with small loudspea"ers playing softly,acoustic interference still causes standing waves - the imperfect fre#uencybalance is e$actly the same but at a lower level. Although higher fre#uencyreflections and echoes are proportionately reduced as you get closer to theloudspea"er, the s"ewed fre#uency response caused by low fre#uency

reflections remains. /i"ewise, adding a subwoofer will not fi$ problems that aredue to poor room acoustics. %hile a subwoofer can be useful to compensate forinade#uate loudspea"ers, it will not solve the problem of an irregular responsecaused by acoustic interference. In fact, a subwoofer often ma"es matters worseby compounding and hiding the real problem.

Another common misconception is that e#ualization can be used to counter theeffects of acoustic problems. )ut since every location in the room respondsdifferently, no single E9 curve can give a flat response everywhere. 0ver aphysical span of ust a few inches the fre#uency response can vary significantly.Even if you aim to correct the response only where you sit, there's a bigger

problem: It's impossible to counter very large cancellations. If acousticinterference causes a d) dip at D( +z, adding that much boost with ane#ualizer to compensate will reduce the available volume ?headroom; by thesame amount. 3uch an e$treme boost will increase low fre#uency distortion inthe loudspea"ers too. And at other room locations where D( +z is already tooloud, applying E9 boost will ma"e the problem much worse. Even if E9 couldsuccessfully raise a null, the large high-9 boost needed will create electricalringing at that fre#uency. /i"ewise, E9 cut to reduce a pea" will not reduce thepea"'s acoustic ringing. E9 cannot always help at higher fre#uencies either. If aroom has ringing tones that continue after the sound source stops, E9 mightma"e the ringing a little softer but it will still be present. +owever, e#ualizationcan help a little to tame low fre#uency pea"s ?only; caused by natural roomresonance, as opposed to pea"s and nulls due to acoustic interference, if used inmoderation. Top

@et another common misconception is that small rooms cannot reproduce verylow fre#uencies, so they're not worth treating at all. A popular ?but incorrect;theory is that very low fre#uencies re#uire a certain minimum room dimension toBdevelop,B and so can never be present at all in smaller rooms. The truth is that

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any room can reproduce very low fre#uencies, as long as the reflections thatcause acoustic cancellations are avoided. %hen you add bass trapping, you arema"ing the walls less reflective at low fre#uencies, so sound that hits a wall orceiling will be absorbed instead of reflected. The net result is e$actly the same asif the wall was not there at all - or as if the wall was very far away - whatever

does come bac" is greatly attenuated due to distance and, therefore, not loudenough to cause as much cancellation. 3ee the sidebar )ig %aves, 3mall5ooms for more elaboration on this topic.

3ome people mi$ using headphones in an attempt to avoid the effects of theirroom. The problem with headphones is that everything sounds too clear andpresent, ma"ing it difficult to find the ideal volume for some trac"s. %henlistening through headphones, a lead vocal or solo instrument can be heard veryclearly, even if it is #uiet, so you'll tend to ma"e it lower in the mi$ than it shouldbe. /i"ewise, it's difficult to assess the amount of reverb and echo being addedelectronically when using headphones.

2ote that standing waves and acoustic interference also occur at higherfre#uencies, such as sustained clarinet or flute tones. @ou can hear the effectand identify the problem fre#uencies and locations fairly easily by playing sinewaves ?not too loudly; through your loudspea"ers. This is also a good way toassess how important bass traps are for your particular studio and control rooms.If you have 3ound1orge, %ave/ab, or a similar audio editor program, it's simpleto create sine wave files at different low fre#uencies for testing. 3pecial &!s thatcontain various tones and pin" noise suitable for room testing and analysis arealso commonly available. To determine the severity of low fre#uency problems,play different sine waves one at a time through your monitors, and then slowly

wal" around the room. It will be very obvious at which fre#uencies the pea"s andvalleys occur, and where they cause the most harm. There's no point in playingfre#uencies below what your spea"ers can produce cleanly - I suggest D( +z, (+z, (( +z, and so forth through maybe ((-=(( +z. If you have a computerconnected to your loudspea"ers, you can download the 2TI 4inirator program,which generates a variety of useful audio test signals. Top

)esides helping to flatten the low fre#uency response, bass traps serve anotherpurpose that is e#ually important: They reduce the modal ringing that causessome bass notes to sustain longer than others, which harms clarity. The ET1 =!BwaterfallB graph below shows the modal ringing in my D'B by 'DB by ' testlab. )oth graphs show not only the low fre#uency response ?the Bbac" wallB ofthe graph;, but also the bandwidth of each room mode and its decay time. As youcan see, adding bass traps lowers the 9 of modal pea"s ?widens theirbandwidth; and also reduces their decay time. %hen the bandwidth of the modesis widened individual bass notes stic" out less than other, adacent notes. Thissolves the problem commonly "nown as Bone note bass.B

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The other change is the large reduction in ringing time ?the BmountainsB comeforward over time;. %ithout traps, some bass notes ring out for as long as C= ofa second, so they muddy other subse#uent bass notes. After adding bass trapsthe ringing time is cut in half or even less, e$cept at the lowest mode which in thisroom is about = +z. )ut even at = +z there's a noticeable, if slight,

improvement in bandwidth and decay time.


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This ET1 graph shows how bass traps reduce ringing by ma"ing it decay faster and lowering the9 of the resonances.

6enerally spea"ing, most rooms need as many bass traps as you can fit andafford. Although it is definitely possible to ma"e a room too dead at midrange andhigh fre#uencies, you probably cannot have too much low fre#uency absorption.The effectiveness of bass traps is directly related to how much of the room's totalsurface area you treat, which includes the walls, floor, and ceiling. That is,covering thirty percent of the surface with bass traps reduces low fre#uencyreflections far more than covering only five percent. It would be great to invent amagical acoustic vacuum cleaner that could suc" the waves out from the air. )ut,alas, the laws of physics do not wor" that way. At the minimum I recommendplacing bass traps in all of the corners. 1or even better results, put additionaltraps on the walls and optionally on the ceiling.

-!B+R.LASS BASS TRAPS

There are a number of ways to create a bass trap. The simplest and leaste$pensive is to install a large amount of thic" rigid fiberglass, spacing it well awayfrom the wall or ceiling. As noted earlier, G(-15H that is four inches thic" andspaced D inches away from the wall can be #uite effective to fre#uencies below +z. )ut many rooms have severe problems far below +z and losingtwenty inches all around the room for thic" fiberglass and a large air space isunacceptable to most studio owners and audiophiles. 1ortunately, more efficientbass trap designs are available that are much smaller. +owever, studios on atight budget can apply rigid fiberglass in the room corners as shown in 1igure =aand lose only the small amount of space in the corners. 3ince bass builds up themost in the corners of a room, this is an ideal location for any bass trap. Top

-igure )a: ( thic$ piece o& )0+ mounted across acorner is e&&ectie to &airly low &re/uencies.

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1igure =a shows the corner viewed from above, loo"ing down from the ceiling.%hen the rigid fiberglass is mounted in a corner li"e this, the large air gap helpsit absorb to fairly low fre#uencies. 1or this application G(-15H is better than G(=because the goal is to absorb as effectively as possible at low fre#uencies.+owever, you can either absorb or deflect the higher fre#uencies by facing the

paper bac"ing one way or the other, to better control liveness in the room. singG( fiberglass that is two inches thic" does a good ob, but using four incheswor"s even better. 2ote that two adacent two-inch panels absorb the same asone piece four inches thic", so you can double them up if needed. +owever, ifyou are using the 15H type you should remove the paper from one of the piecesso only one outside surface has paper. Top

)esides the corners where two walls meet as in 1igure =a, it is e#ually effectiveto place fiberglass in the corners at the top of a wall where it oins the ceiling.%ith either type of corner, you can attach the fiberglass by screwing it to $-inchwood strips that are glued or screwed to the wall as described previously. The

$ ends of these strips are shown as small blac" rectangles in 1igure =a above.0ne very nice feature of this simple trap design is that the air gap behind thefiberglass varies continuously, so at least some amount of fiberglass is spacedappropriately to cover a range of fre#uencies.

%hen mounting G(-15H directly to a wall - not across a corner - you'll achievemore low fre#uency absorption if the paper covered side is facing into the room.+owever, that will reflect mid and high fre#uencies somewhat. 0ne good solutionis to alternate the panels so every other panel has the paper facing toward theroom to avoid ma"ing the room too dead. 8anels attached with the bac"ingtoward the wall should be mounted on thin ?C>-inch; strips of wood to leave a

small gap so the bac"ing is free to vibrate. 1or fiberglass across a corner asshown in 1igure =a, the bac"ing should face into the room to absorb more at lowfre#uencies.

1or a typical unfinished basement ceiling you can ta"e advantage of the gapbetween the support beams and the floor above by placing rigid fiberglassbetween the beams. 3hort nails or screws can support the fiberglass, ma"ing iteasy to slide each piece of fiberglass into place. Then cover the fiberglass withfabric as shown below in 1igure =b. @ou can optionally pac" the entire cavity withfluffy fiberglass one foot thic" and you'll probably get similar results. Top



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-igure )b: )0+ between support beams, coered with &abric.

Treating a BdroppedB grid ceiling is even easier: 3imply lay fluffy fiberglass battson top of the grid, above the ceiling tiles. The thic"er the fiberglass, the better.0ne foot thic" 5= is perfect for this if you have the space. If you don't want to

bother covering the entire ceiling that way, at least put fiberglass batts around theperimeter to treat the important wall-ceiling corners. And since the fiberglass isnot e$posed to the room and doesn't show, you don't need to cover it with fabric.

Another great and ine$pensive way to ma"e a bass trap - if you have a lot ofroom - is to place bales of rolled up fluffy fiberglass in the room corners. Thesebales are not e$pensive, and they can be stac"ed to fill very large spaces. )etterstill, they are commonly available and you don't even have to unpac" them Justleave the bales rolled up in their original plastic wrappers, and stuff them in andnear the room corners wherever they'll fit. 3tac" them all the way up to the ceilingfor the most absorption.

#PT!,!!". T+ A!R .AP

%hile increasing the depth of the air gap does indeed lower the fre#uency rangeabsorbed, for thinner panels it can also reduce the absorption at some higherbass fre#uencies. The ma$imum amount of absorption for a given fre#uencyoccurs when the air gap is C> the wavelength for that fre#uency. 1igure > belowshows the velocity of a sound wave, which is greatest as it transitions throughzero. %hen it reaches the top or bottom of the cycle, the velocity is minimum, butthe pressure is ma$imum. )ecause the velocity is greatest C> wavelength from aboundary, more energy is present to force the waves through the absorbent

material.


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-igure 4: (s a sound wae traels toward a boundary, the pressure and elocity are reset at the boundary. There&orethe wae has a maimum elocity 93: waelength &rom thewall. (t hal& a waelength the elocity is minimum. Then itrises again at *3: waelength. This pattern repeatsinde&initely.

The reason an absorbent material li"e fiberglass wor"s better when spaced awayfrom a surface is that sound waves passing through it have a greater velocitythere. As a wave approaches a boundary, such as a wall, the velocity is reduced,and when it finally hits the boundary, the velocity is zero. Imagine a cue ball as it

approaches the side rail on a pool table. The ball could be travelling (( milesper hour, but at the e$act point where it hits the rail the ball is not moving at all.%ithout motion there's no energy to be absorbed.

/i"ewise, fiberglass placed e$actly at a rigid boundary does nothing because theair particles are not moving there. And since there's no velocity, the fiberglasshas very little effect. As fiberglass is spaced further from the wall, the air particlespassing through it have greater velocity. They are slowed down as they passthrough the fiberglass, which converts the sound energy into heat thereforeabsorbing some of the sound. Top



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-igure 2: (bsorbent material is most e&&ectie when mounted withan air gap e/ual to 93: the waelength o& a particular &re/uency."ut a gap that is ideal &or one &re/uency is not ideal &or all o& thehigher &re/uencies.

As you can see in 1igure above, borrowed from Alton Everest's 4aster+andboo" of Acoustics, absorption for a given gap depth is ma$imum at C>wavelength multiples - in this case starting at around ( +z. It then falls off at ahigher fre#uency where the gap depth e#uals C wavelength. It rises again when

the gap matches =C> of the length of the ne$t higher fre#uency, and so forth. Thisirregular absorption is most severe with thin absorbing materials, and graduallydiminishes as the material is made thic"er. @ou can avoid the reduction inabsorption either by using thic"er rigid fiberglass, or by filling the entire gap withmaterial instead of using only a thin piece spaced away from the wall or ceiling.%hen the entire depth is filled, material is available to absorb all of thefre#uencies whose C> wavelengths fall within that depth. Top



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Although I promised not to use any math, I promise that the following simpleformula is the only e$ception. To determine the best gap depth for a givenfre#uency, you first need to determine the e#uivalent wavelength:

9a;e Length in -eet < &&)3 / -re=uency

Then simply divide the result by > to get the optimum depth. 3o for (( +z thewavelength is =(C(( K .= feet, and C> of that is about . feet. The number=( is the appro$imate speed in feet per second of sound waves travellingthrough air at normal room temperature and humidity.

1or a given thic"ness of absorbent material, the ideal air gap is e#ual to thatthic"ness because it avoids a hole in the range of fre#uencies absorbed. 1ore$ample, if you install fiberglass that is four inches thic" with a four-inch gap,higher fre#uencies whose C> wavelength falls within the four-inch materialthic"ness are absorbed regardless of the gap. And for those fre#uencies whose

C> wavelength is between four and eight inches, the fiberglass is also at theproper distance from the wall or ceiling. This is shown below in 1igure D. Top



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-igure >: The higher &re/uencies ;top< areabsorbed well because their elocity pea$s &allwithin the material thic$ness. The lower &re/uencyat the bottom does not achiee as much elocity soit1s absorbed less.

In practice, you don't necessarily have to measure wavelengths and calculate airgaps, and the first few inches of space yield the most benefit. 4ost people arenot willing to give up two or more feet all around the room anyway, so ust ma"ethe gap as large as you can ustify. If you can afford to fill the gap entirely with

material, all the better. And even though the velocity is indeed highest at C>wavelength, there's still plenty at Cth of the wavelength too. 2ote that the angleat which sound waves stri"e a fiberglass panel can ma"e the panel and its airgap appear thic"er than they really are. 1urther, low fre#uency waves that stri"ean absorbing panel at an angle may be absorbed less than when they stri"e it at( degrees, due to a BgrazingB effect. The e$planations in this section are asimplification and are correct only for a ( degree angle of incidence, which isnot always the case.


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I should mention another popular type of absorber, the tube trap, which isavailable commercially and also as do-it-yourself plans on various web sites.

Although these are often referred to as Bbass traps,B even the largest tubemodels are not very effective below about (( +z, and the smaller ones becomeineffective much higher than that. 4ar"eting hype aside, the real absorption

mechanism in a tube trap is simply the rigid fiberglass inside. The reason a (-inch tube trap wor"s at all down to (( +z is that the tube's diameter serves tospace some of the fiberglass away from the nearest boundary, which helpse$tend its absorption to a lower fre#uency. )ut a tube design is no more effectivethan using plain rigid fiberglass spaced similarly. Top

B+TT+R BASS TRAPS

@et another type of bass trap is the +elmholtz resonator. nli"e foam, fiberglass,and tubes fitted with fiberglass, a +elmholtz resonator can be designed to absorbvery low fre#uencies. This type of trap wor"s on the principle of a tuned cavity

and is often very efficient over a narrow range of fre#uencies. Thin" of a glasssoda bottle that resonates when you blow across its opening, and you have thegeneral idea. Although a +elmholtz design can be very efficient, the downside isthat it wor"s over a fairly narrow range and needs to be rather large to absorbvery low fre#uencies. The range can be widened by filling the cavity withfiberglass, or by creating several openings having different sizes. 0ne commondesign uses a bo$ filled with fiberglass with its front opening partially covered bya series of thin wood boards separated by air spaces. This is called a slatresonator. Another also uses a bo$ filled with fiberglass but has a cover made ofpegboard containing many small holes. Although there is no denying that a+elmholtz trap can be very effective, the fact that it wor"s over a narrow range of

fre#uencies limits its usefulness. %hile it can be sized to absorb the dominantresonant fre#uencies in a particular room, it cannot absorb all the other lowfre#uencies. And broadband absorption is needed to prevent acousticinterference that s"ews the fre#uency response throughout the entire bassrange. Top

0ne of my favorite types of bass trap is the membrane absorber, also called apanel trap because it's made with a wood front panel. 0ne huge advantage ofmembrane traps is that they do not have to be very thic" to absorb very lowfre#uencies. )ecause the bass range spans about four octaves, most panel trapsare designed to wor" over only part of the bass range. Therefore, you will needan e#ual mi$ of trap types, with one intended to absorb the lower bassfre#uencies and the other for the higher bass range. )esides absorbing lowfre#uencies very well, the wood front on a panel trap is reflective at higherfre#uencies. 3o installing enough of them to treat a room properly for lowfre#uency problems will not ma"e the room too dead sounding at mid and highfre#uencies.



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The photo below shows eight panel traps I built for my home studio. )esides thepanels, which are painted white, there are also many G(= fiberglass absorberscovered with tan fabric. 2ot shown are four more panel traps in the rear corners,plus another four on the side walls farther bac" in the room. The photo showsboth types of panel traps ?low-bass and high-bass;, with the thinner units

absorbing the higher bass range. 3ince this is a fairly large room for a homestudio ? by => feet;, many traps are needed to cover a significant amount ofthe room's surfaces. A smaller room would need fewer traps to cover the samepercentage of surface area. Top

This room has an even mi$ of low-bass and high-bass panel traps, and an e#ual number of fiberglassabsorbers to handle the midrange and high fre#uencies. 4ore bass traps are in the rear of the room.

1igure G below shows a cut-away view of a typical wood panel membrane trap.%hen a wave within the effective range of fre#uencies reaches the front panel,

the panel vibrates in sympathy. 3ince it ta"es energy to physically move thepanel, that energy is absorbed rather than returned into the room. The fiberglassthen damps the plywood panel so it doesn't continue to vibrate. %ere the panelallowed to vibrate freely on its own, less energy would be needed to "eep itmoving, so it would absorb less. 1urther, a panel that continues to vibrate on itsown after the source sound stops actually generates sound similar to reverb andthe ringing effect described earlier, and obviously that is not desirable Top



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-igure ?: ound stri$ing the plywood &ront panelcauses it to ibrate. The &iberglass then damps thatibration.

3imilar to an acoustic suspension loudspea"er, panel absorbers li"e this one are

sealed air-tight, and the fiberglass converts the acoustic energy into heat. 2otehow the fiberglass is spaced away from the bac" panel, which is more effectivethan simply attaching it directly against the rear surface. The closer the fiberglassis to the plywood panel, the more effectively it damps the panel's vibration. )ut itis important that the fiberglass not touch the panel because that would restrict itsmovement. 1or a panel trap to absorb as efficiently as possible, the panel mustbe free to vibrate with no restriction other than the damping action of the nearbyfiberglass. Top

There are a few reasons for sealing panel traps. If there's a place for air toescape - let's say at the seam between the front panel and the side of the bo$ -

then pressure from the diaphragm as it pushes into the bo$ will send the wavesout the lea" rather than push them into the fiberglass. Another, more relevant,reason is that a lea" will let the internal pressure escape, reflecting the wavesbac" into the room instead of absorbing them. Thin" of a panel trap as e#uivalentto an open window. If you cut a hole in an outside wall and cover the hole with apiece of heavy cardboard, the cardboard will reflect mid and high fre#uencies butlet lower fre#uencies pass through. Those fre#uencies end up on the outside ofthe wall and so are not reflected bac" into the room. A sealed membrane bass



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trap is similar in that sound passing through the panel goes into the bo$ and doesnot come out. )ut the most important reason a panel trap must be sealed isbecause the air inside acts as a spring, and an air lea" reduces that effect.

Although mounting fiberglass across the corner of a room is best for treating

bass fre#uencies, panel traps wor" on a different principle where the gap doesnot help. 3o with panel traps it's better to put two in each corner, flat against thewall, because that provides twice the surface area of ust one trap mountedacross the corner. )ass traps built from porous materials li"e fiberglass andacoustic foam wor" by absorbing the sound waves as they pass through thematerial. This type of trap is called a elocity absorber because it is velocity?speed; that drives the sound wave into the absorbing material. A wood paneltrap wor"s on an opposite principle, wave pressure, and is considered a

pressure absorber because the wave pressure is greatest at the roomboundaries. @ou can thin" of a wood panel bass trap as being a Bshoc"absorberB for sound waves. As a wave approaches a wall it has plenty of velocity

?the speed of sound; but no pressure. And when it hits the wall there's no longerany velocity but now there's plenty of pressure. This is similar to driving a car intoa tree. @ou can be going D( miles per hour toward the tree - lots of velocity - andthe instant you hit the tree there's no velocity but plenty of pressure Top

AS .##D AS !T .+TS7

0"ay, so how much improvement can you really e$pect after installing basstraps* nless you cover nearly all of the wall and ceiling surfaces with materialthat is ((L absorbent at all problem fre#uencies - which is pretty muchimpossible - you will still have some deviation from a perfectly flat response. )ut

even with a more practical amount of treatment, you can achieve far less severityin the ripples and also increase their bandwidth so the pea"s and dips arebroader, ma"ing them less damaging and less li"ely to affect single bass notes.@ou'll probably still have pea"s and dips that you can measure and identify withsine wave tests, but the music will sound much better, and the bass levels willvary much less around the room. The difference in fre#uency response of a roomwith and without membrane panel bass traps is shown in 1igure below.



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-igure @: That1s more li$e it7 This response plot is &or the same control room shown in-igure 2 but a&ter treatment with wood panel bass traps.

Top

PART (: R##, D+S!." A"D LA1#$T

0ne of the most important properties of a room is its modes, or natural resonantfre#uencies which are related to its length, width, and height. 4ore often than notthe room you use for a studio or home theater has already been built, so "nowingthe modes and other permanent properties of the room is academic at best. Afterall, what's the point in calculating the modes if you can't do anything about them*

And since all listening rooms need treatment at all low fre#uencies, "nowing themodes doesn't even help you determine what type of bass traps you need.8erhaps you are luc"y enough to have the lu$ury of designing an audio room andcontrolling its size and shape before it is built. In that case you can ma"e ameaningful difference in the room's acoustic #ualities by carefully choosingproper dimensions. If not, there's still plenty you can do to ma"e an e$isting roomas good as it can be. Top

R##, S!+S A"D SAP+S

The size and shape of a room determines its natural resonances - often calledroom modes. Every rectangular room has three sets of primary modes, with oneeach for the length, width, and height. If you have an irregular room shape orangled walls, you can average the dimensions to get a rough idea of the modefre#uencies. That is, if the length wall is angled, ma"ing the width ( feet at oneend and feet at the other, you can use as the average for the width



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dimension. 5ooms with irregular shapes, such as an alcove, have more thanthree sets of modes and are more difficult to calculate.

6enerally spea"ing, larger rooms are better acoustically than smaller roomsbecause the modes are spaced more closely, yielding an overall flatter response.

Acoustics e$perts recommend a minimum volume of at least (( cubic feet forany room in which high #uality music reproduction is intended. 1igure belowshows the modes for ust one dimension - let's say the length - of two differentrooms. +ere, the larger room ?top; has a length of feet, so the fundamentalmode fre#uency, which occurs at half the wavelength, is ( +z. 3ubse#uentmodes, similar to harmonics of a note played by a musical instrument, occur at( +z intervals. Even though this creates many little resonant pea"s in theresponse, the pea"s are close together, so the aerage response is fairly flat.

And as one pea" is falling, the adacent pea" is rising to help compensate and fillin the void. ?8lease note that 1igures and ( are appro$imations as drawn in agraphics program, so the shapes of the pea"s and dips are not truly accurate.;Top

-igure : n a large room ;top


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same fre#uencies. In an ideal room, each dimension will contribute pea"s atdifferent fre#uencies, thus creating more pea"s having a smaller distancebetween them. This is shown in 1igure ( below.

-igure &3: The modes &or a room with ideal ratios ;top< gie a more een response oerall than aroom with poor ratios ;bottom


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9idthLengt

h

.(( .> .=

.(( . .>

.(( .D( .==Tab*e (: The ideal room has a ratio o& height, width, and length similar to one o& these.

There are other good ratios, but those shown above are the ones I seereferenced most often. 2ote that when a room has a suspended tile ceiling thereal height, as far as low fre#uencies are concerned, is to the solid surfaceaboe the tiles. /i"ewise, in a basement with e$posed oists the true height is tothe bottom of the floor above, not the bottom of the oists. Top

That said, I believe the importance of room modes is often overstated. @ou don'twant the width to be the same as the depth or an even multiple such as ( feet

by ( feet. )ut the modes ust describe where the resonances will be worst.5egardless of the room's size and shape, standing waves and acousticinterference happen at all low fre#uencies. 3o you still need bass traps thathandle the entire range, not ust the fre#uencies determined by the room modes.

As far as acoustic interference is concerned, the only thing that changes withdifferent room dimensions is where in the room the pea"s and dips at each lowfre#uency occur.

There are many freeware and web-based room mode calculators, but all theones I've seen ust list a table of the modes, so you still have to plot them byhand on semi-log graph paper to get a sense of how close they are to each

other. +ere is a lin" to 4ode&alc ?only G H) to download;, a room modecalculator I wrote that runs in !03 and %indows. It plots the first ten primaryroom modes graphically so you can see how the modes are distributed and howthey relate to one another. The modes for each dimension are displayed in adifferent color, and when two or more modes occur near the same fre#uency, theduplicates are shown on a separate line so one does not obscure the other. Theprogram is easy to use, and pressing 1 displays complete instructions ande$plains how to interpret the results. 3ince the instruction manual containsadditional e$planations about room modes, it is reprinted below in the sidebar5oom 4odes and 4ode&alc. Top

R##, S1,,+TR1

nless you plan to record and mi$ in mono only, the symmetry of your room andloudspea"er placement are very important. If both loudspea"ers are not situatedsymmetrically in a room they will have a different fre#uency response, and yourstereo imaging will not be balanced. In a room that is longer than it is wide, it'sbetter to place the spea"ers near the shorter wall so they fire the long way into

http://www.ethanwiner.com/acoustics.html#tophttp://www.ethanwiner.com/MODECALC.EXEhttp://www.ethanwiner.com/acoustics.html#modecalchttp://www.ethanwiner.com/acoustics.html#tophttp://www.ethanwiner.com/acoustics.html#tophttp://www.ethanwiner.com/MODECALC.EXEhttp://www.ethanwiner.com/acoustics.html#modecalchttp://www.ethanwiner.com/acoustics.html#top


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the room as shown on the left in 1igure below. This puts you farther from therear wall where the low fre#uency pea"s and nulls are most severe.

-igure &&: ymmetry matters7 n a typical stereo miing room, the loudspea$ers are spaced e/ually&rom the walls and corners, and &orm an e/uilateral triangle at the mi position. The arrangement


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shown on the le&t aboe is better than the one on the right because it1s more symmetrical within theroom. The layout on the right also su&&ers &rom a &ocusing e&&ect caused by the wallwall =unctionbehind the listener.

)esides positioning the loudspea"ers symmetrically, you should also place yourconsole and chair so your ears are the same distance from each spea"er.

/i"ewise, acoustic treatment - whether absorption or diffusion - should be appliede#ually on both sides. In many home studios it is not possible to create acompletely symmetrical arrangement, but you should aim for as close to thisideal as possible. Especially in the critical front part of the room where the firstreflections to reach your ears are those from the side walls, and from the floorand ceiling if they're not treated with absorbent material. %hat happens in therear of the room is probably less important. Top

Although the sample rooms shown above in 1igure are rectangular, I preferangled walls and an angled ceiling because that provides deflection whichreduces flutter echoes and ringing. 3ome people argue that parallel walls are

preferred because you can better predict the room modes, and then treat theinevitable flutter echoes with absorption. )ut as I e$plained earlier, simply"nowing the modes is not always that valuable, and with angled walls you canma"e the aerage dimensions comply with the ideal ratios. 1urther, if a room hasparallel walls that must be treated with absorptive material to avoid echoes andringing, you may not be able to ma"e the room as live as you'd li"e. 3ee thesidebar 4reating a Re&lection -ree >one for more related information.

A pea"ed ceiling is better than a flat ceiling because it avoids the echoes andringing that occur when the ceiling is parallel to the floor. )ut a pea" creates afocusing effect, much li"e a parabolic dish, which is less than ideal. 1or this

reason it's a good idea to place absorption or diffusion under the pea"ed portion,as shown in the photo below.

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These 4iniTraps ?commercial acoustic panels; were installedunder the pea"ed ceiling in the author's home recording studio toavoid focusing sound in the room to the area under the pea".Top

0ne somewhat controversial aspect of control room design is soffit mounting themain loudspea"ers. 4ost home studio owners simply put their spea"ers onstands, or sit them on the mi$ing des", and leave it at that. )ut many pro studiosprefer to install the spea"ers into the wall so the front surface of the spea"er

cabinet is flush with the wall. There are sound scientific reasons to use soffitmounting, yet some engineers say it's not necessary or that it gives poorerresults. Those in favor of soffit mounting point out that it reduces reflectionscalled pea$er "oundary nter&erence, or 3)I5, that cause pea"s and dips in thelow fre#uency response. If a loudspea"er is out in the room away from the wall,low fre#uencies from the rear of the cabinet will bounce off the wall behind it andeventually collide with the direct sound coming from the front of the spea"er.?Even though it may not seem obvious, very low fre#uencies do in fact leave aspea"er cabinet in all directions.; 8roponents also claim that soffit mountingimproves stereo imaging by reducing mid and high fre#uency reflections.

I happen to side with those in favor of soffit mounting, yet I also respect theopinions of those who disagree. 0ne thing nobody will dispute is that soffitmounting re#uires a lot more effort If you do use soffit mounting, pleaseunderstand that the spea"ers must be built into the real wall. @ou can't ust applya lightweight facade around the front of the spea"er cabinet and e$pect the sameresults. Top

L!8+ #R D+AD ' 9!% !S B+ST A"D 9+R+7



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If you've ever seen photos of high-end recording studios in magazines, youprobably noticed that the studio room floors almost always use a reflectivematerial li"e wood or linoleum. A hard floor gives a nice ambience when mi"ingdrums, guitar amps, and acoustic instruments. /i"ewise, auditorium stages andschool band rooms always have a reflective floor surface too. As mentioned

earlier, BliveB in this conte$t refers only to mid and high fre#uencies. I cannotemphasize enough the importance of a reflective floor for achieving a naturalsound when recording acoustic instruments. If you record in your living room andyour spouse refuses to let you remove the carpet, get a >- by -foot sheet of C>-inch plywood to put over the carpet when recording. @ou can cut it in half foreasier storage and put the halves ne$t to each other on the floor when needed.

&ontrol room floors are sometimes carpeted, sometimes wood, and often acombination of the two. &eilings in these types of rooms also vary between fullyreflective, fully absorptive, or a mi$ of surface types. There is no one correct wayto treat every room because different engineers prefer a different amount of

liveness. +owever, you should never ma"e a room completely dead because thatproduces a creepy and unnatural sound. The only time you might considerma"ing a room entirely dead is when treating a small vocal booth or a ery smallstudio or control room - smaller than, say, ten by ten feet. %hen a room is verysmall the reflections are too short to be useful and ust ma"e the room bo$ysounding. In that case the best solution is to cover all of the surfaces entirely withabsorbent material and, for a studio room, add any ambience electronically later.Top

In a more typical room I recommend a mi$ of hard and soft surfaces for the walls,with no one large area all hard or all soft. I suggest applying absorbent materialto the walls using stripes or a chec"erboard pattern to alternate between hardand soft surfaces every two feet or so. This ma"es the room uniformly neutraleverywhere. @ou can ma"e the spacing between absorbent stripes or s#uareslarger or smaller to control the overall amount of liveness. If you are using G(-15H rigid fiberglass or an e#uivalent product, you can cover more of the wall andstill control the liveness by alternating the direction of the paper bac"ing. That is,one piece of fiberglass will have the paper facing the wall to e$pose the moreabsorbent fiberglass, and the ne$t piece will have the paper facing out to reflectthe mid and high fre#uencies. In fact, when the paper is facing into the room thelower fre#uencies are absorbed even better than when it is faces the wall.

Alternating hard and soft surfaces is also advisable with wood panel bass traps -simply place a fiberglass absorber between each trap. @ou can see thisarrangement in the photo of my studio ?above 1igure G;, where each type of basstrap alternates with the other type and with fiberglass panels. That is, first is alow-bass trap, then a fiberglass panel, then a high-bass trap, then fiberglass,then low-bass, and so forth. I'll also mention that wood panel bass traps can bemounted horizontally when boo" shelves and other obstacles prevent placingthem vertically. 3ince the corner formed by a wall and the ceiling, or a wall and



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the floor, is ust as valid as any other corner, mounting a panel trap sidewaysnear the top or bottom of a wall is e#ually effective.

0f course, many studios do have large live areas, and there's nothing wrong withthat If the room is big enough to avoid short echoes between closely spaced

walls, having an entire wall reflective can yield a very big sound. And even insmaller rooms a hard floor with one or more bare walls can be useful. 4y celloteacher, who is a total audio neophyte, blew me away with the #uality of arecording she made in her small 4anhattan apartment. 3he recorded whileplaying with her bac" against the corner, facing into the room, using anine$pensive stereo mi"e placed a few feet in front of her cello. The "ey to arealistic and present sound, especially for acoustic instruments, is capturingsome amount of ambience - even when the reverberation of a large space is notappropriate. Top

Although it is often desirable to alternate hard and soft surfaces on the walls, I

often recommend covering the entire ceiling with absorbent material, especially ifthe ceiling is low. )esides eliminating floor to ceiling flutter echoes, full absorptioncan ma"e the ceiling appear acoustically to be much higher. 4ost home studioowners cringe at the thought of ma"ing their ceilings even lower than theyalready are, but it really can help the sound. If you cover the entire ceiling with -to >-inch thic" G(, suspended with strings or wires to leave an air gap, the roomwill sound as if the ceiling were much higher. There's no difference betweenreflections that are reduced by the greater distance of a high ceiling andreflections from a low ceiling that are reduced by absorption. sing thic", densefiberglass e$tends the simulated increase in height to lower fre#uencies. %herethin fiberglass ma"es the ceiling appear higher at midrange and high

fre#uencies, using thic"er and denser fiberglass with an air gap raises theapparent height at lower fre#uencies as well.

Another advantage of full absorption on a low ceiling is that it avoids the combfiltering that occurs when mi"ing drums and other instruments from above.8lacing microphones high over a drum set or string section puts the mi"es veryclose to the ceiling. If the ceiling is reflective, sound will arrive at the mi"es viatwo paths - the direct sound from the instrument and the same sound after beingreflected off the nearby ceiling. %hen the difference in distance is very small, let'ssay one foot, the reflections cause many pea"s and dips in the response, whichare very audible and can sound li"e a flanger effect. ?%hen reflections cause aseries of pea"s and dips, the effect is often called comb &iltering because thefre#uency response plot resembles a hair comb.; Again, reducing strongreflections from a nearby ceiling via total absorption is acoustically identical tohaving a ceiling that's infinitely high. Top

"#!S+ %#"TR#L



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5educing noise and sound lea"age is beyond the scope of this article, but I willshare a few tips studio owners may find useful. If your studio has forced airventilation, be sure to place the microphones away from the vents whilerecording. If the vents have adustable deflectors, set them to direct the air awayfrom where you normally place your microphones. )etter, allow the room to get

to the desired temperature before you start recording so you can turn off theblower. @ou can turn it on again between ta"es if needed. /i"ewise, radiatorsoften ma"e crea"ing sounds due to e$pansion and contraction as they warm upand cool down, so use them before you start recording.

Another troublesome noise source in many studios is the fan noise from acomputer. @ou can buy a low noise replacement power supply from 8& 8owerand &ooling and other companies. Easier, buy a computer from one of the bettermanufacturers because they often have much less fan noise than the cheaperbrands. 4y last three computers were !ells, and they have all been very #uiet.The small premium you pay for a better brand is easily gained bac" by not

replacing the power supply or having to build or buy a sound proof enclosure.

I also attached G(= fiberglass wrapped with fabric to the rear and underside ofmy des", as shown in the photo below ?left;, to absorb the fan noise rather thanreflect it into the room. )etween the !ell's #uiet power supply and the fiberglass,I can record myself playing the cello or acoustic guitar while sitting in front of thecomputer, with the mi"es pointed right at me and the computer, and still pic" upvery little noise. A second piece of G(= ?right; can be placed in front of thecomputer to reduce the noise even further while recording. Top

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0ne easy way to reduce noise from a computer is to line the surrounding surfaces with absorbent material.

If you've done all you can to reduce ambient noise and it's still too loud in arecording, consider using digital noise reduction. 4any programs are availablethat do a remar"able ob of removing any type of steady noise - not ust hiss, but

hum and air conditioning rumble too - after the fact. I use 3onic 1oundry's 2oise5eduction plug-in, but other affordable programs are available that also do ane$cellent ob.

,#R+ R+S#$R%+S

I have tried to ma"e this article as complete as possible, but it is impossible tocover every aspect of acoustics. 4any boo"s have been written about acousticsand studio design, and my goal here has been to cover only the issues that aremost important to recording engineers and audiophiles. 1urther, acoustics is asmuch an art as a science, and surely mine are not the only valid opinions.

1ortunately, the Internet offers many resources for more information including myown Acoustics forum at E9 4agazine, John 3ayer's 3tudio !esign forum, the3AE web site, the Acoustics newsgroup, and Angelo &ampanella's Acoustics1A9. 8erhaps the most valuable resource of all is 6oogle, where you can findweb pages that cover nearly any topic. Top

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