Room Reflections

8/14/2019 Room Reflections

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ROOM REFLECTIONS

CAUSE ACOUSTIC DISTORTION

The audio product created in a studio needs to sound similar in a wide range of listeningenvironments. Thus the audio product must be transferable to these different environments. The

engineer must be aware of the acoustic signature of the room in which the audio product iscreated so that the rooms acoustic signature is not embedded into the audio product.

Since we have no control over the acoustical design of the environments into which the audioproduct will be transferred to, every effort should be made to provide good acoustics in thecreation environment so that the necessary image and signal balances can be executedcorrectly so that countless hours creating the product are not wasted.

The recording industry is conscious of electronic distortion, but acoustic distortion is oftenoverlooked in the pursuit of more electronic equipment.

Experience the difference

andLISTEN TO THE MUSIC,

NOT THE ROOM!

When the sound from a loudspeaker encounters the boundaries of a room, a very complexseries of reflections occur. t is very difficult to isolate the direct sound alone, because thesereflections interact with it and among themselves to produce a wide range of effects, which wecall acoustical distortion. f proper acoustic design is not utili!ed, a room will introduce sonicdistortion which prevents the "ngineer from hearing all of the detail information theloudspeakers and electronics are capable of delivering.

The #coustic $istortion introduced by the room can be so influential that it dominates the overallsonic impression.

The causes of acoustic distortion are:-

Modal Coupling- the acoustical coupling between the speakers and listener with therooms modal pressure variations or room modes.

Speaker-Boundary Interference- the coherent interaction between the direct sound andthe omni%directional early reflections from the rooms ad&acent boundaries.

Comb Filtering- the coherent constructive and destructive interference between the directsound and early reflections.

Sound Diffusion- the spatial and temporal reflection pattern due to mid and late arrivingreflections.

When one stops and realises that the speaker ' room interface is your acoustical microscope, itseems prudent to strive for the ultimate sonic resolution.

ound for ound the acoustical treatment in a room !illmake more of an audible difference than any piece of

electronic hard!are" speaker" or cable


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ACOUSTIC DISTORTION

AFFECTS THE PERCEPTION OF SOUND

#coustic distortion affects three psychoacoustic perceptions, namely(%

Fre#uency $esponse or Timbre- refers to the perception of the harmonic content of themusic and its spectrum.

Imaging- refers to the si!e, and apparent position, of sonic images in the stereosoundstage.

Spatial Impression- is the sense of spatiality, envelopment or sense of immersion in thesonic experience.

#ll three forms of distortion occur across the entire frequency spectrum, but room modes andthe speaker'boundary interactions predominate at low frequencies.

Electronic Equivalent

of Room Colouration

magine secretly inserting a parametric equaliser and digital delay unit into the signal path priorto a critical mix with the circuit set to boost(%

)*+!, *-+! and -*+! by */d0and introduce

ms, )ms and */ms of delay.

The engineer would mix the sound and subconsciously attempt to ad&ust for these effects.magine the engineers surprise when they were informed of the trick and they played back themix with the equaliser and delay switched off 11

This is 23T a practical &oke because the ROOMis playing tricks on the engineer%%&& T'( TIM( ))

The (ngineer !ill al!ays be mi*ing !earing +room coloured+ ear muffs ))

ROOM MODES

Sound waves reinforce and cancel each other as they reflect back and forth between hard walls.This constructive'destructive interference results in resonances at frequencies determined by thedistances between the walls.

4or example an 56 floor to ceiling dimension results in a fundamental room mode of )*+!. This

modal response is characteri!ed by high and low pressure !ones throughout the room. Theloudspeaker placement will accentuate or diminish modal frequencies depending on placement.


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Similarly, an engineer will hear different modal responses depending on where they are seated.

Fig1illustrates how the sound energy is distributed along

a room dimension. The room dimension is shown as afraction ranging from / to *./.7 would be in the center of the room and * would beagainst a wall. "xamining Fig1reveals that thefundamental has no energy in the center of the room.

Fig1

hysically this means that if you !ere sitting inthe middle of the room you !ould not hear this fre#uency

The first harmonic, however, is at a maximum. t can be inferred from this plot that, in the centerof the room all even harmonics are absent and all odd harmonics are at a maximum.

,hen listening to music in a room"the music !ill be modified by the rooms modal response and this

acoustic distortion !ill depend on !here the speakers and the (ngineer are located

#s an example, in Fig2presents the modalfrequency response of a room with a *76dimension. The microphone is located against a

wall perpendicular to this dimension so that allmodes along this dimension will be present. Thefirst three modes are identified as 8*,/,/9,fundamental, 8-,/,/9, first harmonic and 8,/,/9,second harmonic. #t low frequencies the roommodes are widely separated and present apotential problem. Fig2.

The (1,0,0), (2,0,0), and (3,0,0) modes are

identified.

#s the frequency increases, room modes are still present, but their number and density increaseand are not perceived as a problem. This form of modal acoustic distortion is best addressedwith low frequency absorption.

3ne can easily simulate the effect of room modes by maximi!ing a parametric equali!er at onlyspecific frequencies associated with the room6s dimensions. f the room did not introduce anymodification to what is being heard Fig2would be a flat line1

Speaker-Boundary

Interference Response

:oom modes develop as reflected sound interferes with itself. This type of acoustic distortion isdue to the coherent interference between the direct sound from a loudspeaker and the

reflections from the room, in particular the corners immediately surrounding it.


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This distortion occurs across the entirefrequency spectrum, but is more significant atlow frequencies. We refer to it as the Speaker0oundary nterference :esponse or S0:. n

Fig3a loudspeaker is located 6 from eachroom surface with coordinates 8,,9. Thefour virtual images responsible for first orderreflections are also shown. 3ne on theopposite side of the main room boundaries. #virtual image is located an equivalent distanceon the opposite side of a room boundary. Thedistance from a virtual source to the listener isequal to the reflected path from source tolistener. n addition to the four virtual imagesshown, there are ) more. Three virtual imagesand * real image in the speaker plane and images of these above the ceiling and floorplanes. magine that the walls are removedand ** additional physical speakers arelocated at the virtual image positions. Theresultant sound at a listening position would beequivalent to the sound heard from one sourceand ** reflections.

Fig3. Sound from real and virtual speakers comine

to create the speaker!oundar" interference

response.

Fig#. Speaker$%oundar" &nterference 'esponse for

several loudspeaker positions in a corner.

The effect of the coherent interference betweenthe direct sound and these virtual images isillustrated in Fig4.The S0: is averaged overall listening positions with the speaker located6 from one, two and three walls surroundingthe loudspeaker. t can be seen that as each

wall is added, the low frequency responseincreases by .dBand the notch, at roughly*//+!, gets deeper. #t low frequencies there istypically very little absorption efficiency on theboundary surfaces and the notches can bebetween . and /0 dB )#s in the case of room modes, the S0:response in Fig4would be a flat line at /d0.The conclusion is obvious(%

1e2er place a speakers !oofer e#uidistant from the floorand t!o surrounding !alls

The low frequency rise illustrates why one can add more bass by moving the speakers into thecorners of a room.

COMB FILTERING

#nother form of acoustic distortion introduced by room reflections is comb filtering. t is due tointerference between the direct sound and a reflected sound. n ;rofessional


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The location of the first notch is given by the speed of sound divided by twice the total pathlength difference. The spacing between subsequent notches is twice this frequency. 4orexample, a *ms delay corresponds to a path length difference of *.* feet. The first notchoccurs at 7//+!, with subsequent notches *///+! apart. The audible effect of comb filtering iseasy to experience using a delay line and produces various effects such as chorusing orflanging, depending on the length of the delay. Shorter delays have wider bandwidth notches

and thus remove more power than longer delays. This is why microsecond and milliseconddelays are so audible. The effect of a reflection is illustrated in Fig5and Fig6.

n Fig5we illustrate the time and frequency responsesfrom the right speaker only. The upper curve shows thearrival time and the lower curve shows the free%fieldresponse of the loudspeaker.

n Fig6we show the effect of adding a side wallreflection to the sound of the right speaker. 2ow theupper curve shows the arrival time of both the direct

sound and the reflected sound. The lower curve showsthe severe comb filtering that a single reflectionintroduces. f a speaker had a free%field response likethis lower curve, it would never be accepted. =et, manyrooms are designed without reflection control.

Fig.

Time and frequency response of the direct sound from a

loudspeaker.

Fig.

Time and frequency response of the direct sound

combined with a side wall reflection.

The comb filtering is apparent.

The message is to absorb these interfering reflections"so !e can listen to the music"

and not the room3

IMAGING

maging refers to our ability to perceive and accurately locate the instruments, voices andeffects which comprise the soundstage. The factors affecting locali!ation and the acousticsoundstage are reflections from the room6s boundary surfaces which cause comb filtering.These reflections cause frequency response notches and peaks which fool the ear'brain8auditory system9 and degrade our ability to experience the sonic images as they were intended

to be perceived. maging is optimi!ed by an maging #coustic Treatment which absorbs theroom6s first order reflections over a wide range of frequencies.

SPATIAL IMPRESSION

The spatial impression refers to the intangible sensation of openness, envelopment orspaciousness and the feeling that we are part of or immersed in the audio'video event we are

experiencing. >ood


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room. The use of sound diffusing surfaces on the rear wall help to naturally create animpression of ?passive surround sound?.

Thus $iffusive #coustic Treatment is used to create envelopment, widen the sweet spot, addwarmth and naturalness to the sound, and uniformly distribute the sound throughout theroom3

FREQUENCY RESPONSE

(Bass Response)

The perceived bass response in a room is controlled by(%

The free-field speaker response"

The room dimensions"The acoustic coupling of the engineer4speakers to the rooms modal pressure

2ariations"The speaker-boundary interference bet!een the direct sound and ad5acent

reflections"

The internal contents of the room"The acoustical nature of the boundary surfaces and surface treatment and

The hearing response and training of the engineer3

%ny absorption or amplification o2er a fre#uency range"that is introduced by the room"

!ill color the sound that !e hear

The room !ill thus introduce its o!n signaturerather than ha2ing a flat fre#uency response3

Since most of today6s speakers are of high quality and we may not have any control over therooms dimensions, we can focus our attention on proper speaker placement and acousticaltreatment.

;roper speaker placement can minimi!e exciting the room6s resonant frequencies and theinterference between the speaker and reflected sounds from the nearby corner. This can resultin severe cancellations if the speaker is located an equal distance from all boundaries.

#fter locating the speakers 8and engineer9 properly, 0ass #bsorption should be utilised toreduce the room resonances and minimi!e the speaker'boundary interference.

DAMP the MODES

The magnitude of the room mode effects can be reduced by increasing the damping factor of allthe modes by suitable use of low frequency absorbers. The most common are acoustical foamor other porous absorbers which absorb sound by converting sound energy into heat.. The

efficiency of a porous absorber is highest when the sound is travelling at its highest velocity.This point is reached at @ of the wavelength and thus varies with frequency. #t low frequencies


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like *//+!, this distance is about -.7 from the wall. Anfortunately at the wall surface where theporous absorber is usually placed, the particle velocity is !ero. This is where the soundwaveschange direction on reflection and hence the velocity is !ero. Since porous absorbers rely onparticle velocity, they have minimal efficiency at low frequencies.

The point to be taken away is that(%

&%CI16 7$78S %BS7$B($S71 % ,%&&

IS 7I1T&(SS

ACOUSTIC TREATMENTS

MINIMIZE ACOUSTIC DISTORTION AND

IMPROVE THE PERCEPTION OF SOUND

S8MM%$9

f you cant take the room out of the mix, you cant take the mix out of the room 1

f you are building a new room, use good dimensional ratios to provide uniform modalfrequency distribution.

$esign for a symmetrical listening environment for good imaging. ;lace the speakerssymmetrically and on axis for the best response.

Bocate the speaker and engineers position to optimise the acoustical coupling with therooms pressure variations and speaker%boundary interference. i.e. optimise the bass response.

Cinimise first order reflections from the walls, ceiling and floor between the speakers andthe listening position using appropriate absorption or diffusion.

B(,%$(of console reflections and minimise them.

$iffuse rear wall reflections over at least D/E of the surface.

;rovide low frequency absorption on the rear wall to minimise low frequency cancellationeffects.

$amp low frequency modes by applying appropriate low frequency absorbers at maximumroom pressure locations.

CAUTION

%bsorption coefficients of many absorbers are measured and #uoted for $andomIncidence Sound $eflections3

The reflections in a studio" especially from the side-!alls" are mostly at graing angles38nder these circumstances many materials can appear reflecti2e to the higherfre#uencies and hence their performance !ill be belo! that e*pected3

Documents

Room Reflections