Acoustic Design for the Home Studio By Mitch Gallagher

Acoustic Designfor the Home Studio

By Mitch GallagherBy Mitch Gallagher

Acoustics Defined Acoustics Defined acoustics. noun. 1. The scientific study

of sound. 2. The characteristic way in which sound carries or can be heard within a particular enclosed space, for example, a concert hall.

No matter how cool your gear, improving acoustics will result in the biggest sonic improvement you’re going to get.

Good acoustics sound and feel better, and allow you to hear more accurately.

acoustics. noun. 1. The scientific study of sound. 2. The characteristic way in which sound carries or can be heard within a particular enclosed space, for example, a concert hall.

No matter how cool your gear, improving acoustics will result in the biggest sonic improvement you’re going to get.

Good acoustics sound and feel better, and allow you to hear more accurately.

Acoustics versus Sound Isolation

Quality of sound in a room does not address isolation.

Stopping the transmission of sound from one location to another is a different issue altogether.

Soundproof is like waterproof - doesn’t happen without a hefty investment.

Most acoustic treatment materials have little or no sound isolation value.

Quality of sound in a room does not address isolation.

Stopping the transmission of sound from one location to another is a different issue altogether.

Soundproof is like waterproof - doesn’t happen without a hefty investment.

Most acoustic treatment materials have little or no sound isolation value.

Basics of Sound Basics of Sound

Characteristics of Waves Frequency Amplitude Wavelength Phase

Sound WavesSound Waves

Sound travels in waves radiating outward from a vibrating source.

FrequencyFrequency

How fast the wave vibrates. Determines the pitch “A-440” means that the note A has

a frequency of 440 Hz, which means 440 cycles, or wave vibrations, per second

How fast the wave vibrates. Determines the pitch “A-440” means that the note A has

a frequency of 440 Hz, which means 440 cycles, or wave vibrations, per second

AmplitudeAmplitude

Volume or “level” of a sound is measured using decibles

A decibel is the smallest volume change that a human ear can perceive without a reference.

Because the range of levels is so wide, the decibel scale is logarithmic to keep things manageable.

Volume or “level” of a sound is measured using decibles

A decibel is the smallest volume change that a human ear can perceive without a reference.

Because the range of levels is so wide, the decibel scale is logarithmic to keep things manageable.

What does it mean?What does it mean?

You don’t have to make much of a change dB-wise to hear a relatively large change in volume.

A 3 dB increase or decrease is quite a bit.

A 10 dB increase is twice as loud.

You don’t have to make much of a change dB-wise to hear a relatively large change in volume.

A 3 dB increase or decrease is quite a bit.

A 10 dB increase is twice as loud.

How does it equate to something like horsepower or torque?

Sound Pressure Level

How much force the wave exerts thru air. Measured in micro Pascals.

WavelengthWavelength

Measurement of a wave’s physical length. Higher the frequency the shorter the wavelength. Lower the frequency the longer the wavelength. Wavelength in combination with phase affects where

there will be problems in a room

Measurement of a wave’s physical length. Higher the frequency the shorter the wavelength. Lower the frequency the longer the wavelength. Wavelength in combination with phase affects where

there will be problems in a room

Frequency versus WavelengthFrequency versus Wavelength

20 Hz 56.3 feet

60 Hz 18.8 feet

100 Hz 11.3 feet

160 Hz 7.0 feet

320 Hz 3.5 feet

500 Hz 2.3 feet

1 kHz 1.1 feet

2.5 kHz 5.4 inches

5 kHz 2.7 inches

10 kHz 1.4 inches

20 kHz 0.7 inches

PhasePhase Describes the relationship of waves or signals in time Phase is extremely important because waves that are out of phase

by even a small amount can cancel each other, resulting in tonal changes.

Phase problems occur when sound bounces around within a room; the reflecting waves interfere with each other destructively, causing all sorts of problems.

Describes the relationship of waves or signals in time Phase is extremely important because waves that are out of phase

by even a small amount can cancel each other, resulting in tonal changes.

Phase problems occur when sound bounces around within a room; the reflecting waves interfere with each other destructively, causing all sorts of problems.

Identical Waves Cause Very Predictable Phase Interactions

But it can get very complicated…

Reflection ControlReflection Control

Sound waves can be reflected by surfaces in a room and bounce around.

Sound waves can be partially absorbed and bounce around a bit less.

Or they can pass through or around the surfaces in the room and exit to the outside.

Sound waves can be reflected by surfaces in a room and bounce around.

Sound waves can be partially absorbed and bounce around a bit less.

Or they can pass through or around the surfaces in the room and exit to the outside.

Any of these behaviors can be problematic

Sound waves bouncing around can interfere with each other.

Sound waves escaping the room can interfere with others tranquility.

If sound can get out, sound could get in.

Sound waves bouncing around can interfere with each other.

Sound waves escaping the room can interfere with others tranquility.

If sound can get out, sound could get in.

How Sound Behaves in a Room

Frequency of the Sound Wave. The shape and dimensions of the room. Materials of construction and coverings. Placement of doors, windows, and

contents.

Frequency of the Sound Wave. The shape and dimensions of the room. Materials of construction and coverings. Placement of doors, windows, and

contents.

Sound Wave FrequencySound Wave Frequency At lower frequencies wavelengths are long,

and sound waves have a tendency to bend around objects in the room, to pass thru lighter materials, and to spread out in an omni-directional pattern.

At mid and high frequencies (100 Hz and up) they are directional, bounce off hard surfaces, and are absorbed by softer materials.

Bounces if a surface has a dimension equal to or greater than a sound’s wavelength.

Reacts to softer surfaces like a ball as well.

At lower frequencies wavelengths are long, and sound waves have a tendency to bend around objects in the room, to pass thru lighter materials, and to spread out in an omni-directional pattern.

At mid and high frequencies (100 Hz and up) they are directional, bounce off hard surfaces, and are absorbed by softer materials.

Bounces if a surface has a dimension equal to or greater than a sound’s wavelength.

Reacts to softer surfaces like a ball as well.

Sound inside a room bounces off hard surfaces just like a ball thrown against a hard wall.

More surfaces mean more bouncing.

Thrown against a soft surface…

It will stop dead and fall to the ground.

Or bounce off slower. How much absorption occurs

depends on the materials.

It will stop dead and fall to the ground.

Or bounce off slower. How much absorption occurs

depends on the materials.

But Sound Waves Are More ComplexBut Sound Waves Are More Complex A vibrating source transmits waves in

multiple directions. Loadspeakers will have dispersion

specifications. You hear sound directly from the source

as well as sound waves reflected off nearby surfaces.

Different frequencies have different dispersion characteristics.

A vibrating source transmits waves in multiple directions.

Loadspeakers will have dispersion specifications.

You hear sound directly from the source as well as sound waves reflected off nearby surfaces.

Different frequencies have different dispersion characteristics.

Differences in arrival times creates phasing!

Changes in tonality due to phase is called Comb

Filtering

Changes in tonality due to phase is called Comb

Filtering

First Reflections are the Worst!

Sound travels at about 1.14 foot per millisecond.

Sound waves that arrive after one bounce in less than 20 milliseconds will cause the most problems.

That equals a round trip of 20 feet.

Sound travels at about 1.14 foot per millisecond.

Sound waves that arrive after one bounce in less than 20 milliseconds will cause the most problems.

That equals a round trip of 20 feet.

Early Reflections Evil Sidekick… Flutter EchoEarly Reflections Evil

Sidekick… Flutter Echo Upper mid- and high-frequency

waves reflected between two parallel surfaces makes a rattling fast echo known as flutter echo.

Clap your hands in any small room with parallel bare walls… yuck.

Upper mid- and high-frequency waves reflected between two parallel surfaces makes a rattling fast echo known as flutter echo.

Clap your hands in any small room with parallel bare walls… yuck.

Reverberant DecayReverberant Decay Once the sound gets past the first

reflections, the remaining reflections will tend to mush together, creating reverberation.

Too much reverb is usually bad. RT60=time required for reverberation to

drop below 60 dB. RT60 as well as the reverberant frequency

response is very important. Bright ringing reverb, or low booming reverb

are equally undesirable.

Once the sound gets past the first reflections, the remaining reflections will tend to mush together, creating reverberation.

Too much reverb is usually bad. RT60=time required for reverberation to

drop below 60 dB. RT60 as well as the reverberant frequency

response is very important. Bright ringing reverb, or low booming reverb

are equally undesirable.

So what can we do about it?

Absorption and Diffusion.Yea!!!!

AbsorptionAbsorption

Primary method for reducing mid- and high-frequency reflections, flutter echo, and undesirable reverb.

Soft materials placed at the main reflection points

As sound waves pass thru the material some of its energy is converted into heat.

Primary method for reducing mid- and high-frequency reflections, flutter echo, and undesirable reverb.

Soft materials placed at the main reflection points

As sound waves pass thru the material some of its energy is converted into heat.

Best Absorptive materialsBest Absorptive materials

Open cell foam. Glass fiber. The thicker the better. Thicker absorbs more lower

frequencies.

Open cell foam. Glass fiber. The thicker the better. Thicker absorbs more lower

frequencies.

Is there a such thing as too much absorption?

Yes… since absorption affects mainly hi-frequencies too much can create a dark,

dead, or booming, acoustic space.

Yes… since absorption affects mainly hi-frequencies too much can create a dark,

dead, or booming, acoustic space.

Whats the Trick?Whats the Trick? Enough absorption to control the

mids and highs. Coupled with good bass control. Create a balanced response with

an even reverb across the entire frequency range.

Proper placement of absorption.

Enough absorption to control the mids and highs.

Coupled with good bass control. Create a balanced response with

an even reverb across the entire frequency range.

Proper placement of absorption.

DiffusionDiffusion

Scatters the sound wave to reduce first reflection problems.

More or less any irregular surface. Smoothes out the reverb. Very scientific materials for ultra

critical rooms.

Scatters the sound wave to reduce first reflection problems.

More or less any irregular surface. Smoothes out the reverb. Very scientific materials for ultra

critical rooms.

Low FrequenciesLow Frequencies

An entirely different proposition…

Room Modes and Standing WavesRoom Modes and Standing Waves

In a room low-frequency waves reflect between walls and create standing waves, a.k.a. room modes.

A mode creates resonance and longer reverb decay at that frequency.

A mode will also generate modes at each octave above itself.

In a room low-frequency waves reflect between walls and create standing waves, a.k.a. room modes.

A mode creates resonance and longer reverb decay at that frequency.

A mode will also generate modes at each octave above itself.

A room’s modes are determined by its three dimensions.

All rooms have modes A small room is worse because of the

way modes are spaced You can improve the spacing by making

sure the room dimensions aren’t related.

All rooms have modes A small room is worse because of the

way modes are spaced You can improve the spacing by making

sure the room dimensions aren’t related.

Three types of ModesThree types of Modes

Axial - between two surfaces (the biggest mode problems)

Tangential - between four surfaces Oblique - between all six surfaces

Axial - between two surfaces (the biggest mode problems)

Tangential - between four surfaces Oblique - between all six surfaces

Bass Varies Throughout the Room

All the surfaces and spatial factors of a room contribute

to it’s modal response.

All the surfaces and spatial factors of a room contribute

to it’s modal response.

Bass also tends to build up near surface

boundaries.

Bass also tends to build up near surface

boundaries.

Uhh… near the floor, and in the corners.

Figure it OutFigure it OutAssuming a perfectly

rectangular room - its fairly easy to figure out where the

modes will occur. The fundamental frequency of the mode (F) equals the speed of sound (1130 feet per second)

divided by twice the room dimension (D).

Assuming a perfectly rectangular room - its fairly easy to figure out where the

modes will occur. The fundamental frequency of the mode (F) equals the speed of sound (1130 feet per second)

divided by twice the room dimension (D).

F=1130/2DF=1130/2D

Bass response can be shaped with Bass Traps of which there are two

types:

Bass response can be shaped with Bass Traps of which there are two

types: Tuned Absorbers Broadband Absorbers

Tuned Absorbers Broadband Absorbers

Tuned AbsorbersTuned Absorbers

Slatted or Helmholtz devices which function as resonators.

Panel or membrane (say a thin sheet of wood) traps.

Both are designed to solve problems at specific frequencies… they are tuned.

You need to have the modes identified.

Slatted or Helmholtz devices which function as resonators.

Panel or membrane (say a thin sheet of wood) traps.

Both are designed to solve problems at specific frequencies… they are tuned.

You need to have the modes identified.

Broadband AbsorbersBroadband Absorbers

More common. Pourous. Large thick absorbers at

appropriate locations. They catch mid and highs too. But they must be very big to catch

More common. Pourous. Large thick absorbers at

appropriate locations. They catch mid and highs too. But they must be very big to catch

Can you Trap too much?Can you Trap too much?

Nope But too much broad band

absorption could deaden the highs.

Nope But too much broad band

absorption could deaden the highs.

Room DimensionsRoom Dimensions

The size and volume, shape, and ratios of the three room

dimensions have a major impact.

The size and volume, shape, and ratios of the three room

dimensions have a major impact.

Rules of ThumbRules of Thumb

Worst room dimension are a perfect cube

2nd worst is a square room with a different ceiling hight

Also bad two or three dimensions are multiples of each other or of the same number

There are some good ratios

Worst room dimension are a perfect cube

2nd worst is a square room with a different ceiling hight

Also bad two or three dimensions are multiples of each other or of the same number

There are some good ratios

The Golden MeanThe Golden Mean

Ratio of 0.168 16’ x 10’ = ratio of 16:10 24’ x 15” = ratio of 24:15 If all three dimension come close

to the Golden Mean - for example 24’ x 15’ x 9’ modes will be distributed in a good way.

Ratio of 0.168 16’ x 10’ = ratio of 16:10 24’ x 15” = ratio of 24:15 If all three dimension come close

to the Golden Mean - for example 24’ x 15’ x 9’ modes will be distributed in a good way.

If you can’t get the Golden Mean…

Bigger is BetterRectangular is Better

It may be better to build a single large

room than two small rooms

It may be better to build a single large

room than two small rooms

Working in one room can be advantageous.

If your room does not meet this criteria…

its not the end of the world!

With good acoustic treatment almost any room can sound good

enough.

If your room does not meet this criteria…

its not the end of the world!

With good acoustic treatment almost any room can sound good

enough.

Next step…Acoustic Treatments

But Why Not Just EQ?But Why Not Just EQ?

You can take this approach. Some monitors have it built in. Eq can cause phase distortion. Modes can be vary narrow. Low frequency varies thruout the

room. Use acoustic treatment 1st, EQ to

fine tune.

You can take this approach. Some monitors have it built in. Eq can cause phase distortion. Modes can be vary narrow. Low frequency varies thruout the

room. Use acoustic treatment 1st, EQ to

fine tune.

High Frequency AbsorbersHigh Frequency Absorbers

Glass Fiber. In rigid or fluffy. Owens Corning 700 series. Easy to work with but loose fibers

are an irritant. Must be covered.

Glass Fiber. In rigid or fluffy. Owens Corning 700 series. Easy to work with but loose fibers

are an irritant. Must be covered.

Acoustic FoamAcoustic Foam

Cheaper than pre covered glass fiber.

Can be glued or tacked. Easy to cut with heavy shears,

bread knife or electric knife. Sculpting is mostly for looks but

wedges and pyramids do improve their thickness.

Cheaper than pre covered glass fiber.

Can be glued or tacked. Easy to cut with heavy shears,

bread knife or electric knife. Sculpting is mostly for looks but

wedges and pyramids do improve their thickness.

Rating Acoustic MaterialsRating Acoustic Materials

Baseline for comparison is NRC - Noise Reduction Coefficient which shows the overall performance of the material

A more useful comparison is using acoustic coefficients which show the absorptive qualities at different frequency bands.

Baseline for comparison is NRC - Noise Reduction Coefficient which shows the overall performance of the material

A more useful comparison is using acoustic coefficients which show the absorptive qualities at different frequency bands.

Acoustic MythsAcoustic Myths

Egg CartonsEgg Cartons

Not enough mass to do anything. Flammable

CarpetCarpet

Thick with the pad does a little. Does not absorb any lows. Could result in a boomy dead

Any Foam Will DoAny Foam Will Do

True acoustic foam is scientifically shaped.

And its made of open cell material that allows sound to pass thru.

True acoustic foam is scientifically shaped.

And its made of open cell material that allows sound to pass thru.

Furniture Solves Acoustic Problems

Some truth. But Minimal.

Bass Trapping Takes a Lot of Space

To absorb 30Hz takes a porous absorber at least four feet deep!

Bigger is better. You can achieve good performance

with smaller treatments.

To absorb 30Hz takes a porous absorber at least four feet deep!

Bigger is better. You can achieve good performance

with smaller treatments.

EQ Can Solve All Acoustic Problems

Can reduce level of modal peaks. Does not counter spatial

dependence of modes. No effect on time dependence

mode. EQ can’t reduce decay or reverb

time in a room.

Can reduce level of modal peaks. Does not counter spatial

dependence of modes. No effect on time dependence

mode. EQ can’t reduce decay or reverb

time in a room.

If we could predict some of our room’s characteristics we could focus our $ and efforts

on the biggest problem areas.

If we could predict some of our room’s characteristics we could focus our $ and efforts

on the biggest problem areas.

Oh great Swami what will the reverberation of a room be?Oh great Swami what will the reverberation of a room be?

What about a more empirical approach…

Sabine’s reverberation equation was developed at the turn of the centurySabine’s reverberation equation was developed at the turn of the century

RT60=0.049V/Sa RT60=reverberation time, seconds V=volume of room, cu ft. S=total surface area of room, sq. ft. a=average absorption coefficient of

room surfaces. Sa=total absorbtion, sabins

RT60=0.049V/Sa RT60=reverberation time, seconds V=volume of room, cu ft. S=total surface area of room, sq. ft. a=average absorption coefficient of

room surfaces. Sa=total absorbtion, sabins

Reverberation CalculationReverberation Calculation

Given a room 15 x 20 x 8 ft Volume=(15)(20)(8)=2,400cu ft. Surface area=(2)(8)(15)+(2)(15)

(20)=1,160 sq. ft. Find the absorption coefficients for

all of the materials in the room.

Given a room 15 x 20 x 8 ft Volume=(15)(20)(8)=2,400cu ft. Surface area=(2)(8)(15)+(2)(15)

(20)=1,160 sq. ft. Find the absorption coefficients for

all of the materials in the room.

For Our ExampleFor Our Example

Floor covered with heavy carpet. Wall and ceiling are ½” gypsum

board. There is one 2’ 6” x6’ 8” door and a

3’ x 5’ glass window. To simplify we’ll carry out the

calculations for 500 Hz only. Next we build a table.

Floor covered with heavy carpet. Wall and ceiling are ½” gypsum

board. There is one 2’ 6” x6’ 8” door and a

3’ x 5’ glass window. To simplify we’ll carry out the

calculations for 500 Hz only. Next we build a table.

Go here for a sample table of Absorption Coeficients:

http://hyperphysics.phy-astr.gsu.edu/Hbase/acoustic/revmod.html#c4

Compile our findings into a chart…

S A Sa

Material Area Sq. Ft. Absorption Coefficient

Absorption Units, Sabins

Carpet, Heavy 300 0.30 90.0

Door, wooden 17 0.10 1.7

Window 15 0.04 0.6

Gypsum board, ½’

828 0.05 41.4

1,160 133.7

Substituting our know values…

RT60 = 0.049V/Sa = (0.49)(2400)/(133.7)

= 0.88 seconds

RT60 = 0.049V/Sa = (0.49)(2400)/(133.7)

= 0.88 seconds

Acoustic Design for the Home Studio By Mitch Gallagher

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