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Acoustic Designfor the Home Studio
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
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
Characteristics of Waves Frequency Amplitude Wavelength Phase
Sound WavesSound Waves
Sound travels in waves radiating outward from a vibrating source.
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?
How does it equate to something like horsepower or torque?
Sound Pressure Level
SPL
Sound Pressure Level
SPL
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
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
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…
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!
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!
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?
So what can we do about it?
Absorption and Diffusion.Yea!!!!
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?
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…
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.
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
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.
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
lows.
More common. Pourous. Large thick absorbers at
appropriate locations. They catch mid and highs too. But they must be very big to catch
lows.
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…
If you can’t get the Golden Mean…
Bigger is BetterRectangular is Better
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.
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
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
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
room.
Thick with the pad does a little. Does not absorb any lows. Could result in a boomy dead
room.
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
Furniture Solves Acoustic Problems
Some truth. But Minimal.
Some truth. But Minimal.
Bass Trapping Takes a Lot of Space
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
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…
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:
Go here for a sample table of Absorption Coeficients:
http://hyperphysics.phy-astr.gsu.edu/Hbase/acoustic/revmod.html#c4
http://hyperphysics.phy-astr.gsu.edu/Hbase/acoustic/revmod.html#c4
Compile our findings into a chart…
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…
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