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Studio Acoustics
Sound Isolation
An area used for recording sound must be reasonably quiet with some degree of sound isolation. However, the
amount ofbackground noise that can be accepted depends on the kind of studio. For example, apop music studiodoesnt need such a quiet background as a drama studio.
The following table shows the normal levels of background noise, measured in dB above the threshold of hearing,
for common types of studios:-
Type of Studio 100 Hz200 Hz1 kHz5 kHz
General-purpose50 43 28 19
Speech 44 36 23 15
Drama 40 32 19 12
Noise Criteria
Studio designers often use a special chart that defines a studios noise criteria (NC). For example, an area meeting
theNC65 standard has a background noise of less than 65 dB at 4 kHz, although this can rise to 80 dB at 63 Hz. At
the other end of the scale, a studio conforming to the NC15 standard has less than 15 dB of noise, increasing to asmuch as 47 dB at 63 Hz.
A good studio should meet NC15 or NC25. Unfortunately, commercial air-conditioningequipment often increases
the figure to NR40, whilst the ventilation systems employed in offices can even reach NR60.
Materials for Sound Isolation
Most of the background noise in a studio, apart from that produced by air-conditioning equipment or other technica
devices, originates from outside the building. The amount of noise that gets into the studio is dependent on thesolidity of itsstructure. In other words, a studios sound isolation increases in proportion to the total mass of the
material around it. The latter is usually measured in kilogrammes per square metre (kg/m2) of the materials surface
area.
The following table shows the relationship between isolation and total mass:-
Isolation (dB)
Mass (kg/m2) +
17 1.2
20 2.4
22 4.9
25 9.8
27 14.7
30 24.4
34 48.8
37 97.7
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41 146
43 195
44 244
50 488
56 976
Average isolation value over the range 100 - 3200 Hz
+ Total mass measured as mass per unit area
The following table gives approximate total masses for different kinds of materials used in studio construction. The
values for mass are given in kg/m2 per mm thickness of material.
Material Mass (kg/m2/mm)
Aluminium - flat sheet 2.80
Asphalt flooring 1.95 - 2.34
Asphalt roofing - two layers 2.23Block board - laminated 0.46
Block work - hollow, ballast & stone aggregate 1.37
Block work - cellular, ballast & stone aggregate 1.59
Block work - solid, stone aggregate 2.15
Block work - hollow, clay medium density 1.11
Block work - Thermalite 0.77
Brickwork - clay low density 2.00
Brickwork - clay medium density 2.15
Brickwork - clay high density 2.33
Chipboard 0.77Concrete - natural aggregates, 1: 2: 4 mix 2.31
Copper - flat sheet 8.86
Cork - board 0.17
Cork - compressed flooring 0.29
Fibreboard - insulation board 0.27
Hollow clay floor blocks with mortar joints 1.27 - 1.44
Hollow concrete floor units with concrete topping 1.59 - 1.68
Glass - clear plate 2.56
Hardboard 1.06
Lead sheet 11.40Plaster - two coats of gypsum 1.73
Plasterboard - solid core gypsum 0.88
Plywood 0.60
Sand 1.62
Steel - mild steel sheet 7.72
Water 1.00
Wood floor - hardwood strip 0.73
Wood wool slabs- 600 mm wide 0.51
Wood wool slabs - channel reinforced, 600 mm wide0.56
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As you can see, leadis an excellent material for the job, although expensive. Even so, its used in the sandwich-construction of a lead-lined studio door. Unfortunately, although very effective, this kind of door is also
exceptionally heavy and usually requires specialist fixing.
Wall Materials
For every extra 5 dB of sound isolation thats required, the mass of a wall structure must be doubled. The followingtable gives approximate details for different types of fully-sealed walls:-
Wall Type and ThicknessIsolation
(dB)Mass (kg/m2)
50 mm stud frame clad both sides with 6 mm ply 25 10
As above with 12 mm of plaster 30 20
50 mm compressed straw slab, plaster skim 35 50
50 mm hollow clay block 35 50
75 mm clinker block 40 120
Double 50 mm wood wall slab, 50 mm cavity 40 120
112 mm (4.5 in) solid brick 45 250
224 mm (9 in) solid brick 50 450
Double 112 mm (4.5 in) brick, 50 mm cavity * 50 450
450 mm (18 in) solid brick 55 1000
380 mm (15 in) dense concrete 55 1000
Double 224 mm (9 in) brick with 50 mm cavity
55 1000
Coated with 12 mm of plaster
* Coated with 12 mm of plaster on both sides
A Camden partition, consisting of a timber frameworkfaced with one or two layers ofplasterboardon both sides,can also be used an acoustic barrier. This kind of construction can also be attached to a structural wall, forming a
skin that improves acoustic isolation. For the latter, youll need a timber-frame grid, consisting of battens 76 mm
by 50 mm, fitted to the main wall, with vertical battens spaced at 0.6 m and horizontal ones at 1.2 m. Any ends of
the battens that are attached to the buildings structure should be sealed by means of mastic. They should also bebedded in to the wall, again using mastic. Next, a 25 mm thickness ofrockwoolshould be inserted between the
battens and a layer of 12.5 mmfibreboardnailed on top. Finally, a layer of 12.5 mm plasterboard should be fitted,
taking care to overlap the joints with those in the fibreboard, finishing with a fine skim of plaster. The totalthickness added to the original wall will be about 110 mm.
Reverberation
Reflection of Sound
In a perfect recording studio there arent any reflections of sound, so the studio itself doesnt influence the quality
of a recording. Indeed, the perfect place to make a recording is in the open air, where sounds are evenly absorbed
into the air at all frequencies. Unfortunately, in the modern world, its virtually impossible to find a quiet outsideplace for recording.
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When a sound wave meets a solid surface, some of the energy in the wave is absorbedinto the surface, eitherpassing through the material or changing into heat. However, the remainder of the sound is reflectedback into the
air. Such reflections are inevitable and occur when:-
The surface isnt totally absorbent, which is nearly always the case.
The dimensions of the surface (at right angles to the direction of the wave) are greater than the sounds
wavelength.
The latter effect causes low frequencies to pass through a surface, an effect known as diffraction. However, at high
frequencies, most sounds are reflected, sometimes creating asound shadow behind an object. Because of this effec
a square panel of around 300 mm by 300 mm (roughly corresponding to the wavelength of a sound at 1 kHz)reflects hardly any sounds waves below 1 kHz, half the waves at 1 kHz and almost all the waves above 1 kHz.
When a sound passes through an aperture whose size is less than the wavelength of the sound, the fronts of the
normally flat sound waves become curved. If the aperture is larger, or the frequency higher, the wave fronts are
unchanged. Some types ofloudspeakerexploit this effect to obtain a directional characteristic.
Reverberation Time
Any studio with numerous reflective surfaces is said to be lively: when a sound has actually stopped the reflectionscan still be heard, but then gradually fade away. This phenomena, known as reverberation, is measured by the time
taken for a sound to die away to a specified level.
The most common way of measuring reverberation time is known as RT60, which is defined as the time taken for a
sound to fall to 60 dB below its original intensity. You can calculate the theoretical RT60 for a studio using thefollowing equation:-
RT60 = (0.161 V) (S A)
where
RT60 = reverberation time in seconds
V = volume of studio in cubic metres
S = total surface area of studio in square metres
A = average sound absorption coefficient for all surfaces
The total sound absorption, which is S A, can be obtained using the following equation:-
SA = (s1 a1) + (s2 a2) + (s3 a3) + (s4 a4)
where
s1, s2 = surface area for each element of the studio
a1, a2 = absorption coefficient of each element
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Thesound absorption coefficientfor any material is measured insabines, defined as:-
1 sabine = 1 square metre of 100% absorbent material
All of the above equations are only accurate when the average sound absorption coefficient is less than 25%,
although they can still be used for estimates. For precise calculations, the absorption of air may need to be
considered, in which case youll need to use this more accurate equation for reverberation time:-
RT60 = (0.161 V) (-S log(1-A))
Choosing a value for RT60
Ideally, your value of RT60 should give a natural atmosphere and be suited to the kind of recordings that youintend to make in the studio. The actual oroptimum RT60 figures for different environments or recording areas are
as follows:-
Environment RT60 (s) Volume (m3)
Open air very short -Average sitting room 0.5 12 - 20
Drama studio - dead end 0.15 - 0.2 -
Pop music studio 0.3 -
Radio talks studio 0.3 - 0.5 30 - 200
Radio general purpose studio 0.6 - 0.85 250 - 800
Television studio 0.6 - 1.2 3,000 - 15,000
Drama studio - live end 1.2 - 1.5 -
Music studio 0.8 - 1.6 700 - 8,000
Theatre 1.0 -
Concert hall 1.5 - 2.0 15,000 -20,000
Large cathedral 10 - 12 -
A studios RT60 should:-
Be under 0.3 seconds for a studio whose volume is less than 100 m3. This figure is particularly important fo
frequencies up to 2 kHz. Unfortunately, its hard to achieve without using numerous absorbers (see below),
although not so difficult for a large studio.
Be constant between 60 Hz and 8 kHz, although small increases below 125 Hz cant be avoided in smallerstudios. In a large area, where the total volume exceeds 300 m3, there shouldnt be any increase in the value
below 250 Hz. Give a result at 63 Hz thats less than 50% higher than the figure at 250 Hz.
Deviate by less than 10% between 250 Hz and 4 kHz: a high value in this range can result in recordings tha
contain sibilant speech or shrill music.
Sound Reflections and RT60
The mean free path (MFP) for any wave front is given by:-
MFP = 4V S
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where
V = volume of studio in cubic metres
S = total surface area of studio in square metres
The number of reflections (NR) within the time RT60 is given by:-
NR = (c RT60) MFP
where
c = velocity of sound (343 m/s)
For a natural reverberant soundthere should be at least 100 reflections within an RT60 of 0.25 seconds. In a smallstudio of 5 m by 4 m by 3 m the calculated NR is 33.67. This means that such a small studio cant provide ideal
listening conditions, which can also be proven in practice.
Absorption Coefficients
Absorption coefficients for materials at different frequencies are given in the following table:-
Material 128 Hz 256 Hz 512 Hz 1024 Hz 2048 Hz 4096 Hz
Unpainted brick 0.024 0.025 0.031 0.041 0.049 0.070
Painted brick 0.012 0.013 0.017 0.020 0.023 0.025
Plaster on brick 0.020 - 0.020 - 0.040 -
Plaster on lath 0.300 - 0.010 - 0.040 -
Plaster - fibrous with air space above 0.200 - 0.100 - 0.040 -
Unpainted concrete 0.010 0.012 0.016 0.019 0.023 .035
Tiles or glass on solid backing 0.01 - 0.01 - 0.02 -
Glass window - less than 32-ounce 0.30 - 0.10 - 0.05 -
Wood bonding 20 mm thick over air space 0.30 - 0.10 - 0.10 -
Ply or hardboard on 25 mm studs over air 0.30 - 0.15 - 0.10 -
Ply on studs over porous material 0.40 - 0.15 - 0.10 -
Acoustic panelling 0.16 - 0.50 - 0.80 -
Felt 0.13 - 0.56 - 0.65 -
Membrane absorber, wool, 300 mm air 00.87 0.47 0.30 0.15 0.15 0.15
Porous absorber, 0.5% perf, 150 mm air 0.77 0.52 0.38 0.22 0.18 0.17
Porous absorber, 25% perf, 150 mm air 1.10 1.05 1.00 0.98 0.95 0.80
Porous absorber, wire mesh, 150 mm air 1.10 1.05 1.00 0.98 0.98 0.97
Acoustic tile, 25 mm air space 0.14 0.52 0.52 0.61 0.61 0.65
Due to the effects of diffraction, some absorbers seem to have a coefficient thats greater than unity. In practice, a
lower coefficient is obtained when several absorbers are used together.
Typical studio furnishings, together with people, give the following figures:-
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Furnishings 128 Hz256 Hz512 Hz1024 Hz2048 Hz4096 Hz
Linoleum on solid floor 0.05 - 0.05 - 0.10 -
Carpet - medium quality on boards 0.20 - 0.30 - 0.50 -
Carpet on hair felt over solid floor - 0.14 0.35 0.42 0.23 0.34
Curtains in loose folds 0.10 - 0.40 - 0.50 -
Easy chair * - 3.50 4.50 4.50 5.00 -
Theatre seats without audience * 1.30 - 3.00 - 3.40 -
Theatre seats with audience * 2.00 - 5.00 - 5.50 -
Adult human * 1.80 - 4.20 - 5.00 -
* Total sound absorption figures (S multiplied by A)
Each time a sound is reflected from an absorbent material thesound energy is reduced. The amount ofattenuation
provided by materials of different coefficients is shown in the next table:-
Coefficient
(A)Attenuation (dB)
0.5 3.000.6 4.00
0.7 5.10
0.8 7.00
0.9 10.00
An absorber reflecting just 1% of energy will reduce the sound level by
1/10 log (100)
corresponding to an attenuation of 20 dB.
Materials such as wooden floors and plasterboard provide a high coefficient due to a natural mechanical resonance
Unfortunately such resonance is tuned with a high quality (Q), which means that the absorption is often onlyeffective over a narrow frequency band. However, by introducing numerous different resonant frequencies they can
be used to advantage. For example, you can randomise the spacings between the backing studs in partitions or wall
treatments. Its also worth noting the dramatic effect that soft furnishings and people can make to the averagecoefficient.
Acoustic Absorbers
The reverberation time of your studio can be modified by using acoustic absorbers on its walls and ceilings. These
may also be used to reduce resonance, although excessive absorption can reduce a studios RT60 figure to an
unacceptably small value. Common types of absorber include:-
Porous Absorber
This consists of a layer ofporous material, such asglass wool,synthetic waddingorrockwool. A passage of air
must be allowed through the absorber, making cellular materials, such as expanded polystyrene, unsuitable for this
application.
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The thickness should equal at least a quarter wavelength of the sound to be absorbed. For practical reasons, thismeans that this kind of absorber can only be used for frequencies that are above 200 Hz. Typically, a 25 mm
thickness of material is used in front of a similar depth of air space.
A hardboard coverwith 0.5% perforations makes the coefficient fall at frequencies above 250 Hz, whilst 5%
perforations gives a peak coefficient of 1.2 at 600 Hz, again falling at higher frequencies. Without a cover, its
possible to get a coefficient of 0.1 at frequencies above 1 kHz.
Membrane or Panel Absorber
This consists of a flexible membrane placed in front of asealed air space, creating a resonant device. However, thi
resonance is damped, ensuring absorption at all frequencies close to resonance. Membranes can consist of one or
two layers ofroofing felt, wood panelling, hardboardbonded with bituminous roofing feltor other materials.Typical absorption coefficients are given below:-
Frequency (Hz)150 mm
depth
75 mm
depth
100 0.8 0.5125 0.4 0.85
250 0.3 0.4
315 and above 0.2 0.2
Helmholtz Resonator Absorber
This is a hollow box with a neckthat resonates as the mass of air in the neck bounces against the air in the box.With damping, this device can absorb sound near its resonant frequency. This kind of absorber is particularly
suitable for solving problems that cant be fixed by any other means.
Wideband Porous Absorber
This often consists of 150 mm by 150 mm hardboard partitions, facing outwards and attached to a brick wall,covered with a layer ofchicken wire, followed by 25 mm ofrockwoolandperforated hardboard. The coefficient is
substantially flat, giving a figure of 0.8 above 125 Hz and 0.6 at 63 Hz. Using 300 or 600 mm partitions changes th
latter to 0.5 or 0.4 respectively.
Acoustic Box Absorbers
These are similar in construction to a wideband or membrane absorber, but in the form of a box, often 580 mmsquare and 200 mm deep, and favoured by the BBC. When fitted to battens the overall depth increases to 212 mm,
although the space at the back of the boxes can usefully be used to accommodate technical wiring.
To improve the appearance, wooden battens should be placed around the edge of each block of boxes. Technicalboxes and cupboards can be of related dimensions, providing a modular style of studio construction. Theres
usually little benefit in fixing the bottom of boxes lower than 650 mm from floor level.
The following diagrams show how acoustic boxes can be installed in a real studio, using alternate boxes with
different amounts of perforations:-
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In some instances, particularly where resonance is a problem (see below), boxes are better placed in groups of fouror six, with an area of bare wall between them. However, such an exposed wall should always face another group o
boxes on the other side of the studio. This technique uses the sides of the boxes to reflect sound, increasing the
amount ofdiffusion (see below). Having said this, a full complement of boxes should always be used behind astudios monitoring loudspeakers.
Resonance
If any dimension of a room corresponds to half the wavelength of a sound, a standing wave is created. This happen
because the room is tuned to the sounds frequency and resonates with it.
Standing waves can causepressure variations of 20 to 25 dB in different parts of a room. This has unfortunateeffects on monitoring: sometimes certain sounds will be seem too loud, other too quiet.
Resonance also occurs at wavelengths that are multiples of the rooms dimensions. The frequencies that excite such
resonance are at harmonics of the fundamental resonant frequency. Fortunately, the pressure variations produced bsuch harmonics are less serious, particularly at higher frequencies.
Resonant Frequencies
Resonance becomes more complicated when considering all of a rooms dimensions, including its diagonals. In
fact, there are an infinite number ofresonant mode frequencies, although those at higher frequencies arent sosignificant. Such resonant frequencies are set purely by the dimensions of a room, although its possible to reduce
the intensity orquality (Q) of such resonances by installing extra furnishing, acoustic absorbers (see above) and
other treatments.
Such resonances in a small studio can be especially disturbing. Thankfully, they occur at lower frequencies in alarge studio, sometimes in a range almost outside the range of normal hearing.
This table shows the approximate resonances for a room 4.7 m long, 3.4 m wide and 2.5 m high:-
Resonance No.Frequency (Hz) Due to:
1 36.77 Length (L)
2 51.14 Width (W)
3 63.00 L-W diagonal
4 68.60 Height (H)
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5 73.90 2 L
6 77.85 L-H diagonal
7 85.57 W-H diagonal
8 89.70 2 (L+W)
9 93.15 L+W+H
10 100.70 2 (L+H)
For a studio of dimensions 3 m by 5 m by 7 m, the lowest resonant mode frequencies can be calculated using a
series of equations, beginning with:-
f100 = (c/2) (1/3) = 57.2
f010 = (c/2) (1/5) = 34.3
f001 = (c/2) (1/7) = 24.2
f200 = (c/2) (2/3) = 114.4
f020 = (c/2) (2/5) = 68.6
f002 = (c/2) (2/7) = 49
and so on
where
c = velocity of sound (343 m/s)
Ratios of Room Dimensions
Resonance problems can be minimised by choosing ratios of room dimensions that spread the resonant frequencies
over a wide range. Now, at low frequencies sound is radiated in the form ofspherical waves, whilst at highfrequencies it behaves more like light, obeying ray theory. The frequency at which this behaviour changes given
by:-
f = (3 c) d
where
c = velocity of sound (343 m/s)
d = smallest room dimension in metres
This indicates that the most crucial dimension in a studio is the ceiling height.
Some ideal ratios for room dimensions are given in the following table :-
Room SizeHeightWidth Length
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Small 1.00 1.25 1.60
Medium 1.00 1.60 2.50
Large 1.00 1.25 3.20
Minimising Resonance
The worst kind of problem isstructural resonance, caused by elements of the building that are tuned to particularfrequencies. This can be minimised by using techniques such as:-
Floors: laying wood blocks directly onto a concrete sub-floor using pitch mastic
Walls: using clinker or breeze blocks for the inner leaf of all cavity walls Ceiling: applying plaster over builders insulation board
Using Diffusion
One way to reduce resonance involves the introduction of irregular surfaces to break up or disperse standing
waves. To be effective, the depth any diffusing surface must be one seventh of the wavelength of the offending
frequency. In complex areas, such as a television studio, diffusion may accidently be provided by sets, technicalequipment and other hardware.
By using optimum diffusion you can minimise effects such as colouration,flutter-echo and ringing. To do this, you
must ensure that the mean coefficient of absorption of any pair of the three parallel surfaces in a studio does notexceed the ratio 1:1:4. Ideally, the figure should be 1:1:1.
Live End - Dead End (LEDE)
This technique works by providing almost complete absorption in the vicinity of the monitoring loudspeakers. At
the other technical end hard reflective surfaces are used, providing a more comfortable acoustic environment. In a
larger studio the rear wall can also be angled to minimise resonance. Unfortunately, the latter refinement isineffective in small studios.
References
Developments in Recording, Andy Munro, Studio Sound, October 1980
Interior Design, Norman Bone, Studio Sound, 1981
Control Room Acoustics, Andy Munro, Studio Sound, July 1982
Ray White 2004.
http://whitefiles.org/b1_s/1_free_guides/fg1mt/index.htm