Studio Acoustics

8/3/2019 Studio Acoustics

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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


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


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

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