The Recording Environment Part 2

The Recording Environment

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Agenda

• Absorption Coefficients

• The Sabine Equation

• Reverb Calculation Example 1Estimating the Reverberation Time

• Reverb Calculation Example 2Correcting the Reverberation Time

• Control of Interfering Noise

• Absorbers

• Recording Solutions

b. 3 to 1 Rule

c. Working in Mono

d. Comb Filtering

Reverberation

Absorption Coefficients


In architectural acoustics, a measure of the energy lost when a sound wave strikes a given

material is specified by the absorption coefficient.



If we say that a surface material has an absorption coefficient of .25, we’re actually saying that the material

absorbs 25% of the original acoustic energy, while reflecting 75% of the total sound energy at that

frequency.



Question: What is the absorption coefficient of an open window or door?

Answer: 1.00 (100%) as all of the sound would pass through.




Multiplying the total surface area of the room by the average absorption coefficient of the surface

materials tells us how absorbent the room is, and this figure is expressed as absorption units called

'Sabines'.

Note: There are several equations, named after the people who developed them, for calculating the reverb

time based on the absorptivity of a given environment.

This formula was devised by an American physicist, WC Sabine.



Material Frequency 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz

Carpet, heavy, on concrete 0.02 0.06 0.14 0.37 0.6 0.65

Same, on 40oz hair felt or foam

rubber0.08 0.24 0.57 0.69 0.71 0.73

Concrete 0.01 0.01 0.015 0.02 0.02 0.02

Wood 0.15 0.11 0.1 0.07 0.06 0.07

Large panes of heavy plate glass 0.18 0.06 0.04 0.03 0.02 0.02

Chairs, metal or wood seats, each,

unoccupied 0.15 0.19 0.22 0.39 0.38 0.3

Plaster Board, 1/2” 0.29 0.1 0.05 0.04 0.07 0.09

Acoustic tiles 3/4” 0.09 0.28 0.78 0.84 0.73 0.64



Material Frequency 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz


Same, on 40oz hair felt or foam

rubber0.08 0.24 0.57 0.69 0.71 0.73

Concrete 0.01 0.01 0.015 0.02 0.02 0.02

Wood 0.15 0.11 0.1 0.07 0.06 0.07



unoccupied 0.15 0.19 0.22 0.39 0.38 0.3

Plaster Board, 1/2” 0.29 0.1 0.05 0.04 0.07 0.09

Acoustic tiles 3/4” 0.09 0.28 0.78 0.84 0.73 0.64


The Sabine Equation


The maths we need to calculate reverb time states:

RT60 = 0.049Vf or RT60 = 0.161Vm

Sª Sª

Where

RT60 = Reverb time is seconds

Vm = Volume in cubic meters

Vf =Volume in cubic feet

Sª = total absorption, sabins

The Sabine Equation


Reverb Calculation Example 1

Estimating the Reverberation Time


Firstly, we need to work out the surface area of our room


Surface Area

Rear Wall

Front Wall10 ft

16ft

23 ft

23ft x 16ft x 10ft = 3680 cu ft

Area of front and rear walls = 10 x 16 x 2 = 320 ft2

Area of side walls = 23 x 10 x 2 = 460 ft2

Area of floor = 23 x 16= 368 ft2

Area of ceiling = 23 x 16= 368 ft2


Next we need to work out our absorption coefficients


Materials

16ftAbsorption Coefficients of Surfaces (at 125Hz):

Floor (concrete) = 0.01

Rear wall (Plaster board 1/2”) = 0.29

Front wall (Plaster board 1/2”) = 0.29

Side walls (Plaster board 1/2”) = 0.29

Ceiling (Plaster board 1/2”) = 0.29

Rear Wall

Front Wall10 ft

16ft

23 ft


Units (Sª)

Absorption by:

Front & Rear Walls (Plaster board) 320 ft2 x 0.29 = 92.8

Side Walls (Plaster board) 460 ft2 x 0.29 = 133.4

Floor (concrete) 368 ft2 x 0.01 = 3.68

Ceiling (Plaster board) 368 ft2 x 0.29 = 106.72

Total Sabine Units = 336.6

Rear Wall

Front Wall10 ft

16ft

23 ft


RT60 = .049 x 3680

336.6

Using RT60 = 0.049Vf

Sª

RT60 =.53 secs at 125Hz


Size 23 x 16 x 10ft

Treatment None

Floor Concrete

Walls Plaster board 1/2”

Ceiling Plaster board 1/2”

Volume (23) (16) (10) = 3,680 cu ft

MaterialS

Sq ft

Concrete

Plaster board

368

1,148

125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz

a Sa a Sa a Sa a Sa a Sa a Sa

0.01

0.29

3.6

332.9

0.01

0.10

3.6

114.8

0.015

0.05

5.5

57.4

0.02

0.04

7.4

45.9

0.02

0.07

7.4

80.4

0.02

0.09

7.4

103.3

Total Sabins

Reverberation time

(seconds)

336.3 118.43 62.92 53.28 87.72 110.68

0.53 1.5 2.82 3.33 2.02 1.6

a = absorption coefficient for that material and for that frequency.

Sa = S times a, absorption units, sabins.


Size 23 x 16 x 10ft

Treatment None

Floor Concrete

Walls Plaster board 1/2”

Ceiling Plaster board 1/2”

Volume (23) (16) (10) = 3,680 cu ft

MaterialS

Sq ft

Concrete

Plaster board

368

1,148



0.01

0.29

3.6

332.9

0.01

0.10

3.6

114.8

0.015

0.05

5.5

57.4

0.02

0.04

7.4

45.9

0.02

0.07

7.4

80.4

0.02

0.09

7.4

103.3

Total Sabins

Reverberation time

(seconds)

336.3 118.43 62.92 53.28 87.72 110.68

0.53 1.5 2.82 3.33 2.02 1.6

Very High RT


Lets look at this across the frequency spectrum


RT curve for example 1


Reverb

era

tio

n t

ime -

seconds

0

1

2

3

4

125 250 500 1 k 2 k 4 k

Frequency - Hz

Target Values for Reverberation Time

• Broadcast/radio studios: Where speech is the priority, reverberation time needs to be short.

• Orchestra - Microphones would be placed at a distant to capture the full acoustic of the players performing in the

studio. For the players, the studio acoustic would have to be the same as they experience on the concert hall platform;

therefore a longer reverberation time.

• Drama/Popular Music/T.V. - These studios have a short reverb time relative to their volumes. They are acoustically

Dead. If “reverb” is required, it can easily be added electronically. If ever the “reverb” of a studio is captured by the

microphone, it is very difficult to remove electronically from the signal.


Reverb Calculation Example 2Correcting the Reverberation Time


More absorption is needed at midband frequencies.

The goal now is to correct the reverb curve.


Reverb

era

tio

n t

ime -

seconds

0

1

2

3

4

125 250 500 1 k 2 k 4 k

Frequency - Hz

125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz


Same, on 40oz hair felt or foam rubber 0.08 0.24 0.57 0.69 0.71 0.73

Concrete 0.01 0.01 0.015 0.02 0.02 0.02

Wood 0.15 0.11 0.1 0.07 0.06 0.07



unoccupied 0.15 0.19 0.22 0.39 0.38 0.3

Plaster Board, 1/2” 0.29 0.1 0.05 0.04 0.07 0.09

Acoustic tiles 3/4” 0.09 0.28 0.78 0.84 0.73 0.64

3/4” acoustic tile seems to have the right shape.

Selected Absorption Coefficients


Question - What is the area of tile required?


MaterialS

Sq ft

Concrete

Plasterboard

368

1,148



0.01

0.29

3.63

332.9

0.01

0.10

3.63

114.8

0.015

0.05

5.52

57.4

0.02

0.04

7.36

45.92

0.02

0.07

7.36

80.36

0.02

0.09

7.36

103.32

Total Sabins

Reverberation time

(seconds)

336.3 118.43 62.92 53.28 87.72 110.68

0.53 1.5 2.82 3.33 2.02 1.6

Reasonable reverb time with 336.3 sabins

We need to increase sabins at 1 kHz in order to bring the reverb time to a reasonable level

Sabins required = 336.3 - 53.28 = 283.02 sabins at 1 kHz

283.02 / .84 (absorption coefficient of acoustic tile at 1 kHz ) = 337 sq ft of material


MaterialS

Sq ft

Concrete

Plaster board

Acoustic tile

368

811

337



0.01

0.29

0.09

3.6

235.2

30.3

0.01

0.10

0.28

3.6

81.1

94.4

0.015

0.05

0.78

5.5

40.5

262.9

0.02

0.04

0.84

7.4

32.44

283

0.02

0.07

0.73

7.4

56.77

246

0.02

0.09

0.64

7.4

73

215.7

Total Sabins

Reverberation time

(seconds)

269.1 179.1 325.8 322.84 310.17 296.1

0.66 0.99 0.54 0.55 0.57 0.6


0

1

2

3

4

125 250 500 1 k 2 k 4 k

Frequency (Hz) from example 1

Frequency (Hz) from example 2

Re

verb

era

tion tim

e -

seconds


Control of Interfering Noise



Atrium 2nd Floor Studios

Asymmetrical side reflections cause

an acoustic imbalance at the listeners

position

Splaying Room Surfaces

One of the ground rules of studio

design, even more important than

control of amplitude response is the

need for symmetrical reflections

Note that the walls are splayed but retain symmetry!!

White Mark Acoustics

Schematic for Level 2



ATRiuM 2nd Floor Studio

Schematic

!!


In his book ‘Master Handbook of Acoustics’, F. Alton Everest states;

‘Splaying one or two walls of a sound-sensitive room does not eliminate modal problems,

although it might shift them slightly and produce somewhat better diffusion’.


As Gilford stated in his paper, ‘The Acoustic Design of Talk Studios and Listening Rooms

that, “....slanting the walls to avoid parallel surfaces... Does not remove colourations; it only

makes them more difficult to predict.”

The acoustical benefit to be derived from the use of non rectangular shapes in audio rooms

is rather controversial.

Absorbers


High Frequency Absorbers

• HF absorbers are effective at frequencies upwards of 500 to 1000Hz, depending on the

material of which they are made.

• HF absorbers should be used in combination with LF absorbers otherwise the room will

sound dull.

• HF absorbers are principally made of porous material.

• Absorption results from the fact that the velocity of the air particles is damped by the

porous material


High Frequency Absorbers


1.0

0.5

0100 1000 5000 Hz

1.0

0.5

0100 1000 5000 Hz

Frequency

Absorption Coefficient Absorption Coefficient Reverb Time

Frequency

Frequency

porous layer curtains with foldstypical reverberation

frequency response

Mid Range Absorbers

• Mid range absorbers, especially ones effective between 300 and 1000Hz, are created

through the combination of porous absorbers and resonating panels of wood or plaster.



1.0

0.5

0100 1000 5000 Hz

1.0

0.5

0100 1000 5000 Hz

Frequency


Frequency

Frequency

porous layer with

perforated overlay acoustic paneltypical reverberation

frequency response

Mid Range Absorbers

Low Frequency Absorbers

• Low frequency absorbers are effective in the frequency range below 300-500Hz.

• There exist two types of absorbers: Helmholtz resonators, and resonant panels.



1.0

0.5

0100 1000 5000 Hz

1.0

0.5

0100 1000 5000 Hz

Frequency


Frequency

Frequency

vibrating panels resonators typical reverberation

frequency response

Low Frequency Absorbers

Target Values for Reverberation Time

• Broadcast/radio studios: Where speech is the priority, reverberation time needs to be short.

• Orchestra - Microphones would be placed at a distant to capture the full acoustic of the players performing in

the studio. For the players, the studio acoustic would have to be the same as they experience on the concert hall

platform; therefore a longer reverberation time.

• Drama/Popular Music/T.V. - These studios have a short reverb time relative to their volumes. They are

acoustically Dead. If “reverb” is required, it can easily be added electronically. If ever the “reverb” of a studio is

captured by the microphone, it is very difficult to remove electronically from the signal.


Recording Solutions


a. Critical Distance


Source

Critical Distance

Stronger Direct Sound

Weaker Direct Sound

Key

Critical Distance

The distance from the acoustic centre of the

source to the point at which the intensity of the

direct sound equals that of the reverberant field is

called the critical distance.

As we move further away, beyond the critical

distance, into the region where the reverberant

field predominates, we will start to lose sound

quality.

Critical Distance

One rule of thumb

1. Listen to the mic’s output level at a close position.

2. Move the mic farther and farther away until you reach a point where the output seems

to stop getting lower and you’ve found the “critical distance.”

3. Move the mic at least one-half that distance back toward the sound source.

b. 3 to 1 Rule


1 Meter

3 Meter


When multiple microphones are used, the distance between them should be three

times the distance from each microphone to its intended sound source. This is to

prevent interferences, phase cancellation and comb filtering.

The ‘3 to 1’ rule works on the basis of using distance to reduce the pickup of one

sound source in the microphone intended for another source. Because the sound

picked up by the more distant microphone is at least 12dB less than the sound

picked up by the closer one, interference problems are prevented.


c. Working in Mono


Asymmetrical side reflections can be

reduced by working in mono


d. Comb Filtering


If the same source is recorded with two mics at different distances, the sound arrives at the two mics at

slightly different times. When the two mic signals are summed to mono, comb filtering will occur.

References

• Gary Davis, Ralph Jones. (1989) Sound Reinforcement Handbook, Hal

Leonard

• F. Alton Everest, Ken C. Pohlmann. (2009) Master Handbook of

Acoustics, McGraw-Hill

• C.L.S.Gilford. The acoustic design of talk studios and listening rooms.

J.Audio.Eng.Soc. 27. 17-31. (1979).

Reverberation


Properties of Reverberation


The term RT60 refers to the time it takes the reverb to decay by 60dB. RT is measured at the point at

which the reverb decays to -60dB of its peak level.

RT60





[email protected]

k

www.stuartjones.org

mailto:[email protected]

Education

The Recording Environment Part 2