l 18 auditorium and room acoustics

L 18Auditorium and Room

Acoustics

Dekelbaum Concert Hall

at the U MD

Smith Center

Paths of sound in room

Intensity vs. time for pulse

We define the reverberation time T as the time the sound intensity

takes to decay to one millionth of its original steady-state intensity.This is the time taken the sound

level to decay by 10 log(1,000,000)= 60 db

Average Intensity vs. Time

for a constant tone

To calculate T, consider a room with a hole in one wall of area A.

Call the reverberation time T.T ˜ volume V, 1/A

T= K V/AIt has been worked out that, for V in m3 , A

in m2

T= 0.16 V/A

Let us now replace the open window area with an absorbing

material of area S and absorption coefficient a.

Then A= Sa. If there is more than one type of absorbing material, the

A= S1 a1+S2a2 +S3a3+…If you prefer, we can write

T= 0.16 V/(ΣSiai)

We call this new “area” A the “total room absorption”, measured in sabins

(same unit as area, but a somewhat different meaning)

If we wish to calculate T, we need to calculate the total room

absorption- from the walls, floor, ceiling, furniture, drapes, people,

etc.

An empty room with totally reflecting walls, ceiling, floors, etc. has aa) shorter T than one (empty) of comparable dimensions with perfectly absorbing walls, ceiling, floors, etc.b) shorter T than one(empty) of comparable dimensions with open windowsc) shorter T than one (empty) of comparable dimensions with no windows and with carpets covering the wallsd) a longer T than one of comparable dimensions with perfectly reflecting walls , but with people inside.

Examples of Reverberation TimesOne way to respond to the question "What is a good range of reverberation times for concert halls?" is to give examples of some of the most famous halls in the world. For the overall average reverberation times: Vienna, Musikvereinsaal : 2.05 seconds Boston, Symphony Hall: 1.8 seconds New York, Carnegie Hall: 1.7 secondsBut the overall average reverberation time does not tell the whole story. The variation of reverberation time with frequency is also important.

Ideal Reverberation Times

Search youtube for Notre Dame de Paris organ (try a Bach piece)

IntimacyRoom said to be “intimate” when thefirst reverberation arrives within20 ms of the direct sound.

Fullness vs. ClarityRefers to the amount of reflected sound relative tothe amount of direct sound

Warmth vs. Brilliance

Warmth increases withincreasing T for low frequencies

Acoustical Design Problems

1. Focusing of sound2. Echoes3. Shadows4. Resonances5. External noise6. Double-valued T

Focusing of Sound

Occurs with use of parabolic surfaces

either behind performersor at rear of auditorium

Echoes

Highly reflective flat orparabolic wall shapes

Flutter echos fromparallel walls

Standing waves between parallel walls

Resonances

Rectangular practice rooms

Parallel walls (flutter)

Singing in the shower

Double-valued T

Playback room with reverberation

Concert halls with side areas

Auditorium Design

General Design Considerations

1. Visual2. Ventilation3. Acoustical

a. seatingb. stage c. room shaped. room walls

Absorptions (in sabins) Frequency (Hz)

Material 125 250 500 1000 2000

Unupholstered seat 0.15 0.22 0.25 0.28 0.50

Upholstered seat 3.0 3.1 3.1 3.2 3.4

Adult person 2.5 3.5 4.2 4.6 5.0

Adult/upholstered seat 3.0 3.8 4.5 5.0 5.2

Some Absorption CoefficientsFrequency (Hz)

Material 125 250 500 1000 2000 4000

Concrete/brick 0.01 0.01 0.02 0.02 0.02 0.03Glass 0.19 0.08 0.06 0.04 0.03 0.02Plasterboard 0.20 0.15 0.10 0.08 0.04 0.02Plywood 0.45 0.25 0.13 0.11 0.10 0.09Carpet 0.10 0.20 0.30 0.35 0.50 0.60Curtains 0.05 0.12 0.25 0.35 0.40 0.45Acoustical board 0.25 0.45 0.80 0.90 0.90 0.90

Control of TR

Let’s call it T

T = 0.16 V / AWhere:

T = reverberation time in secondsV = room volume in cubic feet

A = total room absorption in sabins

Estimate the reverberation time of this lecture hall

Should there be an enforced control of sound level in listening devices and public performance

venues?a) yesb) no

CNN ping refraction or total internal reflectionhttp://www.cnn.com/2014/04/11/tech/innovation/mh-370-

underwater-sound/index.html?hpt=hp_t1