PH 105

Dr. Cecilia VogelLecture 4

OUTLINE Room Acoustics

direct and early sound precedence effect echoes and anechoic chamber reverberation and reverberation time concert hall acoustics

Sound in a Room Sound in a room is made up of

direct sound, coming

early sound sound that comes from the source,

and reverberant sound sound that reflects

Direct and Early Sound The direct sound will arrive at your

ear after time to=

The early sound will arrive t1=

t2= etc

do

d1

d2

Example Compare the time between direct and

early sound for a lecture hall and a

stadium: Lecture Hall

Assume the listener is 3 m from the source, and the first reflection must travel 6 m.

to= do/v =

t1= d1/v = difference is about

Example Cont’d Compare the time between direct and

early sound for a lecture hall and a

stadium: Stadium

Assume the listener is 50 m from the source, and the first reflection must travel 150 m.

to= do/v =

t1= d1/v = difference is about

Echoes Will we hear echoes in example?

Depends on time lag and whether reflection is clear or muddled

With light, compare reflection from to reflection from a

With sound flat walls reflect

make

rough surface makes

Reverberant Sound Reverberant sound has made

several reflections the sound is

because the 7th reflection from one surface is mixed up with the 10th reflection from another surface, etc.

should be much so direct sound will be

Absorption Reverberant sound is always softer than direct sound

because some of the

Surfaces can reflect sound, but also can ___________ does not absorb much _____________ specifically designed to absorb sound

Anechoic chamber virtually no only

Decay of Sound If 20% of sound energy is absorbed

at each reflection after 1st reflection, down to after 2nd reflection, down to after 3rd reflection, down to etc

Exponential Decay This pattern is called

The reverberant sound in a room tends to

Reverberation time is a measure of

Exponential Decay

Reverberation Time Reverberation time depends on:

V, Larger rooms tend to have

A, the absorption area The more absorbent surfaces to

remove sound energy,

K is a constant: 0.161 for mks units Listen and compare anechoic room,

conference room, reverberant room

VRT K

A

Effective Area A is not the total area of

absorbers most absorbers An open door or window does not

reflect any sound back, so but wood floor only absorbs about

so _____ of wood floor absorbs only as much as

it’s effective area is A = aS = To get total A of room,

Absorption example A 10 mX10mX4m room currently has a

RT of 1.0 s for sounds at 500Hz. What change will be made by covering five 1.2mX0.5m glass windows with heavy drapery?

The current absorption, A, can be found V

RT KA

A =

Example cont’d Lose the absorption of glass, add the

absorption of drapery: the absorption for each is aS, a from

___________, S =

A = 64.4m2 A =

VRT K

ART

Properties of Concert Halls “Intimacy”

means _____________ between direct and early sound is

sounds like “Liveness”

level of adds to

Properties of Concert Halls “Spaciousness”

has to do with sound from a)

b)

Properties of Concert Halls “Clarity”

more important in keep reverb

“Warmth” level of drapery, carpet

“Loudness” total

Properties of Concert Stage “Balance”

keep appropriate relative level of instruments

“Blend” mix of sound from

“Ensemble” can performers hear

SummaryDirect sound arrives first

then early sound (may be echo-y)then reverberation lasts for the reverberation time

RT depends onroom volumesurface area (including occupants)absorption of materials

many qualities must be considered to make a good concert hall