PH 105 Dr. Cecilia Vogel Lecture 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 t o =  The early sound will arrive t1=t1= t2=t2=  etc dodo d1d1 d2d2

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.  t o = d o / v =  t 1 = d 1 / 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.  t o = d o / v =  t 1 = d 1 / 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 7 th reflection from one surface is mixed up with the 10 th 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 1 st reflection, down to  after 2 nd reflection, down to  after 3 rd 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,V,  Larger rooms tend to have  A, the absorption area  The more absorbent surfaces to remove sound energy,  K is a constant: for mks units  Listen and compare anechoic room, conference room, reverberant room

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 A =

Example cont’d  Lose the absorption of glass, add the absorption of drapery:  the absorption for each is aS, a from ___________, S = A = 64.4m 2 A =

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

Summary  Direct sound arrives first  then early sound (may be echo-y)  then reverberation lasts for the reverberation time  RT depends on  room volume  surface area (including occupants)  absorption of materials  many qualities must be considered to make a good concert hall