PH 105 Dr. Cecilia Vogel Lecture 4

OUTLINE  Room Acoustics  concert hall properties

Reverberation Time  Reverberation time  V, volume of room  A, the absorption area = sum of  aS for all surfaces  a from table for material, S = surface area  Na for all seats  a from table for seat, N = # of seats  aV  a from table for air, V = volume  K is a constant: for mks units

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

Whispering Gallery  A curved reflecting surface can focus sound  it sounds like it comes from  There are spots in the science bldg where you hear a voice  think it’s  but actually  The sound was reflected and focused, making an

Diffraction  When a wave meets an obstacle  that doesn’t totally  or one that  it can diffract  or  to get  video

Diffraction Sometimes  Note that a wave will not diffract,  if its wavelength is  Ex: Visible light has wavelengths ____________ than a meter,  do not see through a doorway, or around a tree  Ex: Audible sound has wavelengths ______a meter  you can hear through doorway, or around a tree.  Ex: in lab, to see _________ without too much __________, must use ultrasound  “ultra” meaning high  high freq corresponds to

Superposition  When two disturbances (or waves)  are at the same place at the same time,  total disturbance is the sum of the two.  watch impulsive waves

Interference  Because of superposition,  Waves, when they meet  can add or interfere constructively  so the total is  if they happen to have the same phase  periodic waves, when they meet  can cancel or interfere destructively  so the total is  if they happen to have opposite phase

Beats  Two waves with slightly different frequency (period) go in and out of phase

Interference  Waves from two source,  will have places where they interfere constructively  what does it sound like with sound?  what does it look like with light?  and other places where they interfere destructively  what does it sound like with sound?  what does it look like with light?  video

Summary  Using RT  Concert hall qualities defined  Diffraction is the bending of wave around an obstacle or through an opening  if obstacle or opening is similar in size to wavelength of the wave  Interference is the addition of waves at point where they meet  constructive interference means they add to larger wave (louder)  destructive interference means they add to smaller wave (quieter)