PHYS 105 Auditorium Acoustics Dr. James van Howe Lecture 20 Symphony Hall, Boston When did this song hit the charts, catapulting a new genre to mainstream listening? 1990 1991 1992 1993 Doppler Effect Wavelength gets shorter, Frequency lower Wavelength gets shorter, Frequency higher
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Dr. James van Howe
Symphony Hall, Boston
When did this song hit the charts, catapulting a new genre to mainstream listening?
The truck is moving to the left, how does Doppler think the wavelength and frequency change?
Reverberation times over 5 seconds muddies the sound too much in a concert hall (not very useful for most kinds of music)
-As sound spreads out, intensity gets weaker
-Power is the same at each shell
-So as shell gets bigger, power spreads out over larger sphere
Inverse square Law
Power at different times; bigger sphere is later time
Two ways of showing that expanding sphere on a graph
Intensity (Arbitrary Units)
Distance from Source (m)
Distance from Source (m)
Every time you double the distance, you loose four times the sound, or 6 dB
If the sound source radiates like a point source, only the distance form the source determines sound level (6 dB less for every doubling)
Not true of many brass instruments
Overhead view of a trumpet radiating like a quadrupole not point source
In a room we get multiple reflections from speaker to listener:
These three types of sound are crucial for determining the quality of a concert hall
-Multiple reflections of sound will trick the ear
-First reflection very important for “spatial impression”
Reverberation time: The time it takes for the sound energy in a room to decay by 30 dB intensity (60 dB pressure)
The time it takes to drain a bathtub depends on how much water in tub and how large of a drain
Volume of water
Area of drain
Reverb time is the time it takes to drain the sound out of a room.
Surface area of the drain includes walls, ceiling, floor, people, chairs, etc.
The walls, ceiling, and other objects don’t just absorb all of the sound like a drain, but reflect some back into the room
By using absorption coefficients, we can find how much of the area of the walls, etc, act like a perfect drain
Note that perfect absorption corresponds to ,
Perfect reflection is
In your book (table 23.1), we find that wood floor has at 500 Hz
If I have a 20 m x 10 m wood floor, or 200 m2, it’s as if I have 20 m2 of perfect drain
What is for the concert hall below (shoebox design) at 500 Hz?
Walls: plaster on lath; a=0.06
Ceiling: Acoustic Tile; a=0.83
Floor: Carpet on pad; a=0.57
I just designed a very “dry” hall; a good lecture hall but bad for music