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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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slide1

PHYS 105

Auditorium Acoustics

Dr. James van Howe

Lecture 20

Symphony Hall, Boston

slide2

When did this song hit the charts, catapulting a new genre to mainstream listening?

  • 1990
  • 1991
  • 1992
  • 1993
slide3

Doppler Effect

  • Wavelength gets shorter, Frequency lower
  • Wavelength gets shorter, Frequency higher
  • Wavelength gets longer, Frequency lower
  • Wavelength gets longer, Frequency higher

Stationary

Christian Doppler

The truck is moving to the left, how does Doppler think the wavelength and frequency change?

true or false
True or False

Reverberation times over 5 seconds muddies the sound too much in a concert hall (not very useful for most kinds of music)

slide7

Free Field: Point Source

-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

-Intensity is

Inverse square Law

Power at different times; bigger sphere is later time

slide8

Free Field: Point Source

Two ways of showing that expanding sphere on a graph

40

10000

6 dB

Log Scale

35

8000

6 dB

30

6000

Intensity (dB)

Intensity (Arbitrary Units)

25

4000

20

2000

15

x 2

x 2

10

0

-1

0

0.05

0.1

0.15

0.2

0.25

.04

.02

10

.01

Distance from Source (m)

Distance from Source (m)

Every time you double the distance, you loose four times the sound, or 6 dB

direct sound
Direct Sound

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

multiple reflections
Multiple Reflections
  • Direct Sound: no reflection (typically 20-200 ms)
  • Early Sound: first group of reflections (50-80 ms)
  • Reverberant Sound: thick bunch of reflections

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

early sound first group of reflections
Early Sound (first group of reflections)

-Multiple reflections of sound will trick the ear

  • Early sound arriving within 50-80 ms after direct sound is indistinguishable from the direct sound
  • Later arrival time of early sound = Echo

-First reflection very important for “spatial impression”

  • If first reflection is less than 20 ms, “intimate” feel
  • First reflection from sides of concert hall (lateral reflection) found to be very important to quality of hall
reverberant sound liveness
Reverberant Sound (Liveness)

Reverberation time: The time it takes for the sound energy in a room to decay by 30 dB intensity (60 dB pressure)

calculation of reverberation time
Calculation of Reverberation Time

Bathtub analogy:

The time it takes to drain a bathtub depends on how much water in tub and how large of a drain

Drain time

Volume of water

Area of drain

Drain time

Reverb time is the time it takes to drain the sound out of a room.

enhanced area
Enhanced Area

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

sample problem
Sample Problem

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

10 m

15 m

25 m

sample problem cont
Sample Problem cont.

I just designed a very “dry” hall; a good lecture hall but bad for music