PH 105

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PH 105
Auditorium Acoustics
Dr. James van Howe Lecture 20
Symphony Hall, Boston
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Name the Artist

• Martin Sexton
• Nat King Cole
• Sam Cooke
• Van Morrison

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True or False
The circled area on the spectrogram represents a
vowel
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The vocal tract can be modeled as how many tubes?

• 1
• 2
• 3
• 4

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True or False
Reverberation times over 4 seconds muddies the
sound too much in a concert hall (not very useful
for most kinds of music)
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Sound propagation
Free Field point source propagation, no
reflection, no absorbtion
Goes like if measure pressure, or
if measure intensity
10000
8000
6000
Intensity (Arbitrary Units)
4000
2000
0
0
0.05
0.1
0.15
0.2
0.25
Distance from Source (m)
In each case, doubling the distance gives us 6 dB
less (factor 4 less in intensity)
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Multiple Reflections
In a room we get multiple reflections from
speaker to listener

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

These three types of sound are crucial for
determining the quality of a concert hall
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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) Ex. A
speaker
Not true of many brass instruments
Overhead view of a trumpet radiating like a
quadrupole
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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

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Reverberant Sound (liveness)
Reverberant Level The sound level where the rate
of sound energy supplied by the source is equal
to the rate of absorption by walls, ceilings,
objects, and the air itself
Reverberant sound reinforces direct sound, but
too high of a level will drown out clarity
Should be much lower level than direct sound
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Decay of Reverberant Sound
Reverberation time The time it takes for the
sound energy in a room to decay by 30 dB
intensity (60 dB pressure)
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Video
The tuning of Meyerhoff hall (Baltimore, MD
built 1983)
Clip of The making of a Concert Hall Meyerson
Hall (Dallas, TX built 1989)
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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.
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Enhanced Area
Surface area of the drain includes walls,
ceiling, floor, people, chairs, etc.
The walls, ceiling, and other objects dont 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,
its as if I have 20 m2 of perfect drain
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Sample Problem
What is for the concert hall below
(shoebox design) at 500 Hz?
Walls plaster on lath a0.06 Ceiling Acoustic
Tile a0.83 Floor Carpet on pad a0.57
10 m
15 m
25 m
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Sample Problem cont.
I just designed a very dry hall a good lecture
hall bad for music
If I changed my ceiling to plaster rather than
acoustical tile,
Much better hall, good for music and speaking!
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Sample Problem cont.
What if I take into account the absorption of the
air, the seats, and people in seats?
To take into account the air, we modify the
reverb time equation
m is a constant but only comes into play for
sounds 2,000 Hz
Can give upwards of 100 m2 extra drain area for
orchestral halls
Adding seats and people adds an effective area of
the drain as well
People in upholstered seats give an additional
0.56 m2 per seat at 500 Hz
If I have 200 seats in my auditorium, Ill have
an extra drain area of ?
Less reverb but still o.k. for music
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Reverb time and size of room
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Homework
Skim Chapter 24 Read Chapter 25 Ch25 R2, R15, E4