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Re: [Phys-l] sound stuff



John,

Concerning your response to the Doppler
effect question posed by Anthony L., the frequency
of the sound wave detected by an observer as the
car retreated at the speed of sound would actually
be one-half the emitted frequency (not a frequency
approaching zero). It's interesting to note that
with the relativistic Doppler effect an object that
recedes with a speed approaching "c" would have an
observed frequency approaching zero.

Dave S.

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John
Denker
Sent: Tuesday, April 11, 2006 1:16 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] sound stuff

Anthony Lapinski wrote:
I have a few questions for the group:

1) How do you add two different decibel levels?

Must you convert it to intensity first?

Yes, if the sounds add incoherently.

In the less-common but certainly important case where the sounds add
coherently, you must convert to amplitude and then add the amplitudes
(with due regard to phase).

For example, 65 dB + 80 dB
? 145 dB.

Not a chance.

delta B = 10log(80/65) =
0.9, and so the answer is 65.9 dB?

You need to take the exponential of the dB number, not the log.

10*log(10^6.5 + 10^8.0) = 80.1


2) If a car sounds a horn. If you hear 450 Hz as it approaches and 350
Hz
as it recedes, the actual frequency is about 394 Hz (not 400 Hz).
Conceptually, why isn't the Doppler effect result symmetric for when
the
sound (or observer) is approaching/receding the observer (sound)?

Because 1/x is not a linear function of x.

Consider the extreme case of the car moving very near Mach 1.
The sound of the retreating car goes to frequency=0, a 100% change.
The sound of the advancing car goes to frequency=infinity, a much larger
change.

3) All musical instruments produce overtones (harmonics) except tuning
forks and some electronic pianos. How can the human vocal chords
produce
pure tones? Living things are both delicate and complex, yet it is
easy to
sing into a microphone and see a nice sine wave on an oscilloscope.

This illustrates what Bob Silsbee calls "the peril of the eyeball FFT".

You can't tell by eyeballing the oscilloscope whether the "nice" wave
is a pure sine or not. You can easily detect any aperiodicity, but
not the presense of a harmonic if it is a few dB down.

A human _whistling_ is much more nearly a pure sine than anything coming
out of the vocal cords.

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