wave questions answered

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Fri Mar 18 2005 - 16:49:41 PST 

Message-Id: <C705D6D0-9810-11D9-9AFA-000A95B38012@exploratorium.edu>
From: Paul Doherty <pauld@exploratorium.edu>
Subject: wave questions answered
Date: Fri, 18 Mar 2005 16:49:41 -0800

                Some of the questions that my students have asked are:

 

Can you be killed by a sound wave? I haven't found a report of anyone
killed by a sound wave (although pressure waves have killed people e.g.
pressure waves from a nuclear blast)
I'm considering a sound wave as a continuous pressure variation not a
pulse.The maximum sound intensity in air is about 190 decibels. In a
sound wave the pressure goes above and below atmospheric pressure. At
190 decibels the lowest pressure part of the sound wave goes to 0.
Above 190 dB you get a series of high pressure pulses rather than a
wave.
Acording to NASA

• The shuttle produces acoustic levels of about 188 dB on the launch
platform, 160 dB at the pad perimeter, and 120 dB at the VAB

Sound intensities of 188 decibels have been predicted to be lethal to
people.
They are known to kill animals, small animals absorb sound and overheat
and die. Cells are also disrupted.

At 150 decibels the inner ear is damaged.

Do the little hairs that you lose from loud noises re-grow or are you
“bald” in the ear canals as you get older?

Humans do not regrow hair cells in the inner ear.
Recent studies have found that hair cells can be regenerated in animal
experiments with chickens.

How is it that we lost time off the calendar because of the tsunami?

The surface of the earth changed its radius due to the Indonesian
earthquake, like a rotating skater throwing out her arms changing the
position of a mass in a rotating object changes its rate of rotation.
(Conservation of angular momentum.) The change in rotation rate of the
earth due to the Indonesian quake has been theoretically predicted but
not yet experimentally measured. We should see the rotation change
measured in a few more weeks. It's a tiny change. The change in
rotation rate of the earth due to winds shifting speed as the seasons
change is 30 times larger than the predicted change due to the quake.

Why is it that some instruments cannot play their fundamental frequency?

There are many complex reasons for this.
In the corrugated plastic tube whirly which I play, the frequency
played is proportional to the airspeed. The fundamental is low
frequency and slowest airspeed. At the slowest airspeed the airflow is
laminar and produces no noise to be amplified by resonance. At the
second harmonic the airspeed is faster, the airflow goes turbulent ,
becomes noisy and the resonance of the tube amplifies the sound. Thus
the tube plays the second and higher harmonics but not the fundamental.

Paul D

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