Lewin shows properties of sound


Can you see sound? Visiting science teachers did when Professor of Physics Walter Lewin presented some of the fundamentals of sound and music in "A Feast of Sounds."

This was no conventional feast tempting the senses of taste and smell, but it did make good use of both sight and sound, and touched a good many eardrums. Professor Lewin used compelling visual demonstrations of frequency and resonance to explain sound waves, then showed the potentially destructive effects of resonance by shattering a wine glass with a loud, even tone no opera singer would envy.

Sound is a pressure wave that compresses and decompresses the air to make a vibration, he explained -- a disturbance that propagates similarly to the way unrest moves through a queue of people when one begins elbowing and shoving the next. When that pressure wave reaches your ear, "the eardrum begins to vibrate at the same frequency, and your brain tells you something," he said.

A person whose hearing is excellent hears sound in the 20-20,000 Hz range. (A hertz is one oscillation per second.) As that person ages, he or she may lose the ability to hear frequencies above 5,000-10,000 Hz.

For perspective, Professor Lewin explained that the lowest key of a piano vibrates at 27 Hz; the highest, at 4,000; and the Middle A, at 440 Hz. "Certain combinations of frequencies are very pleasing to us," he said. In other words, they're music to our ears.

To demonstrate harmonics, he used a rubber hose clamped at one end and attached at the other to a mechanical device that moved it up and down while keeping the two ends relatively stable. With the hose stretched in front of a black velvet backdrop, he shone a strobe light on it to arrest movement and capture the hose in several frequencies of oscillation, including the first, second and third harmonics.

Professor Lewin described how musicians manipulate various musical instruments to produce sound. And on an overhead screen, he provided spectral analyses of live musical performances by students on the violin, saxophone and the French horn.

RESONANCE WRECKAGE

The most dramatic demonstration dealt with the destructive potential of resonance, which can occur, for instance, when wind or sound excites an object and causes it to vibrate at a very special frequency to which it is extremely sensitive. He showed a video of the infamous bridge in Tacoma, WA, that began swinging in the breeze, achieved resonance due to the wind and then, when the wind increased, was snapped in two by the torsional forces produced by the oscillations.

In the classroom, Professor Lewin demonstrated the impact of resonance on a smaller scale. He first rubbed a moistened finger around the rim of a wineglass to find the tone, or frequency, that is the resonance frequency of that glass. Then, using an electronic device to reproduce the tone at high volume, he shattered the vessel with sound waves.

A version of this article appeared in MIT Tech Talk on July 15, 1998.


Topics: Physics, Education, teaching, academics

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