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PPoM Physics Of Sound: Basics Concepts

Physics Of Sound: Basics Concepts

Definitions of Sound
Question: If a tree falls in the forest, and there is no one to hear it, will there be a sound?...

This is a very old philosophical dilemma which relies on using the word "sound" for two different purposes. One use is as a description of a particular type of physical disturbance:

Sound is an organised movement of molecules caused by a vibrating body in some medium - water, air, rock or whatever.*

The other is as a description of a sensation:

Sound is the auditory sensation produced through the ear by the alteration ... in pressure, particle displacement, or particle velocity which is propagated in an elastic medium.#

Both these definitions are correct, they differ only in the first being a cause and the second being an effect.

Olson, 1967

The Nature of a Sound Wave
Sound originates when a body moves back and forth rapidly enough to send a coursing wave through the medium in which it is vibrating.

A simple form of sound wave is produced by an explosion of a small balloon of compressed air. By bursting the balloon, potential energy (energy of position) is converted to kinetic energy (energy of motion).

Select the "WAVE" link to see the animated wave front. WAVE

This is a spherical wave. It consists of one condensation and one rarefaction. The wave front is a continuous spherical surface in which the variations for all parts of the surface have the same phase.

Olson, 1967

Sound Generators
Any motion that is repeated is called periodic motion. Examples of periodic motion are the moon's orbit of the earth, a beating heart, the whirring tail of a frightened rattlesnake, the wings of a hummingbird, and the movement of the valve on a revolving bicycle tire. A vibrating body in contact with the atmosphere will produce sound waves. One simple example is a vibrating piston.

Basic Features of Periodic Motion
There are thus three parameters which characterise the basic features of any periodic motion.

1. The period, T is the time required for one complete cycle.
2. The frequency, f is the number of cycles occurring in a given time period.
3. The Amplitude, A. The "extent" of the motion.

Thus the frequency is the inverse of the period (and vice versa), i.e.:

Motion Period (sec) Frequency (cps)
Earth's rotation 24x60x60=86400 0.0000115
Heart beat 1 1
Hummingbird's wings 0.016 62.5
ConcertA 0.0022727 440

Frequency used to be expressed in cycles per second (cps). The unit Hertz (abbreviated Hz) is now used. 1 cycle/second = 1 cycle sec.-1 = 1 Hz.

* Stevens, S.S. & Warshofsky, Fred.: Sound and Hearing : Time-Life Science Library : 1980
# Olsen, Harry F.:Music, Physics and Engineering : Dover Publications, Inc. N.Y., 2nd ed. : 1967.


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