Introduction to Computer Music: Volume One

2. What is Sound?

We live at the bottom of a sea of air. As with the ocean, the farther down one goes, the greater the density of molecules and the greater the pressure it exerts on us. The average atmospheric pressure, as it is called, is measured in many different units, but the one most people are familiar with is barometric pressure (measured in millibars). Altitude, temperature and weather conditions affect barometric pressure (low barometric pressure, for example, is often associated with storm systems). If the micro-pressure level surrounding us remains steady or changes very slowly, we experience silence. Even so, individual air molecules themselves always exhibit at least a minimum rate of motion without causing perceptible pressure changes.

Sound is produced by a rapid variation in the average density or pressure of air molecules above and below the current atmospheric pressure. We perceive sound as these pressure fluctuations cause our eardrums to vibrate. When discussing sound, these usually minute changes in atmospheric pressure are referred to as sound pressure and the fluctuations in pressure as sound waves. Sound waves are produced by a vibrating body, be it an oboe reed, guitar string, loudspeaker cone or jet engine. The vibrating sound source causes a disturbance to the surrounding air molecules, causing them bounce off each other with a force proportional to the disturbance.

The energy of their interaction creates ripples of more dense (higher pressure) to less dense (lower pressure) air molecules, with pressures above and below the normal atmospheric pressure. When the molecules are pushed closer together it is called compression; when they are pulled apart, it is called rarefaction. The back and forth oscillation of pressure produces a sound waves.

Amazing factoid #1: The threshold of human hearing, or the softest perceptible sound, corresponds to a pressure variation of less than a billionth of the current atmospheric pressure (though the threshold of hearing varies according to frequency, as you will see below).

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