Skidmore College

Sound Basics

Figure 1 (Sine Wave)

Sound is really just compressions and rarefactions of air molecules occurring repeatedly and quickly. Think of the normal changes in atmospheric pressure that we all experience every day… usually resulting in some kind of change of weather. If these barometric changes, normally associated with weather, were to occur regularly, say 20 times per second, we might just perceifve that as a low pitch !

The most basic sound wave is a sine wave (based on that good old math function). A sine wave is a “pure” waveform with no harmonics [fig. 1]

The sine wave could be thought of as the plot of the air pressure in time… starting at a zero point aon a Y axis (that zero point being equal to “normal” atmospheric pressure of 14.7 lbs/square inch), moving towards a maximum peak value, returning to the “standard” (zero on Y axis), then descendinhg below the “normal” pressure level (rarefaction) and finally, returning again to the “standard” or zero line of the Y axis.

Few things in life produce pure sine waves. A flute comes close followed by a tuning fork (used by piano tuners) and then, of course, a synthesizer.

A scientist by the name of Fourier discovered that no matter how complex a sound wave is, it can be recreated by simply adding up sine waves. This is called additive synthesis. Most sounds in the real world (called complex sounds) are made up many partials or overtones. Each partial, overtone, or harmonic can be thought of as a sine wave component of the complex sound. By using computers, we can create a large number of these partials and create new sounds “from scratch”. If you have enough time, enough computer memory and a fast enough computer, the sky’s the limit!

A sound can be basically identified by three main qualities: amplitude, frequency, and spectrum. Most musicians would think off these parameters as “volume” (or dynamics), “pitch” and “timbre” (or tone color). The study of how the brain perceives various sounds is known as Psychoacoustics.


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