What is “Sound”?

Sound Waves

Sound is an acoustical energy, a class of physical kinetic energy, is composed of fluctuating waves of pressure in a physical medium such as air or water.

The cycle of an acoustical pressure wave is divided into two halves: one of higher pressure (compression) followed by another half of lower pressure (rarefaction). Louder sounds, ones of a higher amplitude, compress and rarefy the air more than softer (lower amplitude) sounds.

The frequency of a wave is the rate of air pressure fluctuation. This fluctuation must be at a rate of at least 20 cycles per second (cps) and no more than 20,000 cps to be classified as sound. Hertz (Hz) is the unit used to indicate frequency in cycles per second: 20Hz = 20cps. While pitch is more complex (also involving amplitude), higher frequency generally denotes higher pitch of a sound.

A wave’s period is the amount of time required for one complete cycle (compression and rarefaction). This period is expressed in seconds per cycle using this formula: Period = 1 ÷ Frequency.

At sea level on a standard temperature day (59 degrees F or 15 degrees C) sound waves travel through air at the speed of 1130 ft/sec or 344 m/sec. Frequency has no bearing on this speed. Wavelength is the physical distance covered by one full cycle of sound of a given frequency as it passes through the air. Wavelength is found by this equation:

Wavelength = Speed of Sound ÷ Frequency

Acoustic to Electrical Sound

An audio signal represents a sound electrically in the form of a fluctuating current or voltage. This fluctuation is reproduced, within the limits of the specific audio system, at exactly the same rate as the audio signal it represents. Amplitudes of the acoustical sound wave and the audio signal are both scaled proportionately.

An audio signal’s strength, or amplitude, is known as the signal level. Audio systems contain many different operating levels. Level is expressed in decibels (db).


Phase is the time relationship of an audio signal to a known time reference. It is expressed in degrees. One complete cycle of a sine wave equals 360 degrees.

The time reference may be a fixed instant in time, or it may be another signal. In the latter case, the reference signal must resemble the signal being measured for phase. We can only meaningfully compare objects that are alike or, at the least, related.

An audio signal’s output is said to be in phase with the input when the fluctuation of the signals is equal and in the same direction. The output is considered to be 90 degrees out of phase with the input when one signal is at zero while the other signal is at it’s maximum, going in the same direction. Output is considered to be 180 degrees out of phase with the input when both signals are at their maximum but going in opposite directions. These phase relationships can, and often do, change at different frequencies.

-By John Redick

180 degrees out of phase

90 degrees out of phase


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