Return to welcome page

What is Sound?
by Stephen Mackey

Music has been called "the organization of sound towards beauty". Beauty is a subjective concept. The differences in what we each find beautiful vary drastically from one culture to the next.

Sound, however, is a quantifiable physical phenomenon, a class of physical kinetic energy called acoustical energy. It is defined as the reception and transmission of a sound wave. This brings us to an age old question "If a tree falls in a forest and no one hears it, does it make a sound?" Barring philosophical extremism such as "Do squirrels count?" the answer is no, because there has been no reception of the sound wave.

To understand sound we must first identify the components of the sound wave. Acoustical energy is comprised of fluctuating waves of pressure in a physical medium, typically air, but also in solids, liquids, and gases. Sound waves do not travel in a vacuum. A single complete cycle of an acoustical pressure wave consists of one half-cycle of higher pressure (compression) of the air molecules, followed by one half-cycle of lower pressure (rarefaction) of the molecules. Sounds of higher amplitude (louder) compress and rarefy the air molecules to a greater extent than do lower amplitude (softer) sounds.

Frequency is defined as the rate of air pressure fluctuation, and is measured in Hertz (Hz), or cycles per second. A wave must be between 20 and 20,000 Hz to be classified as sound. This range is also considered to be the normal range of human hearing, although hearing responses vary widely according to age and exposure to loud sound levels. Frequency, along with amplitude, corresponds to the musical attribute of pitch.

Period is defined as the amount of time required for one complete cycle of a sound wave. Period is the inverse of frequency and can be found by the following equation:

Period = 1 / Frequency

Sound waves travel through air at a specific rate which depends on air pressure, humidity, and temperature. At sea level, standard temperature (59 degrees Fahrenheit or 15 degrees Celsius) sound travels at 1130 feet/second. The speed of sound is independent of frequency.

Wavelength is defined as the physical distance covered by one complete cycle of a given frequency sound as it passes through air and can be found by the following equation:

Wavelength = (Speed of Sound) / Frequency

"Sound Reinforcement Handbook" Gary Davis & Ralph Jones 1989

Developed by Intertainment Technologies