What is Sound?

Sound is made up of travelling compression waves, or oscillations in pressure.

Acoustics is the scientific study of sound.

Audio is sound within the frequency range which can be heard by human beings, which is generally considered to be between 12 Hz (Hertz: cycles per second) and 20,000 Hz (or 20 kHz). It's this audible range of sound frequencies that I'll be concerned with here.

We experience the frequency of a sound as its pitch: the higher the sound frequency, the higher the pitch (or note if we're talking about music). Some sounds are of too high a frequency to be heard by human ears (ultrasound) and some too low (infrasound). Think of a dog whistle, and then of the serious health concerns of people living too close to industrial wind turbines – Vibro-Acoustic Disease is caused by infrasound noise being transmitted through the ground.





Here is a simplified graphic representation of sound. One whole sound wave is shown, the horizontal axis representing time, and the vertical axis representing the oscillation in pressure from compression (+) to decompression, or rarefaction (−). It is a sine wave, or single pure tone, which doesn't actually exist in nature, but is useful in helping us to understand sound.