Physics of Sound

What is “Audio”?

Audio means “of sound” or “of the reproduction of sound”. Specifically, it refers to the range of frequencies detectable by the human ear — approximately 20Hz to 20kHz. It’s not a bad idea to memorise those numbers — 20Hz is the lowest-pitched (bassiest) sound we can hear, 20kHz is the highest pitch we can hear.

Before you learn how sound equipment works it’s very important to understand how sound waves work. This knowledge will form the foundation of everything you do in the field of audio.

Sound waves exist as variations of pressure in a medium such as air. They are created by the vibration of an object, which causes the air surrounding it to vibrate. The vibrating air then causes the human eardrum to vibrate, which the brain interprets as sound.

Graph of 1 khtzSound waves can also be shown in a standard x vs y graph, as shown here. This allows us to visualise and work with waves from a mathematical point of view. The resulting curves are known as the “waveform” (i.e. the form of the wave.)

The wave shown here represents a constant tone at a set frequency. You will have heard this noise being used as a test or identification signal. This “test tone” creates a nice smooth wave which is ideal for technical purposes. Other sounds create far more erratic waves.

Click here to listen to this tone (22KB wav file)

Note that a waveform graph is two-dimensional but in the real world sound waves are three-dimensional. The graph indicates a wave traveling along a path from left to right, but real sound waves travel in an expanding sphere from the source. However the 2-dimensional model works fairly well when thinking about how sound travels from one place to another.

The next thing to consider is what the graph represents; that is, what it means when the wave hits a high or low point. The following explanation is a simplified way of looking at how sound waves work and how they are represented as a waveform. Don’t take it too literally — treat it as a useful way to visualise what’s going on.

In an electronic signal, high values represent high positive voltage. When this signal is converted to a sound wave, you can think of high values as representing areas of increased air pressure. When the waveform hits a high point, this corresponds to molecules of air being packed together densely. When the wave hits a low point the air molecules are spread more thinly.

In the diagram below, the black dots represent air molecules. As the loudspeaker vibrates, it causes the surrounding molecules to vibrate in a particular pattern represented by the waveform. The vibrating air then causes the listener’s eardrum to vibrate in the same pattern. Voilà — Sound!

Variations in Air Pressure and Corresponding Waveform
Loudspeaker and Waveform
Note that air molecules do not actually travel from the loudspeaker to the ear (that would be wind). Each individual molecule only moves a small distance as it vibrates, but it causes the adjacent molecules to vibrate in a rippling effect all the way to the ear.

Sound Wave Properties

All waves have certain properties. The three most important ones for audio work are shown here:


Wavelength: The distance between any point on a wave and the equivalent point on the next phase. Literally, the length of the wave.


Amplitude: The strength or power of a wave signal. The “height” of a wave when viewed as a graph.
Higher amplitudes are interpreted as a higher volume, hence the name “amplifier” for a device which increases amplitude.


Frequency: The number of times the wavelength occurs in one second. Measured in kilohertz (Khz), or cycles per second. The faster the sound source vibrates, the higher the frequency.
Higher frequencies are interpreted as a higher pitch. For example, when you sing in a high-pitched voice you are forcing your vocal chords to vibrate quickly.

How Sound Waves Interact with Each Other

When different waves collide (e.g. sound from different sources) they interfere with each other. This is called, unsurprisingly, wave interference.


The following table illustrates how sound waves (or any other waves) interfere with each other depending on their phase relationship:

Sound Wave Interactions
  • Sound waves which are exactly in phase add together to produce a stronger wave. (Constructive Interference)
  • Sound waves which are exactly inverted, or 180 degrees out of phase, cancel each other out and produce silence. (Deconstructive Interference)
  • Sound waves which have varying phase relationships produce differing sound effects. (Harmonics/Timbre)