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