Sound is energy which travels through a medium, most often free air. Just like dropping a stone into a pool of water, sound travels in waves from the source.
As sound travels in waves, so we can observe that it has properties (again, like the waves in a pool)
A pure sound is considered as being a sinusoidal waveform. It has certain characteristics: it has amplitude, it has a frequency, it has a velocity, it has a wavelength, it has a phase, it has a harmonic content and it has an envelope.
Amplitude: the distance between the highest and lowest points of the waveform. Shown between B and D in the illustration.
Frequency: how many times, how many cycles, there are of the waveform repeating within 1 second. Shown between A and E in the illustration.
Phase: at what point does the waveform start. (the illustration starts at zero)
Harmonic Content: all the other frequencies which are part of the sound wave. (the illustration doesn’t show any harmonic content.)
Envelope: The Attack, Decay, Sustain and Release of the waveform i.e. the shape of the waveform.
Some of these can be calculated, some can be discovered by measuring the waveform. It’s the comnination of all of the above which gives sounds the characteristics by which we can identify them.
A common calculation is to determine the frequency of the sound. The tale below gives a ready-reckoner for frequency to wavelength conversion.