Measures of waveforms

Below is a sine wave, the sort of wave that describes the changes in pressure brought about by a "pure tone" or sound of a single frequency. We can measure it in some fundamental ways: by its amplitude (the maximum sound pressure* minus the average, or ambient pressure), and by its period. The ambient pressure is at zero on the graph, and the sound pressure is marked as relative to (greater than or less than) this ambient pressure.

From period and amplitude we can calculate the more common, useful measurements like sound pressure level (SPL, in decibels, dB) and frequency, in kilohertz (kHz).

* The amplitude marked here is the peak amplitude. You can also measure the peak-to-peak amplitude, or use more ingenious measures like the "route mean square" (rms) amplitude.