A longitudinal wave. The speaker cone moves back and forth and pushes the air next to it. Each particle of air moves back and forth about a fixed position, along the direction the wave travels. Where the particles are pushed together the air is at a higher pressure (a compression); where they are spread apart it is at a lower pressure (a rarefaction). The pattern, not the air itself, travels to the right.

The graphs. The upper graph is a snapshot: the pressure at every point along the air at this instant. Its crests line up with the compressions, where the particles are closest together. The lower graph is the pressure at one point, the microphone, as time goes by. The distance between two crests in the upper graph is the wavelength λ. The time between two crests in the lower graph is the period T = 1/f.

v = f λ, v = 331.3 + 0.606 θ m/s (θ in °C)

Measuring the speed. The wave moves one wavelength in one period, so v = λ/T = f λ. The speed of sound depends on the air (mainly its temperature), not on the pitch or the loudness. So a higher note has a shorter wavelength, and a warmer day gives a longer wavelength for the same note. Try both.

Loudness. The sound level in decibels is 20 log(p_rms/20 µPa). Ordinary speech is about 60 dB, with a pressure change of only about 0.03 Pa out of 101 000 Pa. The picture shows the particles moving far more than they really do (the real movement is far less than a thousandth of a millimetre). For the same loudness, a lower note makes the particles move further than a higher note.

The animation is slowed down by a factor of 250, so a 262 Hz note repeats a little more than once a second. The distances are drawn at a true scale of 150 pixels per metre.