to air. When the medium under consideration is air, it is possible to express E in terms of the undisturbed air pressure, P, since it is this pressure that supplies the restoring force which returns the air molecules to their equilibrium positions. The speed of sound in air is: where d = air density & P = air pressure (a measure of the elasticity of the air). It is interesting to note that in air an increase in pressure causes a corresponding increase in density and so the ratio of P to d remains constant. Since P/d can be shown to be proportional to the temperature (in kelvin), the speed of sound in air is actually proportional to the square root of the kelvin temperature. The pitch of a sound (how high or low it is) depends on its frequency. The higher the frequency, the higher the pitch. For a sound or note of specific frequency, like that produced by a tuning fork, the pitch is the same as the frequency. However, for a complex sound such as a chord played on a piano, the pitch is not so easily defined. It can no longer be taken as identical to the frequency of the sound, since the sound contains several nearly equal amplitude waves of various frequencies. The loudness of a sound depends upon the amplitude of the wave that produces it. The greater the amplitude, the louder the note, because more energy is used to produce a larger amplitude.