Sound is a longitudinal wave because it moves air molecules (or the medium that it is moving) in the same direction that it is traveling. Therefore, the density of air molecules increases and decreases. Areas of high pressure and high density are considered condensation and low density and low pressure areas are called rarefaction. Areas of rarefaction are considered the wave troughs and areas of condensation are wave peaks. Therefore, regions of condensation are separated by a full wavelength as are regions of rarefaction. The speed of sound depends on the medium through which it is traveling. Sound generally travels fastest in solids because solids are generally stiffer than gases and therefore provide a larger restoring force and acceleration. The speed of sound in a gas is dependent on temperature, and at room temperature the speed of sound is approximately 343 m/s.
The frequency of sound is infinite. However, the human ear can only hear sounds between the frequencies of 20-20,000Hz. Infrasonic sounds occur at smaller frequencies than the human ear can hear and ultrasonic sounds occur at larger frequencies than the human ear can hear. A sound wave is a pure tone if is is a single frequency. Most sounds are some combination of pure tones and many sounds – especially in music – are combinations of pure tones that are harmonically related. The term pitch is used to describe the combination tones.
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