Particularly interesting waves are sound waves . These pressure waves propagate through the air and, if they reach an ear, they induce the same vibration in the eardrum. The eardrum vibrations are then interpreted by the brain as sound, and the pitch of the sound is determined by the vibration frequency. Most of the results we had obtained for traveling waves on a string also apply to sound waves.
Sound speed is the smallest in gases, followed by liquids and then solids.
More precisely, one has ,where T the temperature measured in Celsius. If we move a large distance away from the sound source, even though the wave is still a spherical wave, the sphere curvature will be very small and, if one considers a small enough region, it can be neglected. In this approximation we are dealing with a plane wave . Because of the non-linear response and the very wide range, it is customary to compare sound levels on a logarithmic scale. The reason for using decibels is that it has been shown that 1 dB is roughly the smallest change in sound intensity that a normal human ear can detect.