Sound propagates through compressible media such as air, water and
solids as longitudinal waves and also as a transverse waves in solids
(see Longitudinal and transverse waves,
below). The sound waves are generated by a sound source, such as the
vibrating diaphragm of a stereo speaker. The sound source creates
vibrations in the surrounding medium. As the source continues to vibrate
the medium, the vibrations propagate away from the source at the speed
of sound, thus forming the sound wave. At a fixed distance from the
source, the pressure, velocity, and displacement of the medium vary in
time. At an instant in time, the pressure, velocity, and displacement
vary in space. Note that the particles of the medium do not travel with
the sound wave. This is intuitively obvious for a solid, and the same is true for liquids and gases (that is, the vibrations of particles in the gas or liquid transport the vibrations, while the average position of the particles over time does not change). During propagation, waves can be reflected, refracted, or attenuated by the medium.
The behavior of sound propagation is generally affected by three things:
The behavior of sound propagation is generally affected by three things:
- A relationship between density and pressure. This relationship, affected by temperature, determines the speed of sound within the medium.
- The propagation is also affected by the motion of the medium itself. For example, sound moving through wind. Independent of the motion of sound through the medium, if the medium is moving, the sound is further transported.
- The viscosity of the medium also affects the motion of sound waves. It determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.
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