Sound can be described as a pressure disturbance that travels through the constituent particles of a medium: as one particle of the medium is perturbed by the pressure, it exerts a force on an adjacent particle, thus perturbing that particle in turn. The speed of sound reflects the speed at which the disturbance is passed from particle to particle, or the distance it travels in time. This form of pressure disturbance is propagated through the medium exclusively as a longitudinal wave in the case of gases and liquids, i.e., a type of wave in which the displacement of the medium is parallel to the propagation of the wave. In this case, the speed of sound refers to the distance a compression or a rarefaction point travels per unit time. In solids however, sound may propagate as a transverse wave, i.e., a type of wave in which the displacement of the medium is perpendicular to the direction of propagation of the wave. Unlike light, which can travel through a vacuum, sound waves require a medium to travel though and their propagation speed will not depend on wave properties such as period, frequency or amplitude, but on the inertial and elastic properties of the medium through which they travel, such as bulk modulus, elasticity and density. If the medium is a homogeneous gas, the speed of sound is constant and can be calculated using:
where &ggr; is a constant specific to the gas through which sound is propagating, R is the universal gas constant (8.314 J/mol K), T is the temperature (in units of kelvin) and M is the formula weight of the gas. In air, the speed of sound is approximated at normal atmospheric pressure by: = 331.5+0.6 T m/s, where T is in Celsius units, and which yields 340 m/s. The speed of sound propagating through a liquid or a solid can be obtained using:
where B is the bulk modulus of the propagating medium and its density. Generally speaking, the more dense the medium, the faster the speed of sound, and the following expression is usually valid for the speed of sound when other properties are equal: in solids > in liquids > in gases. For example, at standard temperature, the speed of sound in water is 1485 m/s, compared to 1522 m/s in sea water, 5790 m/s in bulk stainless steel and 4700 m/s in bulk brass.
The slow speed of sound in air relative to that of light (i.e. 340 vs. 300,000,000 m/s) makes it possible to approximate the distance of thunderstorms by calculating the delay between observing lightning (the light wave) and hearing thunder (the sound wave). For example, if thunder is heard five seconds after lightning, then the sound wave has traveled an approximate distance of 340 m/s x 5 s 1,700 m and the storm is located about a mile away. Echo, or the reflection of a sound wave from a barrier, is another time delay perception used to estimate distances with the speed of sound. The time delay between a voiced cry, shout or holler and its echo corresponds to the time required by the original sound wave to travel the round-trip distance to the barrier and back. It is then possible to calculate the one-way distance to the barrier responsible for the echo. This phenomenon is used by bats to navigate and hunt. They produce short bursts of ultrasonic sound waves that reflect off various bodies and which they can hear to estimate distances in their surroundings. The underlying mechanism is a consequence of the Doppler effect, or of the change produced in the frequency of a sound tone due to the relative motion between the source of the sound and the observer. Similar to bats, underwater SONAR uses the travel time of sound waves to determine distance. This makes the speed of sound in water critical to whales, dolphins, and other marine creatures that use sonar. Likewise it is also critical to submarines and oceanographic researchers who use SONAR to map the ocean floor.
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