Sound is a type of mechanical wave created by oscillations in pressure. As with any wave, the speed of sound depends on the properties of the medium through which it travels. Between materials classified as solid, liquid, or gas, sound will travel fastest in solid mediums and slowest in those that are gas. As a reference point, the standard speed of sound in air is approximately 340 m/s.

Over the years, inventors have found ways to incorporate and utilize sound waves for a variety of purposes. Police cars and ambulances, for example, use sirens to emit high-pitched audible sound waves that warn surrounding vehicles of their approach. Submarines use inaudible sound waves in Sound Navigation And Ranging, more commonly known as sonar. While submerged, active sonar technology allows submarines to determine the location or speed of other underwater objects simply using reflected sound waves.

One notable property of sound is that the emitted frequency and the observed frequency are not always equivalent. Depending on how the source and the receiver are moving relative to one another, the observed frequency of sound may be higher or lower than emitted sound, a phenomenon known as the Doppler effect. In equation form, the change that occurs when the source and the receiver are moving relative to one another can be represented as a ratio of detected versus observed frequencies:

Here, v equals the speed of sound in the medium, v_s is the speed of the source, and v_d is the speed of the detector (also referred to as the receiver).
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