Because light travels very fast, almost instantaneously, it is possible to estimate the distance between an observer and a thunderstorm from the time difference between the lightning flash and the thunderclap . In imperial units sound travels at approx. A compressional or longitudinal wave travels as a series a compressions and rarefactions which correspond to the peaks and troughs in a transverse wave. The amplitude corresponds to displacement of molecules by the sound wave and the human ear is sensitive to amplitudes in air corresponding to around a thousand millionth of a centimetre. Above about 130 dB the loudness of sound is progressively perceived as pain.
Unwanted sound is noise .
Noise is probably the most pervasive form of pollution and can cause a great deal of anguish to some people whereas other people are unaffected. Legislation protects people from exposure to dangerous levels of noise and time limits are set for exposure to prescribed levels of noise and ear protectors must be worn in some occupations. Sound like light can undergo reflection and refraction . Reflection of sound gives rise to the echo phenomenon which is reflected sound that can be easily distinguished from the original pulse of sound. It is necessary to stand about 20 metres from a large reflecting barrier to hear an echo.