Technical Guide - Sound Measurements: Introduction
Ancient civilisations understood sound in the context of music and architecture but the first measurements of sound were concerned with its speed. In 1627 Francis Bacon [1] discussed the possibility of comparing the speed of sound with that of light (which he knew to be immeasurably high) by comparing the time taken for the sound of a church bell to travel one mile with that taken by a simultaneous light signal (an interrupted lighted taper) over the same distance. However he only had his own pulse as a timing mechanism and it was the French mathematician Mersenne [2] some nine years later who published the first value. Mersenne was also the first to point out that knowledge of the speed of sound enabled the range of enemy cannon to be determined simply by timing the interval which elapsed between the flash and the arrival of the sound. Methods for determining the speed of sound have developed over the years, see Delaney [3] for an excellent review, and it is perhaps surprising that the subject can still cause controversy [4,5,6], but now the subject is inextricably linked with measurements of other gas properties [7], and those making acoustical measurements are usually content to take the values derived by experts in that field [8].
It was not until the 19th century that quantitative measurements of sound pressure began to be made and only in the 20th century that those measurements were augmented with other information to produce scales designed to indicate human annoyance due to sound or risk of damage to the human auditory system. It should be noted that most measurements of sound in air have some link to human perception. A Concorde fly past at NPL is loud enough to halt all conversation, but the same equivalent continuous sound pressure, for the same duration, could result from the release of just 4 joules of sound energy in a normal living room (enough to raise the temperature of 5 ml of water by 0.2 C). In general, sound in air contains very little energy and if humans were not sensitive to it then there would be little need to measure it. In contrast, ultrasound in water can carry significant amounts of energy and has many industrial uses.
The challenge in most acoustical measurements is that the physical parameter of interest is a small and rapid fluctuation in a gas. For example, human hearing is sensitive to pressure fluctuations at frequencies from 20 Hz to over 15 kHz, at pressures from as small as 20 µPa. Although the fluctuations are small they cover a wide range with an energy content changing by a factor of 1012 between the human threshold of hearing and the pain threshold.