Guide To Acoustics
Sound is a form of energy, produced when a source generates waves (rapid variations in pressure) within a medium. The sound is heard when the waves reach a receiver. The number of waves radiated every second is described as the frequency of the sound (measured in Hertz, Hz), which determines the pitch: doubling of the frequency of a sound increases the pitch by one octave. See Comparative Sound Waves below.
The loudness of a sound depends upon the size of the pressure changes in the medium. The basic measure of loudness is the sound pressure level, which is recorded in decibels (dB). The sound pressure level is logarithmic, so a doubling of the actual sound pressure produces a 6dB increase in sound pressure level.
The human perception of the loudness of sound depends upon its pitch: the ear is less responsive to very high and very low pitched sounds compared to those in the middle range (500 - 4000Hz). Specialist reports indicate that when a noise level is reduced by 10dB, the human ear perceives it to be half as loud.
Recording the sound level as a single value is good enough for most everyday uses, but is not sufficiently accurate for use in acoustics because the single value can mask substantial variations in sound levels at different frequencies. For these purposes, sound levels are measured across a range of 6 octaves (centred on 125, 250, 500, 1000, 2000, 4000Hz) in bands 1/3 of an octave wide. The results can be expressed in a number of ways, including response curves such as those found in Acoustic Performance — Ceilings and Acoustic Performance — Walls.