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Last edited 27 Dec 2019
Sound (or audio) frequency is the speed of the sound’s vibration which determines the pitch of the sound. Sound is caused by vibrations that transmit through a medium such as air and reach the ear or some other form of detecting device.
A frequency of 1 Hz refers to one wave cycle per second, while 20 Hz refers to 20 per second, where the cycles are 20 times shorter and closer together.
A ‘frequency band’ is a continuous range of frequencies between stated upper and lower limits. An ‘octave band’ is a frequency band in which the upper limit of the band is twice the frequency of the lower limit. A ‘one-third octave band’ is a frequency band in which the upper limit of the band is 2 times the frequency of the lower limit.
The sound-absorbing characteristics of materials vary significantly with frequency. Low-frequency sounds, below 500 Hz, tend to be more difficult to absorb whereas high frequencies sounds, above 500 Hz, are easier to absorb. A material's sound absorbing properties can be expressed by the sound absorption coefficient, alpha, as a function of frequency, where alpha ranges from 0 (total reflection) to 1.00 (total absorption).
Similarly, the sound insulation of materials varies with frequency. Low-frequency sounds tend to be attenuated less by passing through sound insulating materials than high-frequency sounds. As a result, the sound attenuation properties of materials are generally measured at a range of frequencies representative of normal human hearing and this is then compared to a reference frequency profile such as that defined in BS EN ISO 717-1 Acoustics. Rating of sound insulation in buildings and of building elements. Airborne sound insulation.
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