Last edited 27 Aug 2015

Sound frequency

Sound is caused by vibrations which transmit through a medium such as air and reach the ear or some other form of detecting device.

Sound intensity is measured in Decibels (dB). This is a logarithmic scale in which an increase of 10 dB gives an apparent doubling of loudness. Sound pitch is measured in Hertz (Hz), the standard unit for the measurement for frequency. The audible range of sound for humans is typically from 20 Hz to 20,000 Hz, although, through ageing and exposure to loud sounds the upper limit will generally decrease. As well as intensity and frequency, sound also transmits information. For example, music or speech, transmit information which people may perceive differently from other sounds.

Approved document E, Resistance to the passage of sound defines ‘frequency’ as:

The number of pressure variations (or cycles) per second that gives a sound its distinctive tone. The unit of frequency is the Hertz (Hz (formerly called ‘cycles per second’).

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 2⅓ times the frequency of the lower limit.

The sound absorbing characteristics of materials varies 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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