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Acoustic Terminology

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Acoustic Terminology
Sound Sound is vibrational disturbance,exciting hearing mechanisms,transmitted in a predictable manner determined by the medium through which it propagates.(to be audible the disturbance must fall within the 20 to 20,000Hz range)
Noise Basically too much of the above.
Decibel (dB) Unit for measuring sound derived from the Latin deci meaning tenth and bel from early researcher Alex Bell. Ten times the logarithm (to the base of ten) of the ratio of two mean square values of sound pressure,voltage or current.
Spectrum The audible range of sound frequencies arranged in sequence according to cycles per second (Hz). In our work we use 8 octave bands 63 to 8000Hz.
Octave Band That part of the sound spectrum included between the limiting frequencies of an octave.(an octave has a frequency ration of 2 eg 63-125- 250 etc)
Frequency The number of complete cycles per second (Hz) which characterises any periodic phenomenon.
Sound Pressure Level (SPL) This is the measured or calculated sound pressure which exists at a distance from the sound source due to the sound energy (power) and stated in decibels (dB) either as an overall level or in octave bands .Is often quoted as an ‘A’ scale figure.
Sound Power Level (SPwL) Where the SPL is affected by conditions surrounding it ie presence of barriers,wind speed and direction or ground cover the SPwL is the noise energy AT THE SOURCE.
Ambient Noise The sound pressure levels associated with a given environment .Ambient noise is usually a composite of sounds from near and far sources none of which are particularly dominant.
Audible Frequency Range The range of sound frequencies normally heard by the human ear.
‘A’ Scale This is the scale derived to measure sound pressure level (SPL) .It is weighted electronically in the frequency range below 1000Hz to compensate for the reduced sensitivity of the human ear to low frequency sounds at low pressure levels.(There is also B and C scales rated lower but not generally used on building services work).
‘A’ Sound Level This is a measured or calculated SPL referenced to the A scale eg 85dBA sometimes used as a measure of loudness.(For example a fan 65dBA at 3 metres)
Noise Criteria (NC Level) A system of rating how noisy an occupied space is with readings given in dB in each frequency. The curves from these are not used in Europe now.
Noise Rating (NR ) This is the standard introduced by ISO in 1970 to which we now rate our noise requirements.This uses the 1000Hz band as a reference point and you will note for example NR55 is 55dB at 1K.
Breakout Noise Noise emitted through a surface eg Fan or AHU Casing
Free Field Sound from a source free from obsticles ie walls,buildings etc.
Flanking Noise bypassing measures to reduce it.
Reverberation Time (T ) Acoustical absorption of a room measured in seconds.
Self Noise Noise generated by an attenuator or louvre by air velocity.
Dynamic Insertion Loss Attenuator acoustic performance with airflow.
Static Insertion Loss Attenuator acoustic performance with no airflow.
Cross Talk Audible voice communication between compartments.Often a problem in ducts passing between rooms.

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