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AURACLE ACOUSTICS  -  Glossary of terms

This page provides brief definitions of some key acoustical terms

frequency f   =   the number of cycles per second of a vibrating object or medium

Hertz (Hz)   =  the unit of frequency equal to one cycle per second

       1 kHz     =   1000 Hz

sound pressure level or SPL =  20 log10 (Prms / P0) dB

    where Prms is the sound pressure at the measurement point

    and P0 is the reference pressure, 0.00002 Pa

sound power level or SWL =  10 log10 (W / W0) dB

    where W is the sound power of a source

    and W0 is the reference acoustic power, 10-12 watt

dB(A) -  the sound pressure level as measured with a sound pressure level meter using an A-weighting network which differentiates between sounds of differing frequencies in a similar way to the human hearing system

dB LAeq,T -  the equivalent continuous A-weighted sound pressure level having the same energy as a fluctuating sound over a specified time period T -  this parameter is used for the assessment of residential development sites in the context of PPG24

dB LA10,T -  the A-weighted sound pressure level which is exceeded for 10% of a specified time period T -  this parameter is frequently used for road traffic noise monitoring and prediction, although LA10,T figures are often converted to LAeq,T figures for purposes of PPG24 assessments

dB LA90,T -  the A-weighted sound pressure level which is exceeded for 90% of a specified time period T -  this parameter is often taken to represent the background noise level, particularly in the case of BS4142 assessments

dB LAmax -  the maximum A-weighted sound pressure level recorded during a noise event or noise monitoring period  - the sound level meter time-weighting (fast or slow) is normally stated

LEP,d -  the daily personal noise exposure level as defined in the Noise at Work Regulations: 1989 is the total exposure to noise in the workplace experienced by an individual over the working day, taking into account noise levels in various areas of activity and the time spent in these areas  - no account is taken of ear protection when calculating the daily personal noise exposure level

Vibration Dose Value (VDV) as defined in BS6472: 1992 is calculated by taking the fourth root of the integral of the fourth power of acceleration after it has been frequency-weighted.  The frequency-weighted acceleration is measured in m/s2 and the time period over which the VDV is measured is in seconds.  This yields VDVs in m/s1.75.

An estimated VDV (eVDV) can be calculated approximately for a given time period, for example a 16-hour day, using the expression:

    eVDV   =   1.4 x a(rms) x t0.25

    where

    eVDV   =  the estimated VDV in m/s1.75

    a(rms)   =   the rms acceleration in m/s2

    t            =   the total duration of vibration exposure in s

However, BS6472 notes that this procedure will underestimate the true VDV where the crest factor of the vibration exceeds a value of about 6.

 

Copyright © 2003-2006 Owen Clingan - Auracle Acoustics

 

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