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 log_{10} (P_{rms} / P_{0}) dB
where P_{rms} is the sound pressure at the measurement point
and P_{0} is the reference pressure, 0.00002 Pa
sound power level or SWL = 10 log_{10} (W / W_{0}) dB
where W is the sound power of a source
and W_{0} is the reference acoustic power, 10^{12} watt
dB(A)  the sound pressure level as measured with a sound pressure level meter using an Aweighting network which differentiates between sounds of differing frequencies in a similar way to the human hearing system
dB LAeq,T  the equivalent continuous Aweighted 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 Aweighted 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 Aweighted 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 Aweighted sound pressure level recorded during a noise event or noise monitoring period  the sound level meter timeweighting (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 frequencyweighted. The frequencyweighted acceleration is measured in m/s^{2} and the time period over which the VDV is measured is in seconds. This yields VDVs in m/s^{1.75}.
An estimated VDV (eVDV) can be calculated approximately for a given time period, for example a 16hour day, using the expression:
eVDV = 1.4 x a(rms) x t^{0.25}
where
eVDV = the estimated VDV in m/s^{1.75}
a(rms) = the rms acceleration in m/s^{2}
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 © 20032006 Owen Clingan  Auracle Acoustics
