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3. Equivalent Continuous Sound Level (Leq)


Sound levels tend to fluctuate, and as such an ‘instantaneous’ measurement like sound pressure level cannot fully describe many real-world situations. A summation can be made of the measured sound energy over a certain period, and a notional steady level can be calculated which would contain the same total energy as the fluctuating sound. This notional level is termed the equivalent continuous sound level Leq. Leq can be determined over any time period, which is indicated as Leq,T where T is the time period (e.g. Leq,24h).


In mathematical terms, for n discrete sound level measurements, Leq is given by: Leq,T = 10 log10 (t1 x 10L1/10 + t2 x 10L2/10 +… tn x 10Ln/10)/T where t1 = time at level L1 dB; t2 = time at level L2 dB; and T = total time


4. Frequency Weighting Networks


Frequency weighting networks, which are generally built into sound level meters, attenuate the signal at some frequencies and amplify it at others. The A-weighting network approximately corresponds to human frequency response to sound. Sound levels measured with the A-weighting network are expressed in dB(A). Other weighting networks also exist, such as C-weighting which is nearly linear (i.e. unweighted) and other more specialised weighting networks. Variables such as Lp and Leq that can be measured using such weightings are expressed as LAp / LCp, LAeq / LCeq etc.


5. Percentile or Statistical Levels (LN)


Sometimes it is useful to calculate the level which is exceeded for a certain percent of a total period. Background noise is often defined as the A-weighted sound pressure level exceeded for 90% of the specified period T, expressed L90,T. Road traffic noise is often characterised in terms of LA10,18h.


EAOW


Noise Prediction Methodology P41388


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