Sound Exposure Level

Although L_eq measurements provide practical results for fluctuating noise, an L_eq measurement does not remove ambiguity in the case of transient noise (e.g. aircraft flyover or a vehicle drive‐by). In such a case the SEL is very useful and corresponds to an equivalent SPL L_eq normalized to one second, combining both loudness and duration in a single metric. Therefore, SEL is numerically equivalent to the total sound energy while L_eq is proportional to the average sound power. For example, an A‐weighted noise level of 80 dB lasting one second would have a SEL = 80 dB but if the noise event lasted two seconds, the SEL would be 83 dB. SEL can be used to either characterize a single noise event or compare the energy of noise events which have different time durations. Sometimes A‐weighted sound exposure level is referred to ASEL and L_AE.

Comparing the definition of SEL with Eq. (6.5), we see that

(6.6)

where T is the integration time in seconds. Some integrating sound level meters offer an automatic SEL mode.

EXAMPLE 6.6

What is L_eq over eight hours for a machine performing 500 operation cycles each with SEL = 105 dB?

SOLUTION

We have that 8 hours = 8 × 3600 = 28 800 seconds. Therefore, from Eq. (6.6)

EXAMPLE 6.7

For a typical day a lane of traffic in a highway located at a distance of 50 m from a receiver carries a total of 20 000 vehicles of all types. Assume that during the day trucks constitute 10% of the total of vehicles and automobiles 90%. For the example, suppose that the average ASELs at a distance of 50 m are 80 dB for trucks and 65 dB for automobiles. (a) Determine the 24‐hour average SPL, and (b) Determine the effect of banning trucks from this highway.

SOLUTION

1. We have that 24 hours = 24 × 3600 = 86 400 seconds. We obtain a number of 2000 trucks and 18 000 automobiles per day. First, we need to obtain the energy‐combined SEL for all the events:Therefore, using Eq. (6.6)
2. The computation without trucks gives

Therefore, banning trucks from this highway will produce an A‐weighted noise level reduction in L_eq of 64.7 − 58.2 = 6.5 dB.