Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
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-5 -4 -3 - 2 - 1 0 1 2 3 4 5
Distance from CPA. km
Fig. 5.1. Transmission Loss vs. Distance from CPA for a Moving Source.
CPA 300 m from Receiver.
apparently fluctuate in level because of their motion, which causes the range
and thus the TL to change with time. The effective received level from a
moving source can be estimated using the same procedure used to determine Leq
for a fixed source that fluctuates in time [see Eqs. (5) and (6) in Section
3.1.21. Figure 5.1 shows the typical bell shaped curve that describes the
change in transmission loss for sound from a source traveling along a straight
line past a fixed receiver (a received level curve would have the same shape).
A 15 Log R transmission loss characteristic was assumed in Fig. 5.1. The
stepped curve shows the 5 dB incremental approximation used to estimate the
effective TL for the entire closest point of approach (CPA) sequence. Since
measured data (and some model outputs) often are not describable with an
analytic function, an incremental integration process was used. This process,
in effect, determined an equivalent constant sound level which contains the
same acoustic energy during the-time interval of the CPA sequence as the
actual time-varying received level. The difference between the received level
when the source is at CPA and the effective level for the entire CPA sequence
is Cm, the moving source correction factor. The appropriate values for this
factor were found by analyzing the TL information for each of the lease areas
which were studied. The results showed that while this factor is area
specific, it is not frequency dependent.