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

Report No. 6945
BBN Systems and Technologies Corporation
Ts is the time duration of a single sequence
T is the time duration of the exposure period.
P
If a time-varying source produces most of its output within 5 dB of the maximum
level, even though its output sequences are not identical, Eq. 7 may be
simplified to the form:
L = Lm + 10 Log (Tm/T )
e q
P
where Lm is the median level of the highest exposure class
Tm is the total time during which the exposure level was within k2.5 dB
of Lm during the exposure period.
(Note that for this case, the time ratio - T,/Tp.)
3.1.3 Source spatial characterization
Man-made noise is often produced by a moving source. A standard procedure
has been established to determine the effective source output with
respect to receiver locations which may be either fixed or moving. This is
done by measuring the source output at a standard reference range. The
resulting sound level spectrum is called the source level (Ls) and is usually
specified at a range of 1 m. For many sources which are too large to measure
accurately at a range of 1 m, the local transmission loss is calibrated so
measurements made at greater ranges can be corrected to an effective range of
1 m. For aircraft noise measurements where atmospheric absorption is an
important factor in addition to the geometric spreading loss, it is customary
to use a flyover altitude of 1000 ft (300 m) as a reference range to avoid
large potential errors in estimating atmospheric absorption loss corractions
back to a range of 1 m.
From the viewpoint of a stationary listener, a moving source becomes a
source with a,fluctuating output level even though the actual output of the
source may be constant. As a result the procedures developed in this report
for application to fluctuating sources will also be relevant for use with
moving sources. Source level spectra for sources which are usually moving are
based on measurements made at the time of the closest point of approach (CPA)
and range corrected using the CPA distance.
3.2 Natural Background Noise
There is a very large volume of literature on the subject of natural
background noise (ambient ~oise) for both deep ocean and shallow water.
Studies of ambient noise have ranged from treatments of specific environments
(e.g., open ocean, island areas, harbors, near-shore or coastal regions and
arctic regions) to concern with understanding specific source characteristics
and physical mechanisms. Classical references on the subject are Knudsen et
a1 . ( 19441, Wenz ( 1962) and Ur ick ( 1983) . As one might expect, many causes of