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

Report No. 6945
BBN Systems and Technologies Corporation
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The time duration required for the CPA sequence must also be compared to
the reference period. This is done as a separate correction process.
Received levels significantly below the level at CPA do not contribute much to
the total energy received during the CPA sequence. By neglecting contributions
20 dB or more below the level at CPA, it is possible to calculate a specific
time required for completion of the sequence as determined by the speed of the
source. The range from the receiver at which the received level is 20 dB
below that at CPA is obtained using the measured or modeled TL characteristics.
The effective time duration of the CPA sequence is approximately
equal to the travel time required for the source to cover two of the 20 dB
range intervals or:
where
Rm = Maximum effective range where TL is 20 dB greater than at CPA (km)
S = Speed of the source (km/sec)
When these modifications are incorporated into the SNC Model, Eqn (20)
can be rewritten as follows:
SNC = Ls - TLr - Cm + 10 Log{(Tt)(Tf)(Pe)(Ns)} dB re 1 vPa at 300 m (22)
Eq. (22) was used to calculate the SNC ratings for the major sources identified
in the study for each of four OCS planning areas - Chukchi, Norton Basin,
North Aleutian and Shumagin. The effect of two different sound propagation
conditions was also considered if appropriate. In order to simplify the
analysis, the procedure followed previously in presenting a summary of source
level characteristics is also used here in that the dominant bandwidths and
maximum 1/3 octave band levels are used rather than the full 1/3 octave
spectra. The SNC Model spreadsheet layout is shown in the example in Table
5.2. (Complete tabular SNC results for all of the areas studied are presented
in the next section in combination with the SER Model results.)
The following description gives detailed information on the organization
and terminology of the SNC tables.
Standardized Noise Contribution Model
(Description of terms and data entering procedure)
Source - Description of category or specific name of source.
Type - Fixed (remains stationary), Local (moves less than 600 m
in 2 hours), Moving (moves more than 600 m in 2 hours)
Dominant Bandwidth - The frequency band including the 1/3 octave Band
with the highest level in the source level spectrum and
bounded by the 1/3 octave bands with levels within 10 dB.