Page 32 - Winter 2009

Fig. 1. This diagram is taken from ANSI/ASA S12.64-2009/Part 1 and shows the hydrophone geometry for Grades A and B. DCPA is the distance at the “Closet Point of Approach” that is set equal to the ship’s length, but no less than 100 meters. This insures the measurements are being taken in the far-field. The distances d1, d2 & d3 are the hydrophone depths, which are functions of DCPA. Grade C is similar to this figure except the top and bottom hydrophones are deleted. Used with permission from the Acoustical Society of America, © Copyright 2009.
Distance determination (ranging) is necessary to nor- malize the sound pressure levels to a distance of 1 meter. The final results are “source” sound pressure level (SPL) values normalized from typical measure-
ment distances of 100 meters (or at
least one ship’s length) to the refer-
ence distance of 1 meter. In actuali-
ty, the underwater sound pressure
levels are affected by other factors
besides distance from the ship to
the hydrophones. These other fac-
tors (that are ignored in this stan-
dard) include the presence of the
free surface and bottom reflections.
The resulting quantities are thus
considered “affected source levels,”
but referred to within S12.64 as just
source levels. The resulting data
could alternately have been report-
ed in the form of sound power lev-
els (Lw). However, all previous
work by the U.S. Navy and many
others has used the “SPL at 1
meter” parameter. Early in the stan-
dard development, it was decided
that the final resultant would use
this existing parameter and not
create any new acoustic metrics.
The section on “Measurement
Procedures” includes requirement
conditions for a suitable test site,
sea state conditions, hydrophone deployment, test course, vessel operations, and the test sequence. Another requirement that was agreed upon early in the standard development was that no special ocean location be specified. For the most part, naval underwater test ranges use very selective loca- tions which have deep water and low background noise.
Since no specific open ocean location or locations are required, the standard has requirements for water depth and background noise. Other factors related to site selection are the proximity to main shipping traffic, bottom type and space available for maneuver- ing. The standard does not pro- vide hard and fast sea state (or weather) requirements other than recommendation that testing be conducted with wind less than 20 knots (10.28 meters/second). However, the measurement sys- tem needs to meet background
noise requirements so measurement of a quiet ship in rough seas may not be possible. On the other hand, measurement of a noisy ship might be accomplished.
Fig. 2. This diagram is taken from ANSI/ASA S12.64-2009/Part 1 and shows three possible hydrophone deployment configurations that could be used for any Grade. Other configurations or combinations of these three configurations are certainly possible as long as the basic requirements of the standard are achieved. Used with permission from the Acoustical Society of America, © Copyright 2009.
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