Page 33 - Winter 2009

Fig. 3. This diagram is taken from ANSI/ASA S12.64-2009/Part 1 and shows a top down view of the ship’s “Test Course” configuration for port approaches to the hydrophone(s). The starboard approach is simply in the opposite direction. The Data Window Length (DWL) and Data Window Period (DWP) are defined by equations in the box in the lower left corner. Both DWL and DWP are bound by the “Start Data” and “End Data” points. U.S. Navy typical- ly uses COMEX and FINEX as points for starting data and ending data collection. For this standard COMEX and FINEX define points where ship operations must be maintained during the test run. Used with permission from the Acoustical Society of America, © Copyright 2009.
Hydrophone deployment must follow a strict set of geo- metrical constraints that depend on the measurement Grade (A, B or C) and ship length. Figure 1, extracted from ANSI/ASA S12.64-2009/Part 1, shows the configuration for Grades A and B. The configuration for Grade C is similar except the top and bottom hydrophones are eliminated. Figure 2, again extracted from the standard, shows three typical deployment configura- tions that could be used for each of the Grades.
Figure 2(i), support vessel deployed, is the simplest con- figuration, but has limited low frequency performance because sea state motion from the surface is transferred to the hydrophones. A very soft suspension system along with other features is required to get useful measurements below 100 Hertz. Figure 2(ii) shows a configuration where the hydrophones are bottom anchored and the cable is kept tight by a sub-surface buoy. This approach is more complicated but greatly aids in low frequency performance. Figure 2(iii) combines elements of the other approaches and adds the use of telemetry. Although more technology is required, this con- figuration results in better data and has the potential to elim- inate the need for a support ship.
Figure 3, also extracted from the standard, is a top down view of the vessel’s test course for port approaches. The ship’s noise is measured as it approaches the hydrophones which are suspended vertically in the water column. At COMEX (Navy term for “commence exercise”) the vessel must main- tain a constant speed and course until FINEX (“finish exer- cise”) is achieved. SPL measurements are taken between the COMEX and FINEX points. Most Navy ranges start and end
data acquisition at the COMEX and FINEX locations. Grades B and C which may use simpler methods will then take measurements within “Start Data” and “End Data.” points. The difference in distance between these two points is called the “Data Window Length” defined in the standard based on the length of the ship. The “Data Window Period” (time) is based on ship length and ship speed.
The section on “Post Processing” includes specification of the Data Window Length/Period, background noise adjustments, sen- sitivity adjustments, distance nor- malization and combination of data from multiple hydrophones and multiple runs. The final results are reported in one-third octave bands, sound pressure level in decibels rela- tive to one (1) micro-Pascal normal- ized to one (1) meter distance. These units are consistent with the U.S. Navy databases collected over the last forty years.
The background sound adjustment uses standard decibel subtraction between SPL with the ship going by the hydrophone less SPL with the ship at least 2 kilo- meters away. Distance normalization uses standard spherical spreading from the acoustic center of the ship to each hydrophone. This was one of the most discussed elements as underwater sound propagation is much more complex than the classic “20 log(distance)” relationship. The committee settled on this method, as using anything else would be very complicated and depend on many parameters related to the ocean water and bottom. Further, many elements of the stan- dard help insure spherical spreading is correct including: the short distances between the ship and hydrophones, large
water depth and use of multiple hydrophones.
The “Post Processing” section also describes how data
from three hydrophones (Grades A & B only) are combined into one data set for each run and then how multiple data sets are combined into one resulting sound pressure level (SPL) relative to 1 microPascal at 1 meter as a function of one-third octave bands. These results may then be compared with data from other ships or underwater noise criteria such as that found in the International Council for the Exploration of the Sea (ICES) Cooperative Research Report for fisheries research vessels.2
During the development of ANSI/ASA S12.64/Part 1, the Working Group discussed the need to expand the project to address additional issues and it was agreed that these would be addressed in additional parts of the standard or as new Annexes in future revisions of Part 1. These areas include: a measurements application guide (likely to be an informative annex), narrowband measurements, shallow-water measure-
32 Acoustics Today, October 2009