Limitations; Cavitation; Machinery Noise - Teledyne SeaBat T Series Operator's Manual

ACTIVE SONAR USAGE
It is recommended to change the SVP 70 filter mode to 0 when convenient and reset
the SVP Filter to SVP Normal.
For access to filtered and unfiltered values, please see the DFD messages R7510 and R7610. Refer
to the Data Format Definition document for information (see Appendix E Reference Documentation).
6.5

Limitations

Physical limitations in the functionality of the system have to be taken into consideration when
operating sonar systems, including SeaBat systems. Some of the limitations are absolutes; others
can to some degree be overcome.
The following subsections provide examples of different kinds of limitations.

6.5.1 Cavitation

Some reduction in the transmitted source level is expected to be caused by cavitation. For means
of compensating for this, see subsections in 6.2 above.
6.5.1.1 Propeller Cavitation
Propeller cavitation can be a significant concern for two reasons. First, the frequency of propeller
cavitation noise is in the operating frequency band of most sonar equipment. Second, the source
levels of the propellers installed are expected to be high unless special quieting techniques have
been implemented.
Propeller energy typically propagates to the forward area sonar systems via a direct or hull-
grazing path. The other arrival path of propeller energy is by the bottom bounce path. This path
will be more significant in shallow water depths (less than 500 meters). The potential impact can
be further quantified for particular bottom depths of proposed operational test sites.
6.5.1.2 Appendage Cavitation
Appendage cavitation can result from rough or unfaired edges on the hull and can even be
present at lower speeds. If the source is located near the sonar installation area, it can be a
significant noise deficiency. It is difficult to predict the occurrence of appendage cavitation without
under-hull drawings, pictures, or visual inspection of the ship when in dry-dock.

6.5.2 Machinery Noise

Machinery noise should not be a major factor for sonars that operate at frequencies above 5kHz.
Mechanical machinery noise is usually prevalent at 2kHz and below, though it is occasionally
detected at higher frequencies when fluid flow through pipes or cavitating pumps are involved.
Typically, the most important consideration for machinery noise is the proximity of the acoustic sensor
to the machinery source. There has been one observed case where diesel engine piston frequency
harmonics have been the controlling noise source at 12kHz, but in this situation, the engine room and
the sonar compartment shared a common bulkhead. Normally, acoustic devices are sufficiently
distanced from major machinery noise sources that machinery noise does not interfere.
A concern remains that there may be some auxiliary machinery (or other unusual equipment)
located near the sonar that could produce noise interference. An inspection of the installation site
and ship drawings to verify the position and proximity of all shipboard equipment should be
conducted to determine if there is a potential for machinery interference prior to conducting at-sea
testing. For means of compensating for this, see subsections in 6.2 above.
SeaBat T-Series Subsea Operator's Manual
Version 7
Page 50 January 31, 2019