Page 388 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

380 CHAPTER 14 Underwater Acoustics
Wide beam
Receiving elements
FIGURE 14.7
Multielement transducers.
Narrow beam
Receiving elements
Transmitter element
Transmitter element
14.3.8 Medium beam/narrow beam transducer
The transducer used in USBL (ultrashort baseline—Europeans use the term SSBL or “super short baseline”) mode (acoustic positioning) normally consists of three different groups of elements. This is to be able to calculate a three-dimensional bearing to the transponders.
• The beam width of a transducer can be changed during operation. This is achieved by combining more transducer elements in series/parallel.
• A more narrow beam gives higher directivity (higher gain and higher noise suppression from outside the beam) but will give a smaller signal “footprint.”

• For the narrow beam transducer in Figure 14.7, the 3 dB point in wide beam mode is 160 , 
while in narrow beam mode at the 3 dB point it is 30 .
• When using the transducers in SBL (short baseline) or LBL (long baseline) mode, no angle
measurements are done and only the R-group (reference) is used. Dedicated SBL/LBL transducers containing only one element or one group can be used.
14.4 Acoustic noise
14.4.1 Environmental
Noise from thrusters and propellers from surface vessels is the dominating environmental noise source. This noise is approximately 40 dB above normal sea noise. Common for all noise sources is that the noise level drops approximately 10 dB per decade with increasing frequency.
14.4.2 Noise level calculations
The noise level at the system detector is calculated by the following equation:
where
B 5 detector bandwidth,
DI 5 directivity of transducer.
N5ðN0 10logðBÞDIÞ