STRATABOX
™
Operations and Maintenance Manual
INTRODUCTION 1-5
©
2006 SyQwest Incorporated
Cavitation is a result of the pressure on the face of the transducer reducing to a level that permits the
water to boil (i.e. turn from a liquid to a gas). This is directly related to the flow around the transducer as
described above. Cavitation is also related to the depth, source level, and frequency of the transducer
being used. As the depth of the transducer increases, static pressure increases to levels that will prevent
cavitation from occurring. If the transducer is mounted properly, with flow noise and cavitation levels
considered, this phenomenon should never affect an echo sounder.
Cavitation may also occur on a ship’s propeller. As the speed of the water increases over the surface of
the propeller, turbulence causes changes in the static pressure of the water. At this point, if the pressure is
low enough, boiling will occur. This process releases a large quantity of bubbles into the water, causing
noise that can affect the performance of an echo sounder. Care should be taken to mount a transducer as
far as possible from this noise source.
Ambient Noise
Ambient noise is always present in the ocean. This noise is caused by both natural or human-made
events. For echosounding principles, ambient noise sources that are significant include hydrodynamic
boundaries, ocean traffic and biological transmitters.
Hydrodynamic noise results from natural phenomenon such as wind, waves, rain or currents. The levels
produced vary greatly, but are largely related to the sea state level. In severe storms, hydrodynamic noise
can reach levels that make reception of a transmitted signal very difficult, if not impossible.
Ocean traffic noise is dependent on the number of ships, the distance from noise sources, and the current
propagation conditions due to thermoclines and sea state. Ocean traffic generally produces acoustic noise
that is not within the frequency bandwidth of most navigational echo sounders, except when ships pass
very near to the receiving transducer.
Biological noise producers include marine life such as shrimp, fish, and mammals. Mechanical
movements of crustaceans, with their hard shells, may produce significant noise when in large schools.
Likewise, certain fish species produce noise by reflecting noise off their swim bladder.
Transducer Properties
Properties of a chosen transducer affect sonar performance which directly relates to depth measurements.
The source level, directivity index, and beam width of a transducer have a relationship with how the sonar
system performs under different conditions.
Source Level
Sound is created by a transducer from a mechanical vibration of the ceramic material of which underwater
transducers are made. The movement of the transducer face creates alternating regions of high and low
pressure, resulting in an acoustic wave. The amount of energy transmitted into the water column is
defined as the acoustic source level. A source level is expressed in units of decibels that describe the
intensity of sound relative to a reference intensity at one unit distance from the sound source. For the
user, a source level is useful for selecting a transducer for a particular use.
The amount of movement on the face of the transducer is linearly related to the voltage difference created
across the ceramics of the transducer. Normally it would seem desirable to put as much power into the
water as possible to achieve maximum depth performance. Cavitation, however, which results from high
source levels and small transducer depths below the sea surface, is generally the limiting factor when
driving a transducer. The electrical and mechanical design of the transducer, which has a maximum
recommended input power level, must also be considered.
