April
2018
Pathfinder DVL Guide
Page 44
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
Mounting Considerations
Well Mounting
: The effects of ringing will last longer if the transmit signal becomes trapped inside the
transducer well. This can occur because the well itself is ringing with the transducer or when windows
covering the opening of the well reflect the signal back inside the well.
The window causes the transmit signal to reflect back into the well due to the difference in acoustic
impedance between the window and the water. When the transmit signal is reflected in the well it be-
comes trapped and this results in longer ringing periods. To keep from processing this signal, the
blanking period would have to be increased. This is possible for Water Profile using the
and for Bottom Track using the
To mitigate the effects of ringing inside the transducer well, a sound absorbing material can be mated with
a mounting plate or a clamp to reduce acoustic coupling increasing the ringing period. Also, the inside
walls of the well can be lined with a sound absorbing material to damp the ringing effect. Using stand-
ard neoprene wet suit material has been found to work well.
Windows Use Considerations
Windows can be used to produce overall performance improvements in vessel-mounted DVLs. There
are several advantages and disadvantages to consider before using an acoustic window.
Advantages
•
Well will not fill with air bubbles caused by the ship moving through the surface water.
•
Flow noise is reduced.
•
The well can be filled with fresh water to limit corrosion.
•
Barnacles cannot grow on the transducer faces. Barnacle growth is the number one cause of
failure of the transducer beams.
•
The transducer is protected from debris floating in the water.
Disadvantages
•
The range of the DVL may be reduced because the window can and will absorb some of the
transmit and receive energy.
•
The transmit signal could be reflected into the well, causing the well to “ring”. This will
cause the data being collected during the ringing to be biased. Some vessels have reported a
loss in range as great as 50 meters. As noted, the ringing may be damped by applying
sound absorbing material on the well walls (standard neoprene wet suit material has
been found to work well).
•
The transmit signal could be reflected off the window and back into the other beams.
If the well has excessive ringing (due to reflection from the window or ringing within the mechanical
structure itself) the DVL can be instructed to keep from processing this signal. To do this, the blanking
period of the instrument would have to be increased. This is possible for Water Profile using the #
and for Bottom Track using the #
Temperature differentials should also be considered because if the water temperatures inside the window
and outside the window are not the same, all four beams will be refracted and actual velocity components
will be rotated into a new coordinate system. In particular, some of the horizontal velocity will appear
as a vertical velocity