Pathfinder DVL Guide
April
2018
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
Page 45
Recommended Practices for Window Use
Window Orientation
:
The acoustic window should be flat and parallel to the transducer mounting plate.
Note this is not an absolute requirement but can make the transducer susceptible to differentials in water
temperature as well as variations in absorption characteristics.
If the window is at an angle to the DVL transducer bottom edge, it will change the absorption. While we
do not have experience with different angles, we have had customers use domes or a window that fol-
lows the contour of the ship bottom, without noticeable degradation of the data.
Window Material
:
The acoustic properties of the window include an acoustic refractive index (which
should be as close as possible to that of water) and insertion loss (which should be as small as possible)
and speed of sound. There are two acoustic refractive indices: one for shear waves and one for plane
waves. The acoustic refractive indices are simply the ratios of speed of sound in water to speed of sounds in
the material. Insertion loss combines absorption and reflection of sound, and it depends on both the
thickness and the material properties of the window. In particular, care should be taken to avoid
using a window thickness equal to odd multiples of shear mode quarter-waves (Dubbelday and Ritten-
meyer, 1987; Dubbleday, 1986). Refer to Selfridge (1985) and Thompson (1990) for more information.
Note that the speeds of sound in plastics decrease with increasing temperature and that causes the
resonant frequencies to shift. This can be a large effect. Neither Selfridge nor Thompson provides much
information on the temperature coefficients of sound speeds in materials.
Our experience has shown that Polycarbonate windows are very good for use as a DVL window. The thick-
ness of the materials depends on the frequency you intend to use and the weight of the water inside the
window as noted above. Table 10 is provided as a guide for choosing the maximum thickness window to
use.
Table 10:
Window Thickness
Frequency
Thickness
600
0.25 inches
One concern with window selection is that it has be able to support the weight of the water
inside the well once the ship is dry-docked. TRDI recommends that you always fill/drain the
well at the same time that you are either filling/draining the dry dock area.
Spacing between window and transducer
:
The primary geometrical factor in design of windows is the re-
flection of a beam into another beam, causing crosstalk between the beams. The distance of the trans-
ducer from the window should be at least 0.25 to 0.5 inches. The optimum distance for the bottom of
the transducer assembly from the window is 0.25 inches ±0.125 inches. The optimum does not match
the recommended. Never allow the transducer to touch the window. If installed farther than 0.25 to
0.5 inches, then it is imperative to insure that the window aperture is large enough to clear the convex
beams. The farther away the transducer is from the window, the more the sound is reflected off of one
beam and then reflected into another beam.
Acoustically-absorbing well lining
:
A sound absorbing material should be used inside the well to minimize
the effects of sound ringing within the well. The material should be a minimum of one wavelength thick
(include the sound speed of the absorbing material when calculating the size of a wavelength). The wave-
length within the absorbing material can be calculated using the following formula:
DVL
m
m
f
c
=
λ
Where
m
c
= Speed of Sound in the material
DVL
f
= Center Frequency of your DVL unit
m
λ
=
wavelength of the signal in the material.
Approximate wavelengths of sound in seawater are given below in Table 7. Using standard neoprene wet
suit material has been found to work well with 300 and 600hKz frequency DVLs.