Method and apparatus for sensing acoustic signals in a liquid

An apparatus for sensing acoustic signals in a liquid wherein an omnidirectional hydrophone exhibits a hemispheric response pattern. A hydrophone, including a transducer element and a transmitting cable, is mounted such that the transducer element is adjacent the forward face of an acoustic baffle including a layer of sound-absorbing material. The rear face of the acoustic baffle is positioned adjacent an acoustic shield including a layer of sound-reflecting material. The transmitting cable is positioned in an aperture in the acoustic baffle and the acoustic shield, and passes beyond the rear face of the acoustic shield. A flow fairing is positioned adjacent the forward face of the acoustic baffle and encloses at least the forward portion of the transducer element.

FIELD OF THE INVENTION 
The present invention relates generally to the field of underwater 
acoustics, and particularly to the use of a sound-absorbing baffle to 
alter the isonification pattern of an omnidirectional hydrophone. 
BACKGROUND OF THE INVENTION 
The present invention is directed to achieving a hemispheric isonification 
pattern from a wide-band omnidirectional hydrophone mounted on a 
sound-absorbing baffle. The invention obtains hemispheric directional 
response and uniform sensitivity over a broad band of frequencies from a 
hydrophone designed to exhibit a spherical directional response. 
In the field of underwater acoustics, it is desirable to have a hydrophone 
that exhibits a hemispheric directional response. Hemispheric directional 
response results from a hydrophone that detects only those sound waves 
emanating from points within a 180.degree. sector defined from the 
receiving element of the hydrophone. Additionally, it is desirable for 
such a hydrophone to exhibit a uniform open circuit receiving response 
over a wide frequency band; that is, the hemispheric response pattern 
should be relatively uniform over a broad range of frequencies. Such a 
device is particularly useful for determining the distance between an 
array of such hydrophones and an underwater object. Devices of this type 
might typically be mounted in some fashion on a submarine, and thus must 
also be designed to withstand pressures exerted at submarine depths. 
Certain commercially-available hydrophones are particularly well-suited to 
this type of application, possessing an optimum combination of several 
competing performance parameters including depth capacity, charge 
sensitivity and diffraction field size. One such hydrophone consists of a 
thin-walled spherical shell of piezoelectric ceramic waterproofed by a 
thin shell of rubber or polyurethane encapsulant. This type of hydrophone, 
however, normally exhibits a spherical isonification pattern, detecting 
sounds emanating from points within a 360.degree. arc defined from its 
receiving element. 
Previous efforts to achieve hemispheric isonification of a spherical 
hydrophone while maintaining desired performance characteristics have met 
with limited success, and typically have involved mounting the hydrophone 
on a sound-reflecting surface. While the reflecting surface blocks sound 
waves emanating from points behind the hydrophone, thus achieving a 
substantially hemispheric isonification pattern, a significant problem is 
presented by the sensing of standing waves in the vicinity of the 
hydrophone generated by sound impinging on the face of the reflecting 
surface nearest the hydrophone. This problem is greatest at higher 
frequencies, where short wave reflections tend to destroy the uniformity 
of the hemispherical pattern. Prior approaches to solving this problem 
involved the use of multiple hydrophones, each of which exhibit 
satisfactory response characteristics over a limited frequency range. To 
achieve broad band response with limited interference from standing waves, 
it was necessary to switch from one hydrophone in the array to another 
hydrophone as the frequency varied. 
The present invention solves the standing wave problem by mounting an 
omnidirectional hydrophone adjacent to an acoustic baffle, resulting in 
hemispheric directional response and uniform frequency response over a 
broad range of frequencies. The present invention offers the additional 
advantages of simplicity of design and fabrication. 
SUMMARY OF THE INVENTION 
The present invention provides an apparatus for sensing acoustic signals in 
a liquid. The invention is directed to achieving a hemispheric 
isonification pattern from a wide-band hydrophone designed to produce a 
substantially spherical isonification pattern, while avoiding unwanted 
interference caused by standing waves in the vicinity of the hydrophone. 
The apparatus of the present invention includes an acoustic baffle. The 
acoustic baffle may be constructed of any sound-absorbing material known 
in the art, such as syntatic foam or rubber, which exhibits sound 
dampening characteristics sufficient to prevent reflection of sound waves 
across a desired frequency range. A preferred syntatic foam composite 
consists of small chunks of lossy lead-filled rubber mixed with tiny glass 
microspheres and epoxy to form a solid homogenous mass. When positioned 
behind the spherical transducer element of a typical omnidirectional 
hydrophone, the acoustic baffle absorbs sound waves that do not impinge 
directly on the forward hemisphere of the transducer element, thereby 
ensuring that such sound waves are not reflected back towards the 
transducer element and detected by the rear hemisphere. 
The apparatus of the present invention also includes an acoustic shield. 
The acoustic shield may be constructed of any sound-reflecting material 
known in the art, such as stainless steel, brass or lead, which exhibits 
sound reflecting characteristics sufficient to prevent penetration by 
sound waves across a desired frequency range. The acoustic shield may 
comprise a distinct structure, such as a backing plate, or may be part of 
a structure on which a hydrophone is mounted, such as the hull of a 
vessel. When positioned on the opposite side of an acoustic baffle from a 
spherical transducer element, the acoustic shield reflects sound waves 
that would otherwise impinge on the rear hemisphere of the transducer 
element. Thus, the combination of an acoustic baffle and an acoustic 
shield ensures the hydrophone only registers sound waves impinging on its 
forward face, thereby achieving hemispheric isonification. 
The apparatus of the present invention may optionally include a flow 
fairing. The flow fairing may be constructed of any sound-permeable 
material known in the art, such as polyurethane or rho-C rubber, which 
offers little or no obstruction to sound waves in a desired frequency 
range transmitted through water. Such a flow fairing should be constructed 
in accordance with generally-accepted hydrodynamic principles to minimize 
any turbulence generated by moving water contacting the flow fairing. Such 
a flow fairing would typically present a tapered leading edge. 
An embodiment of the present invention includes an acoustic shield, 
comprising a steel backing plate, an acoustic baffle, comprising a layer 
of syntatic foam, and a flow fairing, comprising a hydrodynamically-shaped 
layer of polyurethane. The rear face of the acoustic baffle is secured to 
the forward face of the acoustic shield, and an aperture passes through 
both layers. The hydrophone is mounted such that the base of the 
hydrophone's transducer element is flush with the forward face of the 
acoustic baffle and the hydrophone's transmitting cable extends through 
the aperture. The entire apparatus may then be affixed to the exterior 
hull of a vessel. 
The acoustic shield prevents sound waves emanating from points behind the 
hydrophone from being detected. Likewise, the acoustic baffle prevents 
sound waves emanating from points in front of the hydrophone, but which do 
not impinge directly on the forward hemisphere of the hydrophone, from 
being detected. Additionally, and unlike prior art applications, the 
acoustic baffle of the present invention does not generate a significant 
amount of unwanted standing waves. Sound waves that do not impinge 
directly on the hydrophone surface are absorbed by the acoustic baffle 
instead of being reflected as standing waves. This apparatus, including 
both absorbing and reflecting components, thus enables a spherical 
hydrophone to exhibit a truer hemispheric isonification pattern than was 
possible in the prior art. 
In another embodiment of the present invention, the acoustic shield 
actually comprises a portion of the hull of a vessel on which the 
hydrophone is mounted. 
In yet another variation on this embodiment, the entire apparatus may be 
suspended from a structure by means of a bracket. The bracket may be 
fixedly attached to the structure, or may be rotatably attached to a drive 
means on the structure. For rotatable attachment, the bracket ideally 
contains an offset portion between its upper and lower ends of sufficient 
degree to align the core of the transducer element with the bracket's axis 
of rotation. The acoustic shield comprises a sheet of acoustic-reflecting 
material substantially larger than the diameter of the transducer element 
to ensure shielding from sound waves emanating behind the transducer. The 
acoustic baffle comprises a sheet of acoustic-absorbing material of 
substantially the same size as the acoustic shield.

DETAILED DESCRIPTION 
FIG. 1 depicts an apparatus for sensing acoustic signals in a liquid 
according to the present invention. A wide band omnidirectional hydrophone 
includes a spherical transducer element 1, a mounting stem 2 and a 
transmitting cable 3. The apparatus includes an acoustic baffle 4 
positioned adjacent to an acoustic shield 5. In mounting the hydrophone, 
the spherical transducer element 1 is positioned such that the base of the 
sphere is flush with the forward face of the acoustic baffle 4. The 
mounting stem 2 and the transmitting cable 3 extend from the spherical 
transducer element 1 into an aperture passing through both the acoustic 
baffle 4 and the acoustic shield 5. The transmitting cable 3 is adapted to 
carry electrical signals to and from the spherical transducer element 1. 
FIG. 2 provides a schematic of an omnidirectional hydrophone capable of use 
with the present invention. A commercially-available example of such a 
device is the EDO Western Model 6600 transducer, which is effective as a 
receiving hydrophone over a frequency range of 500 Hz to 400 kHz. The 
transducer element 1 comprises a sphere of lead-titanate-ziconate ceramic 
material encapsulated in urethane and secured to a stem mount 2. A 
transmitting cable 3, comprising a twisted pair with a braided shield, 
extends from the stem mount 2. A transducer of this type exhibits an 
essentially spherical response pattern with only minor deviations in its 
vertical directivity caused by the stem mount 2. While the present 
invention is described with reference to this particular type of 
hydrophone, it will be apparent to those skilled in the art that other 
hydrophones are equally compatible with the present invention. 
Referring again to FIG. 1, the acoustic shield 5 is constructed of a 
sound-reflecting material, such as stainless steel. Stainless steel is 
particularly well-suited to this application since it presents a high 
level of acoustic impedance as a result of its density. The acoustic 
shield 5, in addition to providing a rigid surface for supporting the 
acoustic baffle 4, shields the hydrophone from unwanted background noises 
such as might emanate from the interior of a vessel to which the apparatus 
is attached. 
The acoustic baffle 4 may be constructed of any composite material, such as 
syntatic foam, which exhibits limited variation in acoustic properties at 
depths up to and exceeding submarine depths (typically from one thousand 
to two thousand feet). A preferred syntatic foam known in the art consists 
of small chunks of lossy lead-filled rubber mixed with tiny glass 
microspheres and epoxy to form a solid homogenous mass. This composite 
presents a high loss factor for impinging sound waves over a wide 
frequency range. The acoustic baffle 4 absorbs sound waves impinging in 
the vicinity of the spherical transducer element 1 so that the hydrophone 
is isonified only on the hemispheric surface opposite the acoustic baffle 
4. 
In one embodiment of the present invention, the apparatus comprises a 
discrete unit that may be attached to a structure. The acoustic shield 5 
and the flow fairing 6 may be adhesively bonded to respective faces of the 
acoustic baffle 4. The apparatus may then be detachably fastened to the 
exterior of a structure by means of fasteners, with the transmitting cable 
3 passing into the interior of the structure. 
Alternatively, the individual layers of the apparatus could be housed in a 
frame or detachably secured to one another with fasteners. Such a 
configuration would permit individual elements of the apparatus to be 
replaced without the need to replace the entire unit. 
FIG. 3 shows an example that demonstrates some of the features and 
advantages of the present invention. Other features and advantages will be 
readily apparent to those skilled in the art. 
Referring to FIG. 3, the acoustic baffle/acoustic shield portion of the 
apparatus is configured as in FIG. 1. A spherical transducer element 1 is 
mounted such that its base is flush with the forward face of the acoustic 
baffle 4. Here, the acoustic baffle 4 comprises a 2 foot by 2 foot square 
of syntatic foam approximately 3 inches thick. It will be apparent to 
those skilled in the art that the thickness of the acoustic baffle 4 may 
be varied depending on the sound-absorbing characteristics of the 
particular material from which it is constructed. 
The acoustic baffle 4 is positioned adjacent to the acoustic shield 5, 
which comprises a 2 foot by 2 foot steel plate approximately 1 inch thick. 
Again, it will be apparent to those skilled in the art that the thickness 
of the acoustic shield 5 may be varied depending on the sound-reflecting 
characteristics of the particular material used. 
The stem mount 2 and transmitting cable 3 pass through an aperture in the 
acoustic baffle 4 and the acoustic shield 5. A flow fairing 6 is 
positioned adjacent to the forward face of the acoustic baffle 4 to shield 
the spherical transducer element 1 from unwanted interference caused by 
water turbulence. The flow fairing 6 comprises a 2 foot by 2 foot layer of 
polyurethane with an impression in its rear face to accommodate the 
spherical transducer element 1. The forward face of the flow fairing 6 is 
bubble-shaped, presenting a tapered leading edge in the vicinity of the 
transducer element 1. 
The transmitting cable 3 is detachably connected to a preamplifier 7, which 
is itself secured to the rear face of the acoustic shield 5, to enhance 
the acoustic signals detected by the hydrophone. The preamplifier 7 may be 
any suitable device known in the art. A preamplifier cable 8 connects the 
preamplifier 7 to means for monitoring the detected acoustic signals (not 
shown). 
A mounting bracket 9 suspends the other components of the apparatus. The 
lower end of the mounting bracket 9 is attached to the rear face of the 
acoustic shield 5, while the upper end forms a mounting flange 10. The 
mounting flange 10 may be rotatably attached to a rotation drive means 
(not shown) on a vessel (e.g., a submarine or a surface-going ship). The 
mounting bracket 9 contains an offset portion such that its axis of 
rotation 11 passes through the core of the spherical transducer element 1. 
FIG. 4 provides an isolated view of the spherical transducer element 1 to 
more clearly illustrate this orientation. 
FIG. 5 shows a possible variation for a listening apparatus according to 
the present invention. This embodiment also includes an acoustic shield 5 
and an acoustic baffle 4; however, the flow fairing does not comprise a 
layer completely covering the acoustic baffle 4 as it did in FIG. 3. 
Instead, the acoustic baffle 4 contains a recessed portion 12 housing the 
spherical transducer element 1. This recessed portion 12 is filled with 
polyurethane to shield the hydrophone from unwanted interference produced 
by water turbulence. 
FIG. 6 shows a cross-sectional view of a portion of yet another embodiment 
of a listening apparatus according to the present invention. In this 
embodiment, the acoustic baffle 4 includes a counterbore 13 comprising a 
recess in the upper surface of the acoustic baffle 4. The length and width 
of the counterbore 13 are preferably substantially larger than the 
circumference of the spherical transducer element 1, while the depth of 
the counterbore 13 is approximately equal to the radius of the transducer 
element 1. Transducer element 1 is positioned directly over the center of 
the counterbore 13, and is supported by a polyurethane column 14 formed as 
part of the flow fairing 6. 
Counterbore 13 functions as an acoustic trap to prevent sound waves from 
impinging on the lower hemisphere of the transducer element 1, thereby 
improving the performance of the listening apparatus of the present 
invention. As shown in FIG. 6, an acoustic wavefront 15 impinges the 
acoustic baffle 4 at an angle of approximately 45.degree. to the upper 
plane of the acoustic baffle 4. Although acoustic baffle 4 is designed to 
absorb the acoustic wavefront 15, it is possible that in the absence of 
counterbore 13 some portion of the acoustic wavefront 15 might be 
reflected off of the acoustic baffle 4 and onto the lower hemisphere of 
the transducer element 1, resulting in degradation of the desired 
hemispheric response pattern. The possibility of such an undesirable 
degradation is eliminated by this embodiment. The counterbore 13 ensures 
that any portion of the wavefront 15 reflected off of the acoustic baffle 
4 glances off at an angle sufficient to bypass the transducer element 1. 
The present invention has been shown to cause a normally omnidirectional 
hydrophone to exhibit a substantially hemispheric isonification pattern 
over a frequency range of 30 kHz to 200 kHz. FIG. 7 graphically depicts 
the actual isonification pattern achieved by the example apparatus of FIG. 
3 for incoming sound waves at a frequency of 60 kHz. This and similar 
directional response patterns were obtained by producing short bursts of 
single-frequency tones using a sound-producing transducer stationed 
approximately two meters from the apparatus. The spherical transducer 
element of the apparatus faced the sound-producing transducer, while the 
acoustic baffle/acoustic shield were positioned perpendicular to an axis 
formed between the transducer element and the sound-producing transducer. 
For each frequency tested, the hydrophone was rotated incrementally 
between each repetitive sound burst to generate a locus of points on a 
polar plot that shows the resulting output voltage of a preamplifier 
coupled to the hydrophone in decibels as a function of the rotation angle. 
It was necessary to synchronize the single sine wave cycle sample taken 
near the beginning of the resulting burst of voltage from the preamplifier 
to account for the delay caused by the sonic travel time between the 
sound-producing transducer and the hydrophone. 
As can be seen, the hemispheric isonification pattern is slightly deformed 
at the outer regions of the base of the hemisphere. This deformation is 
apparently due to the reflection of sound waves impinging the acoustic 
baffle at shallow grazing angles, and becomes more prominent as the 
frequency of the impinging sound waves increases. 
The present invention achieves the goal of limiting the generation and 
detection of unwanted standing waves in the vicinity of the hydrophone. 
Table 1 below lists predicted standing wave ratios (VSWR) and reflection 
losses (RL) for an apparatus such as that shown in FIG. 3 for impinging 
sound waves across a range of frequencies. The data was generated using 
circuit analysis and modeling techniques well known in the art. 
TABLE 1 
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Frequency (Hz) VSWR RL 
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10000 1.75 -22.62 
15000 7.33 -4.77 
20000 1.27 -36.96 
25000 5.29 -6.64 
30000 2.00 -19.12 
35000 4.44 -7.97 
40000 2.57 -14.27 
45000 3.83 -9.29 
50000 2.82 -12.90 
55000 3.28 -10.94 
60000 2.73 -13.35 
65000 2.78 -13.09 
70000 2.47 -14.95 
75000 2.40 -15.44 
80000 2.25 -16.60 
85000 1.20 -41.40 
90000 2.27 -16.46 
95000 2.37 -15.60 
100000 2.47 -14.92 
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The standing wave ratio describes the ratio of reflected to incident sound 
waves, and is calculated as follows: 
##EQU1## 
where .alpha. is the complex coefficient of reflection for the acoustic 
baffle/acoustic shield combination. The return loss is the ratio of the 
reflected sound wave over the incident sound wave for the acoustic 
baffle/acoustic shield combination, and is calculated as follows: 
EQU RL=20log.sub.10 (.mu..sup.2 +.upsilon..sup.2), 
where .mu. and .upsilon. are the real and imaginary components respectively 
of the complex coefficient of reflection for the mounting apparatus. The 
fluctuations in the values in Table 1 are caused by variations in the 
wavelengths of sound transmitted through the various layers at varying 
frequencies. 
While the present invention is described with reference to specific 
embodiments, it will be apparent to those skilled in the art that many 
modifications and variations are possible. Accordingly, the present 
invention embraces all alternatives, modifications and variations that 
fall within the spirit and scope of the appended claims, as well as all 
equivalents thereof.