Underwater acoustic projector

The underwater sound projector disclosed herein employs, as radiating surfaces or pistons, stiff, lightweight panels whose mass is substantially less than the inertial component of the radiation impedance over the operating frequency range. The panels are driven in opposition by a plurality of linear actuators, e.g., piezoelectric stacks, distributed essentially uniformly over the panels so that each stack drives an essentially equal area of the panel and flexing of the panel is avoided. The compliance reactance of the actuators is made to cancel the inertial reactance of the radiation impedance at all frequencies within an at least decade-wide frequency band. In this way, operation, similar to resonance with its high efficiency, is achieved continuously over a wide frequency band.

BACKGROUND OF THE INVENTION 
The present invention relates to an underwater sound projector and more 
particularly to such a projector which operates efficiently over a wide 
frequency range. 
For towed array, active sonar systems such as are employed for 
anti-submarine warfare (ASW), it is highly desirable that the transducers 
used in the array be operable over a wide band of frequencies with high 
efficiency. It is also desirable that the transducers have a physical 
configuration that lends itself to underwater towing with low drag. 
While various expedients have been proposed for broadening the response of 
some transducer designs, most prior art transducers, in fact, operate in a 
mode which involves a fixed-frequency mechanical resonance of the 
transducer itself, with the resonance frequency slightly modified by the 
radiation impedance. Examples of such transducers are the so called 
bending moment transducers of the type disclosed in U.S. Pat. Nos. 
3,150,347, 4,972,390 and 5,204,844. Various electromagnetic low frequency 
transducers have been devised which have fixed-frequency resonances with 
relatively broad responses, but these have typically entailed bulky and 
heavy physical configurations. 
Among the several objects of the present invention may be noted the 
provision of an underwater sound projector which is operable efficiently 
over a wide range of frequency; the provision of such a transducer which 
is efficiently operable over a range of frequencies spanning three 
octaves; the provision of such a projector which provides a configuration 
suited for underwater towing; the provision of such a projector which 
provides desirable directivity characteristics; the provision of such a 
projector that can be neutrally buoyant; the provision of such a 
transducer which is highly reliable and which is of relatively simple and 
inexpensive construction. Other objects and features will be in part 
apparent and in part pointed out hereinafter. 
SUMMARY OF THE INVENTION 
The underwater sound projector of the present invention is adapted for 
radiating sound energy over a range of frequencies into a body of water in 
which the projector is immersed. A pair of stiff lightweight plates are 
employed as complimentary, aligned and spaced apart pistons with their 
peripheries being flexibly sealed to exclude water from the space between 
them. A plurality of linear actuators, e.g., piezoelectric stacks, are 
provided between the pistons for driving them in opposition thereby to 
radiate sound energy into the body of the water, the inertial component of 
the radiation impedance being substantially greater than the mass of the 
panels over the range of frequencies of interest. 
In accordance with one aspect of the present invention, the compliance of 
the linear actuator is such that 
EQU C.sub.m .alpha.=1 
where C.sub.m is the combined mechanical compliance of the actuators and 
.alpha. is the product circular frequency times inertial component of the 
radiation impedance, over the frequency range where .alpha. is 
substantially constant. One method of making the pistons is to fabricate 
them as honeycomb cored panels. Another method is to employ an aluminum 
plate grooved to allow individual sections to align with respective 
actuators.

Corresponding reference characters indicate corresponding parts throughout 
the several views of the drawings. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 and 2, the projector illustrated there employs a 
pair of pistons 11 and 13 which are set into corresponding recesses in a 
circular frame 15. While frame 15 is shown as including a central web 17, 
this web may be omitted in some arrangements since the pistons are driven 
in opposition as described hereinafter. The pistons may be described as 
complimentary, aligned and spaced apart. Flexible diaphragm seals 21 and 
23 retained by clamp rings 22 and 24 are provided for flexibly sealing the 
piston panels so as to exclude water from the space between them. As will 
be understood, sliding or O-ring seals might also be employed. 
In accordance with one aspect of the present invention, the pistons 11 and 
13 are constructed as relatively stiff, lightweight plates. In the 
embodiment of FIGS. 1 and 2, the plates are made up of honeycomb cored 
panels. As may be seen in FIG. 2, the panels comprise outer and inner 
skins of stainless steel, designated by reference characters 25 and 27 
respectively, separated by an aluminum honeycomb 29. As is understood by 
those skilled in the art, such a construction is highly resistant to 
bending since the skin panels take up the tension and compression forces 
of bending while the honeycomb maintains the desired spacing between the 
skins. 
The pistons 11 and 13 are driven in opposition by a plurality of 
piezoelectric stacks 31 which are distributed essentially uniformly over 
the panels so that each stack drives an essentially equal area of the 
panel. Magnetostrictive or other types of linear actuators might also be 
used. Combined with the inherent stiffness of the panels, this distributed 
arrangement essentially eliminates flexing of the panels. In the 
illustrated embodiment, the stacks 31 work against the central web 17 but, 
as will be understood, in other arrangements where the web is omitted, a 
longer stack might be employed where each piston is, in effect, driven 
with respect to the opposite piston. 
As illustrated, the stacks 31 are set into recesses in the piston panels 
formed by flanged cylindrical sockets 33 and are clamped by through bolts 
34. These sockets facilitate the coupling of driving forces from each 
stack to the corresponding local area of the honeycomb panel while 
maintaining the panel's structural integrity. These sockets also allow the 
two pistons 11 and 13 to be closely spaced, thereby making the overall 
projector thinner. As described in greater detail hereinafter, the 
piezoelectric stacks 31 are configured to provide a compliance or spring 
constant which is matched to the change in the inertial component of the 
radiation impedance with frequency over the operating frequency range. 
FIG. 3 illustrates a rectangular projector configuration which is 
particularly well adapted for inclusion as a transducer in a towed 
underwater array. For such an application, the rectangular pistons 51, set 
in a frame 53, may, for example, have a height of 5 meters and a width of 
1 meter. Such a configuration gives significant directivity in the 
vertical dimension, which is useful in avoiding ocean bottom reflections, 
while being essentially omni-directional in azimuth over the working 
frequency range of 400 Hz to 3000 kHz. Again, piezoelectric stacks 55 are 
distributed essentially uniformly over the pistons so that each stack 
drives an essentially equal area of the honeycomb panel. Arrangement of 
the stacks within recessed flanged cups is essentially the same as in the 
construction of FIGS. 1 and 2. 
As will be understood, the piston construction employed in the preferred 
practice of the invention inherently provides a relatively thin panel, so 
that the transducer as a whole is relatively thin, e.g., 0.17 meters. 
Thus, the transducer itself provides a good approximation of a fin, which 
can be relatively easily towed, rather than having to be fit into a 
flooded tow body as is the case with most prior art projectors intended 
for the same applications. 
FIG. 4 is a graph illustrating calculated and normalized radiation 
impedance for a 1 meter by 5 meter radiating piston such as is employed in 
the projector illustrated in FIG. 3. The resistive component of the 
radiation impedance is represented by the curve 41 while the reactive or 
inertial component is represented by the curve 43. The abscissa values are 
the products of acoustical wave number and piston width. As may be seen, 
the inertial component drops off significantly after a maximum at about 
1.5, corresponding to 360 hertz. While there are various discontinuities 
in the behavior of the reactive component, the general behavior can be 
characterized as a slope (reference character 44) indicating that the 
radiation reactance decreases inversely with increasing frequency. The 
asymptotic frequency dependence of the reactive component can be expressed 
as follows: 
##EQU1## 
where a and b represent the projector width and height, .rho. represents 
the mass density of water and c represents the speed of sound in water. 
In accordance with an important aspect of the present invention, the 
compliance reactance of the piezoelectric stacks is selected to cancel the 
mass reactance of the radiation reactance such that 
EQU C.sub.m .alpha.=1 
Where C.sub.m is the combined mechanical compliance of the actuators and 
.alpha. is as defined above. 
With this matching of compliance or spring constant with the inertial 
component of radiation impedance, a behavior essentially equivalent to 
resonance in terms of transduction efficiency is obtained over a wide 
range of frequencies. This can be explained in the following manner. 
In general, resonant behavior occurs when the reactive impedance in the 
system is equal to zero. 
EQU I.sub.m (Z.sub.rad)+I.sub.m (Z.sub.mech)=0 
Z.sub.mech is the mechanical impedance of the pistons and the actuators. 
The piston mass is M.sub.p. 
EQU I.sub.m (Z.sub.rad)+.omega.M.sub.p -1/.omega.C.sub.m =0 
and further if the radiation reactance is in the range described above: 
EQU .alpha..omega.+.omega.M.sub.p -1/.omega.C.sub.m =0 
However, if the mass of the piston (Mp) is made substantially less than the 
inertial component of the radiation reactance in the frequency range of 
interest, the corresponding term in the above equation drops out, and 
there remains. 
EQU .alpha.C.sub.m =1 
so that resonant type behavior becomes pervasive over the frequency range. 
The limit on this behavior is when, at higher frequencies, the mass 
reactance of the projector exceeds the radiation mass, i.e., the inertial 
component of the radiation reactance. 
However, as will be understood from the foregoing explanation and the graph 
of FIG. 4, this condition of pervasive resonance can exist over a quite 
substantial frequency range, e. g. over three octaves. Over this range, 
the projector will exhibit relatively high efficiency in the conversion of 
electrical energy to acoustic energy. Not only is this useful range 
considered to be substantially greater than that available with prior art 
arrangements, the physical configuration of the projector is well-suited 
for underwater towing as described previously. 
While it is desirable that the entire effective face area of the projector 
move in controlled fashion, the use of a completely unbending diaphragm in 
practice causes some difficulty in establishing equal loading of a 
multiplicity of piezoelectric stacks. Unequal loading may, in turn, cause 
stress waves across the width of the diaphragm since its width can be 
large as compared to the wavelength of the projected acoustic signal. 
The embodiment of FIGS. 5 and 6 employs the same arrangement of 
piezoelectric stacks as the embodiment of FIGS. 1 and 2. However, rather 
than a honeycomb piston, the piston is constructed as an aluminum plate 60 
which is divided by milled slots 61-64 into four regions 71-74 which are 
of equal area and each of which encompasses three of the piezoelectric 
stacks. The central region is circular and the other three regions are 
arcuate,each extending one third of the region around the central region. 
The milled slots 61-64 extend most of the way through the aluminum plate 
60 so that the remaining thickness provides some flexibility allowing a 
small relative movement between the different regions. Accordingly, as the 
through bolts 34 draw the aluminum plate down against the piezoelectric 
stacks, each region is to some extent free to line itself with the heights 
of its three respective piezoelectric stacks so that equal loading of each 
stack can be provided. 
Although the resultant overall projector may, in one sense, be regarded as 
an array of four pistons, the advantages of the present invention are 
obtained so log as the relationship 
EQU C.sub.m =1 
is maintained. In this regard, it should be understood that the inertial 
component of the radiation impedance is based on the overall area of the 
projector rather than the area of each region of the piston. In similar 
fashion, it can be understood that the projector can be constructed of a 
plurality of separate elements with their edges in close proximity, again 
observing the desired relationship and determining inertial component of 
the radiation impedance based on the overall area of the array. 
While the use of a solid aluminum plate lowers the high frequency response 
somewhat since the mass of the piston begins to approximate the inertial 
component of the radiation impedance at a lower frequency, it is still 
possible to achieve very wide range response, i.e., over several octaves. 
A further alternative for the construction of the piston for use in a 
projector in accordance with the present invention is to construct the 
piston of a high strength composite, e.g., using carbon fibers so as to 
achieve high strength with relatively low mass without having the 
difficulties attendant with localized points of attachment in a honeycomb 
structure. 
In view of the foregoing it may be seen that several objects of the present 
invention are achieved and other advantageous results have been attained. 
As various changes could be made in the above constructions without 
departing from the scope of the invention, it should be understood that 
all matter contained in the above description or shown in the accompanying 
drawings shall be interpreted as illustrative and not in a limiting sense.