US 3706967 A
A high efficiency, broadband acoustic projector for deep water applications in excess of twelve thousand feet. The projector comprises an active element consisting of a plurality of piezoelectric staves electrically connected in series. The staves are physically arranged in a cylindrical assembly prestressed in a fibrous glass wrapping, and installed in an annular, acoustically transparent, neoprene boot. Air-exhausted oil fills the boot to completely immerse the active element and thereby couple the radiated energy thereof to the surrounding water. The center cavity of the projector is open at both ends for free-flooding. Broadband response is achieved by close-coupling the active element's cavity and hoop resonances whereby they reinforce each other across a broad range of operating frequencies.
Description (OCR text may contain errors)
United States Patent Retina, .i r.
 UNDERWATER ACOUSTIC PROJECTUR  Assignee: The United States of America as represented by the Secretary of the Navy  Filed: Jan. 21, 1971  Appl. No.: 108,488
Thousand [521 TU.S.C11.....7.V. ....340/10,s1 0/s.2, 31079.1
 int. Cl. ..H04b 13/02  Field oi Search ..340/8, 10, 11; 310/82, 8.3, 310/9.1
 References Cited UNITED STATES PATENTS 3,624,429 11/1971 Behrendt ..3l0/9.l
3,043,967 7/1962 Clearwaters ..340/10 Primary Examiner-Benjamin A. Borchelt Assistant Examiner-H. J. Tudor Attorney-R. S. Sciascia and Henry Hansen  ABSTRACT A high efficiency, broadband acoustic projector for deep water applications in excess of twelve thousand feet. The projector comprises an active element consisting of a plurality of piezoelectric staves electrically connected in series. The staves are physically arranged in a cylindrical assembly prestressed in a fibrous glass wrapping, and installed in an annular, acoustically transparent, neoprene boot. Air-exhausted oil fills the boot to completely immerse the active element and thereby couple the radiated energy thereof to the surrounding water. The center cavity of the projector is open at both ends for free-flooding. Broadband response is achieved by close-coupling the active elements cavity and hoop resonances whereby they rein- 3,262,093 7/1966 Junger et al ....340/10 force each other across a broad range of operating 3,230,505 1/1966 Parker et al.. ....340/l0 frequencies, 3,230,504 1/1966 l-loran et al... ....340/l0 3,263,209 7/1966 Madison ..340/10 7 Claims, 5 Drawing Figures 23 I? rtfi f |2\; A: 4/41 M: 2| 32F z. *3 2s"" f I :1 'I IO I/ KS 5/ I40 F; m '4 r- 12 I4 PATENTED 19 3. 706. 967
sum 1 OF 2 INVENTOR.
NICHOLAS RENNA Jr.
BY \Mm ATTORNEY PMENIEDMM m2 3,706,967
SHEET 2 OF 2 INVENTOR. NICHOLAS RENNA Jr.
ATTORNEY UNDERWATER ACOUSTIC PROJECTOR STATEMENT OF GOVERNMENT INTEREST BACKGROUND OF THE INVENTION The present invention relates to underwater acoustic projectors, and more particularly to an underwater acoustic projector capable of operating at substantial depths over a wide range of frequencies with relatively high efficiency.
Advanced sonar systems employing deep refractive paths for extended detection ranges have provided impetus to the development of acoustic projectors employing piezoelectric ceramic active elements which are more efficient at ocean depths in excess of twelve thousand feet. Omnidirectional acoustic projectors used in initial experiments at such depths employed annular arrays of active elements with both ends closed to form a center cavity. The pressures on the inner and outer surfaces of the elements were equalized by completely filling the center cavity with oil and by providing pressure transfer means between the outer and the inner faces of the elements. Hence, the active element was subjected to a stress equal only to its operating depth which was in the order of magnitude considerably less than the stress to which an air-filled cavity projector was subjected at the same depth. Later-developed projectors achieved direct pressure equalization at the active elements by free-flooding the inner cavity. While this last configuration ideally provided pressure equalization, its efficiency over a desired frequency range of 2 to 3 kHz. was relatively poor.
SUMMARY OF THE INVENTION Accordingly, it is a general purpose and object of the present invention to provide a novel and improved underwater acoustic projector suitable for operating at substantial depths while achieving broad frequency response in a predetermined range. Another object is to provide a broadband projector having relatively high efficiency within such range.
These and other purposes and objects are accomplished according to the invention by close coupling the cavity and ring resonances of a cylindrical acoustic projector so that they augment each other to yield efficient operation over a predetermined frequency band. The projector includes a plurality of piezoelectric staves electrically connected in series and physically arranged in a cylindrical array. The array is prestressed in a fiberglass wrapping and inserted, in spaced relation, in an annular neoprene boot. Air-exhausted oil fills the boot and completely immerses the array to acoustically couple the radiated energy to the ambient sea water. The center ofthe boot is free-flooding thereby enabling theprojector to operate at any ocean depth.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents a side view of a preferred embodiment of an underwater acoustic projector according to the invention;
FIG. 2 represents a top view of the projector;
FIG. 3 represents an enlarged transverse cross-sectional view of the projector taken along the line 33 of FIG. 1;
FIG. 4 represents a fragmentary top view of piezoelectric staves in the projector; and
FIG. 5 represents an enlarged longitudinal cross-sectional view of the projector taken along the line 5-5 of FIGS. 2 and 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT Normally, the major resonance of a short annular transducer with the center cavity filled with air is the fundamental hoop or ring resonance frequency f of the active element where w, angular ring resonance frequency,
Y Youngs modulus of circumferentially poled active element,
p density of active element, and
Zi= mean radius. For operation at great ocean depths, however, it is necessary to free-flood the center cavity with the sea water in order to equalize the hydrostatic pressure on all sides. As a result, the cavity resonance frequency f also becomes significant. Through theoretical and experimental investigation, an equation has been developed for cavity resonance f, in terms of active element parameters and the bulk modulus of the fluid contained within the cavity as where B bulk modulus of medium, Y Youngs modulus of the active element, t= thickness of active element, and .Q is a dimensionless frequency parameter given by for values of h/a between 0.40 and 8.0. The Youngs modulus used in Equation (3) is valid for low frequency, but must be corrected if it is desired, as in the present invention, to operate the active element at frequencies near the ring resonance frequency, Equation l Thus, the final form of the equation is:
lOGOll where After solving for Q, the cavity frequency is Thus, it is apparent that close coupling the ring and cavity resonances of a water-filled cylindrical acoustic projector within or near a desired broadband operating frequency is predicated on the particular active element height and radius, stiffness, and mass of the water filling the cavity. In view of the foregoing design limitations, the present invention will now be further described.
Referring now to the drawing, there is shown an underwater acoustic projector according to the present invention which includes an acoustically transparent annular boot 10, preferably of neoprene, having inner and outer concentric sides 11 and 12 respectively and a bottom 13. Six molded neoprene pads 14 between sides 11 and 12 are vulcanized in place to the bottom 13 at equally spaced intervals. Of course it is understood that pads 14 could also be formed integrally with boot l0. Pads 14 are preferably spaced from either inner and outer sides 11 and R2 to provide additional communication between the spaces defined by the confronting sides of adjacent pads 14. An arcuate groove Ma concentric with boot is formed in the upper surface of each pad 14 and registers with the lower end of a cylindrical active element 16 spatially positioned within boot 10.
Active element 16 is secured in position at the upper end by a molded neoprene top cover 17 which includes an annular ring 18 sealingly engaging the confronting surfaces of sides 11 and 12 adjacent to the top of boot 10, and six equally spaced pads 19 extending downwardly. An arcuate groove 21 concentric with boot it) is formed in the lower surface of each pad 19 and registers with the upper end of cylindrical active element 16. Two openings 22 in diametrically opposite sides of ring 118 communicate with the enclosed volume of boot 10 for filling and venting the space surrounding active element 16 with db castor oil 25. Plugs 23 seal the openings 22.
An external electrical connector 24 mounted on the top surface of cover 17 provides for signal input to terminals (not shown) on active element 16.
Referring now to FIGS. 3 and 4, the active element 36 is shown as comprising a right circular cylindrical array of 54 piezoelectric staves 26 contiguously bonded together with confronting sides of adjacent staves. The staves 26 have a hoop prestress of about 4000 psi induced by covering 27 formed about the outer cylindrical surface of the array. Covering 27 is preferably layers of fiberglas roving wrapped around the element under constant tension, normally six to nine pounds, built up with an epoxy filler for a prestress about 50 percent greater than desired to offset the relaxation which occurs during subsequent oven curing of the wrapped element.
A projector constructed in accordance with the foregoing description was based on a configuration yielding a cavity resonance of 2.1 kHz. and a ring resonance of 3.2 kHz. The ring/cavity resonance ratio f,/f was minimized at 1.48, h/a to less than L0 and t/d to approximately 0.04. More specifically, the active element 16 was a low-loss lead zirconate lead titanate ceramic, 11 inches outside diameter (d), 4.3 inches high (h), 0.40 inch thick (1) inserted in a boot 10 which is 12.25 inches outside diameter, 8.25 inches inside diameter and 6.75 inches high. With this configuration a measured efficiency of 60 percent at 2.3 kHz. and percent at 2.8 kHz. was obtained rendering the projector ideally suited for broadband operation in deep water applications.
it will be understood, of course, that various changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.
What is claimed is:
ll. An underwater acoustic projector, comprising:
an acoustically transparent annular boot having concentric walls sealingly closed at one end to form an annular trough around a cylindrical open-ended cavity;
a cylindrical piezoelectric element spatially positioned within the trough;
an annular cover mounted on the other end of said boot sealingly enclosing the trough; and
castor oil completely filling the unoccupied space in the trough.
2. An underwater acoustic projector according to claim 1 further comprising:
said element including a circular array of lead zirconate lead titanate staves bonded together and an outside fiberglass covering inducing a hoop prestress in said element.
3. An underwater acoustic projector according to claim 2 further comprising:
said element having a h/a ratio equal to or less than 1.0, where h length of element, and a inside radius of element.
4. An underwater acoustic projector according to claim 3 further comprising:
said element having a t/d ratio of substantially 0.04
where t= thickness of element, and d outside diameter of element.
5. An underwater acoustic projector according to claim t further comprising:
said boot being formed of neoprene and including a plurality of circumferentially spaced support pads at the closed end of said boot formed to receive confronting portions of one end of said element.
6. An underwater acoustic projector according to claim 5 further comprising:
claim 6 further comprising:
said cover further including openings for filling and venting the space within said boot, and plugs sealingly inserted within said openings.