Barrel stave projector
This invention relates to a &#34;barrel-stave&#34; projector which efficiently generates high power underwater sound at low frequencies. The projector is also of small size, of light weight, and is capable of being produced at a low manufacturing cost. 
     It is known to construct underwater acoustic transducers using a flextensional concept such as that discussed in applied acoustics (3)(1970) Page 117 in an article by L. H. Royster entitled &#34;The Flextensional Concept: A New Approach to the Design Of Underwater Acoustic Transducers&#34;. A transducer of this type consists of a ceramic piezoelectric, or piezo-magnetic driver, a mechanical shell and an internal and external fluid medium. The shell is slit lengthwise forming curved beams or staves. The name &#34;flextensional&#34; implies that the flexural vibration of the curved stave is driven by the extensional vibration of the piezoelectric, or piezo-magnetic, driver. The volume velocity of the vibrating stave is much greater than that available from the driver, and the resonance frequency is much lower. 
     However, such a known transducer is quite expensive to manufacture and not very amenable to mass production. Moreover, disassembly of the transducer for repair or for any other reason risks its complete destruction. There are various configurations produced utilizing the flextensional concept, and with some of these configurations, the hydrostatic pressure due to submergence tends to generate tensile stresses in the ceramic driver, which itself is very weak in tension. 
     In the present invention, the vibrating staves curve inwardly rather than the conventional outwardly. The ceramic driver that is utilized is a stack of rings coupled to stiff, flat end plates to which the staves are attached. The end plates are preferably polygonal and, after assembly, the outer surface of each of the staves could be machined so that the finished projector is of round cross-section between the end plates. This shape allows for the easy application of a tubular rubber boot for sealing purposes, such as waterproofing. If preferred, the driver and the end plates can be open to the medium (with suitable sealing) so that electrical and support cables can pass through the driver. This allows for a very compact unit. 
    
    
     The invention will now be described with reference to the accompanying drawings in which: 
     FIGS. 1A and 1B show two perspective views of a preferred embodiment of the barrel-stave projector of this invention without a rubber boot, with FIG. 1B having some of the staves removed to show the inner construction; 
     FIG. 2 is a cross-sectional view along a longitudinal axis of FIG. 1 with a rubber boot in place and without the upper and lower end caps of FIG. 1; 
     FIG. 3 is a plan view of the projector of FIG. 2; 
     FIGS. 4A and 4B are cross-sectioned elevational and plan views of a ceramic stack piezo-electric driver for the projector, with the electrical connections shown schematically; and 
     FIGS. 5A, 5B and 5C are side elevational, front elevational, and plan views of a single stave of the projector of FIG. 1. 
    
    
     Referring to the drawings and in particular FIGS. 1A and 1B, the &#34;barrel-stave&#34; projector consists of a driver 1 manufactured from piezo-electric ceramic; an enclosure around the driver 1 which consists of staves 2, and two polygonal end plates 3. The end plates are shown specifically as being octagonal in this embodiment. The staves 2 are secured to the end plates 3 with an adhesive, one suitable adhesive being epoxy resin, and bolts 4, retained in threaded holes 4a in the end plates. The bolts have countersunk heads which recess into suitable countersunk holes in the staves so that the heads do not protrude above the general immediate surface level of the staves. Between adjacent staves there are gaps 5 which permits the staves to vibrate freely in flexure. If it is wished to close off the inside of the driver 1, end caps 6 and 7 can be used to cover openings in end plates 3. The end caps can be of any suitable design, being made to fit partly within each opening within the end plates, or being large enough to surround the opening. They may be sealed with O-rings or bonded to the end plates if disassembly is not a requirement. A single bolt (not shown) through the center of the driver can alternatively be utilized for pulling the end caps towards each other for sealing purposes. Note that no electrical wires or connections have been shown in FIGS. 1A and B, but such wires could be passed through one of the end caps through a sealing grommet. 
     Referring to FIGS. 2 and 3, the end plates 3 having opening 3a are clamped together across the ceramic driver 1 by relatively small bolts 10 which mainly serve as an assembly aid to provide ruggedness when handling during assembly. The end plates will normally be bonded to the ends of the ceramic driver 1. No electrical connections are shown in these Figures. Referring to FIG. 3, the outer surface of the staves is machined to form a circular cross-section throughout the length of the staves, and gaps 11 between the staves at the end regions only are filled with a potting plastic such as epoxy. A tubular rubber boot 8 is fitted over the stave section of the projector to prevent the external acoustic fluid from entering the space 9 between the staves and the driver 1. A light coating of ethylene glycol or other suitable liquid is wiped onto the staves before the boot is placed thereon, so that the boot 8 will be in good acoustical contact with the staves. The enhanced vibration amplitude of the staves is thus transformed into an enhanced volume velocity in the external acoustic fluid. 
     The projector of FIGS. 2 and 3 is a free flooding design, the interior of the driver 1 being open to the surrounding medium. Therefore a sealing coating will be required on the inner surface of the driver. If the surrounding medium is to be water, a suitable waterproofing coating could be applied to the inner surface of the driver. Alternatively, if the projector of FIG. 2 is not to be free flooding, a pair of simple end caps can be utilized such as those shown in FIGS. 1A and 1B. 
     The driver 1 is shown in more detail in FIGS. 4A and 4B and is comprised of rings 12 of piezo-electric ceramic which will typically be a lead zirconate-titante material, each ring being plated on its flat surfaces with conductive electrodes and axially poled to render it piezo-electrically active. The rings 12 are bonded together by epoxy, or another suitable adhesive, in a cylindrical stack and connected electrically in parallel, by wire conductors 13 and 14. Short pins 15 are soldered or conductively cemented into notches 16 in the rings 12 and conductors 13 and 14 are then soldered to these pins. There are other methods of connecting the conductors to the rings, the method shown being only one of the preferred ways. The wiring harness is potted within a plastic 17, the plastic preferably being epoxy. This potting of the harness prevents the connections from suffering from fatigue failure which could be caused by the severe vibration conditions under which the driver operates. If the driver is to be free-flooded with water, the potting must be thicker than required if the driver is not to be free-flooded, and the potting should extend around the whole of the inner surface of the driver. The conductor 13 leads to the &#34;high&#34; side of an electrical cable 18, and conductor 14 leads to the &#34;low&#34; side, which is common to the metal parts of the projector. Therefore in the structure shown, connection to the &#34;low&#34; electrode of the upper ring of the driver can be made via the metal structure. 
     A single stave is shown in FIGS. 5A, 5B and 5C in its, as installed, shape. Countersunk holes 19 accommodate the heads of the bolts 4 and the outer surface of the staves is rounded to present a circular cross-section finished body. 
     This invention is restricted only by the scope of the claims which follow.