Abstract:
A flexural disk transducer is located in a shell or enclosure primarily for underwater use. Compression chambers adjacent to the transducer capture sound from bands on the disk and waveguides are used to direct the sound forward along a single propagation axis. An acoustic delay line is built into one waveguide to compensate for phase differences between the bands.

Description:
BACKGROUND 
     1. Technical Field 
     The field relates to flexural disk transducers and more particularly to a shell or enclosure providing improved efficiency in operation. 
     2. Description of the Problem 
     Flexural disk transducers are usually constructed as metal/ceramic bi-laminar (or tri-laminar), vibratile electroacoustic transducers. Early versions were placed in housings in which they were mounted along their perimeters and which included an acoustic shield which left only the central portion of one major transducer surface of the piezoelectric ceramic exposed to the transmission medium such as water or air. This was done due to inner and outer portions of the disk operating out of phase with one another when the disk was operated in its free fundamental resonance mode. Shielding a portion of the disk mitigated destructive interference between the different sections of the disk. 
     Use of such shields was known to reduce the efficiency of such devices. This loss of efficiency was addressed in U.S. Pat. No. 4,190,783 which was directed to device for use in air in which the shield or plate was displaced from the transducer surface and sized so that sound produced along the peripheral edge reached a central aperture in phase with sound produced at the center of the device. The plate functioned to introduce a time delay for the sound generated by the peripheral portions of the disk allowing them to be constructively added to vibrations generated in the center of the disk. In this way most of the sound energy produced along one face of a disk could be captured. 
     Flexural disk transducers have been applied to underwater applications as well, particularly as high frequency acoustical sources. In such applications it has been supported along its edges so that the disk vibrates in a flexural mode similar to the bottom of an oil-can when depressed to force out oil. 
     SUMMARY 
     A sound generating and propagating device includes a flexural disk transducer having front and reverse major surfaces and a primary resonant frequency of operation. A ring compression chamber is located adjacent a band on the front major surface and a band on the reverse major surface to capture sound generated off either or both bands. First and second waveguides are connected to the ring compression chamber with the first waveguide providing coupling of sound captured from the band on the front major surface to the environment forward along a propagation axis and the second waveguide providing for coupling of sound captured from the band on the reverse major surface forward along the propagation axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Understanding of the following description may be enhanced by reference to the accompanying drawings wherein: 
         FIG. 1  is a perspective view of a flexural disk transducer shell. 
         FIG. 2  is a perspective cutaway view of the flexural disk transducer shell of  FIG. 1 . 
         FIG. 3  is a partial cross section view of the flexural disk transducer shell. 
         FIG. 4  is a perspective of a flexural disk transducer. 
         FIG. 5  is a perspective cutaway view of a flexural disk transducer as located in the transducer shell. 
         FIG. 6  is a cross-section view of a flexural disk transducer as located in the transducer shell. 
     
    
    
     DETAILED DESCRIPTION 
     The FIGURES illustrate an acoustic transducer assembly  10 . Assembly  10  comprises a shell  12  which has a cylindrical section  22  opening outwardly toward one end to define an assembly mouth  24 . A perimeter mouth  18  and a center mouth  20  are displaced inwardly from assembly mouth  24  and channel sound into the assembly mouth for constructive summation. Shell  12  may be made of any suitable material such as, but not limited to, aluminum. 
     Perimeter mouth  18  is defined between an outer phase plug  14  and cylindrical section  22 . Center mouth  20  is defined between the outer phase plug  14  and an inner phase plug  16 . Referring to  FIGS. 1 and 2  it may be seen that outer phase plug  14  has a generally toroidal shape, but one which is elongated in a direction parallel to a propagation axis “A” for sound from the device. The outer phase plug  14  is pointed at one end in its direction of elongation. The outer phase plug  14  defines a hollow central core with the propagation axis corresponding to the center of the hollow center core. The pointed end of the outer phase plug  14 , adjacent to and between the perimeter and center mouths  18 ,  20 , forms a wedge. Inner phase plug  16  is located centered on the propagation axis in the hollow center of the outer phase plug  14 . Inner phase plug  16  is generally cone or bullet shaped and pointed toward the open end of shell  12  corresponding to a combined or common mouth  24  from waveguides  28  and  30 . Sound from the device is emitted outwardly from combined mouth  24  or forward along the propagation axis A. 
     Outer phase plug  14  is nested in the bowl of an open semi-torus formed by section  26  of shell  12 . Section  26  has a semi-circular cross section and forms a closed loop extending between the cylindrical section  22  and a base element to inner phase plug  16 . A gap is left between the base of the outer phase plug  14  and the inner surface of the section  26  to form a serpentine waveguide  28  for the reverse major surface  50 B of the flexural disk  34 . The gap between the outer phase plug and the inner phase plug defines a straight waveguide  30  for the front major surface  50 A of the flexural disk  34 . The throats  29  and  31  (See  FIG. 5 ) to waveguides  28  and  30  are juxtaposed across a ring compression chamber  46 . Acoustic transducer  10  has a design wavelength at its selected operating frequency which defines the relative lengths of the serpentine waveguide  30  and the straight waveguide  30 . Serpentine waveguide  30  is nominally one half wavelength longer than straight waveguide, or some odd whole number multiple of half a wavelength, to produce in phase sound at combined mouth  24 . A typical length for straight wave guide  30  could be ¾ of a wavelength though other lengths are possible beginning generally with a minimum length of ¼ wavelength. For a straight wave guide of ¾ wavelength the serpentine waveguide  38  is generally constructed to have a wavelength of 1 and ¼ wavelengths. Serpentine waveguide  30  reverses the direction of propagation of sound introduced at its throat  29 . While the depicted embodiment is intended for use in are it may be modified for underwater use in which case both of waveguides  28  and  30  are flooded. 
     A flattened cylindrical cavity  34  which includes compression chamber ring  46  is provided within shell  12 . The central portion of cylindrical cavity  34  is defined by a gap between inner phase plug main body  40  (See  FIG. 3 ) and inner phase plug base  32 . It extends to include a small notch radially outside of ring compression chamber  46  in a gap between an inner section to outer phase plug  14  and the main body  39  of the outer phase plug. 
     Suspended within cylindrical cavity  34  is a flexural disk transducer  36 . Flexural disk transducer  36  is supported at its center on a central shaft  42  inserted through a hole through inner phase plug base  32  into inner phase plug main body  40 . A screw  56  and ring  58  complete the suspension assembly and a cap  44  closes the hollow central core of the inner phase plug main body  40 . Wiring connections to the flexural disk transducer  36  are not shown but are conventional. Sound is generated from both major faces  50 A and  50 B of the disk transducer  36  but 180 degrees out of phase. 
     Ring compression chamber  46  captures sound from both major surfaces  50 A and  50 B (front and reverse) of flexural disk transducer  36 . The regions of capture correspond to bands on the major surfaces substantially displaced from the center of the flexural disk transducer  36  and substantially adjacent to the outer perimeter of the disk. Sound generated from one face is 180 degrees out of phase with sound produced from the face opposite. 
     Referring to  FIGS. 4-6  the flexural disk transducer  36  comprises several layers. A center carrier  54  is made of either metal alloy or carbon-fiber cross grained resin impregnated laminated composites. Carbon-fiber has stiffer lower mass characteristics for highest efficiency. Materials such as Kevlar or fiberglass could be substituted. Layers  52  are piezoelectric crystals and, if like kind, are electrically polled opposite. It is possible that layers  52  could be different types of crystals exhibiting usable piezoelectric properties in which case they are electrically poled to produce a summed bending function to the carrier  54  at a desired frequency. The outer layers  48  are a micro mesh stainless steel screen with perforations  51  to lower mass in the matrix. The layers are glued to one another with electrically conductive adhesives. A carbon-fiber carrier  54  uses a metalized thin film on both sides to allow it to efficiently pass electricity to the crystal  52  surfaces adhered to it. If an electrically conductive carrier is used the metalized thin film is not used. Stiffer lighter weight composites result in higher frequency of natural resonance. A metal alloy carrier  54  can be aluminum or other material depending on the tuned resonance chosen for the target frequency, heavier softer alloys result in lower frequency of natural resonance. 
     Flexural disk transducer  36  is bolted (See  FIG. 3 ) to the center of the transducer assembly  10  so that the disk is essentially pinned in the middle forcing the bi-morph to move the outer surface area in a toroid fore and aft direction with the application of electrical voltage and current supplied to the core carrier  54  (negative) and the outer mesh stainless screen  48  on both sides of the crystal wafer stack. Like type piezoelectric crystals  52  are polled opposite to allow the electrically opposite condition of one crystal to reinforce the direction of the other crystal. Many variants of piezoelectric ceramics are commonly used depending on the specific application use of the transducer. One such crystal material is lead zirconate titanate. 
     The highly resonant disk transducer with a selected frequency of resonance matched to several other acoustic elements in the topology. The use of a center pinned transducer matrix where the voltage applied to the transducer excites the wafer to move in a toroid bending function with no dampening of the excited ring transducer outside of the center mounting point. Piston like modes are set up in the resonant crystal which tends to remain linear in acoustic phase around its outer circumference. Transducer acoustic load is harvested from the area of largest peak xmax (piston+/−travel) which is also the area of largest surface square area resulting in efficiency increases as compared to a typical piezoelectric element that is pinned around the outer edge and the acoustic energy is harvested from the center of the wafer the point of highest xmax but also the area of smallest square surface area. Front/back harvested area is coupled to a compression chamber  46  of size to provide good acoustic impedance leverage and an increase in velocity. It is possible to harvest sound energy from just the front or reverse face  50 A,  50 B. 
     Compression chamber  46  is coupled to the differential waveguides with the forward waveguide being shorter in length than the waveguide for the reverse major surface by ½ the wave length (or a odd whole number multiple thereof) of the primary resonance of the transducer. This makes the waveguide for the reverse major surface (in the present embodiment the serpentine waveguide  28  serves this function) a time delay or phase adjustment element. The waveguide for the reverse major surface bends its path forward to be summed to the front wave. When the device is operated at its resonant frequency sound passing through the two waveguides arrives in phase for wave summing and coherent in-phase acoustic propagation. There exists an option to mis-tune the rear wave to front wave at a given frequency to narrow or widen the acoustic beam generated by virtue of phasing the concentric acoustic apertures in a mechanical beam forming. Alternately one of waveguides could contain an adjustable length design (such as a valve or slide arrangement) to allow the end user to mechanically change the effective length of the waveguide relationship between the inner and outer waveguides.