AU1102183A

AU1102183A - Piezoelectric transducer apparatus

Info

Publication number: AU1102183A
Application number: AU11021/83A
Authority: AU
Inventors: Jonathan Robert Bost
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1981-12-30
Filing date: 1982-12-03
Publication date: 1983-07-15
Anticipated expiration: 2002-12-03

Description

PIEZOELECTRICLOUDSPEAKERCOUPLEDWITHRESONANTSTRUCTURES

Background of the Invention

This invention relates to piezoelectric electro- acoustic transducers, and more particularly, to an improved piezoelectric acoustic transducers apparatus which exhibits an enhanced or broadened frequency response.

Description of the Prior Art

Recently, piezoelectric transducers such as ono- orphs have been increasingly used in signalling devices such as pagers and other alerting apparatus which employ an essentially single tone alert signal. A monomorph includes a ceramic disk bonded to a metallic backplate thus forming a bender. The monomorph resonates at a predetermined frequency when excited with electrical energy and exhibits a frequency response similar to the classical L-C tuned circuit about a predetermined center resonate frequency. An essentially single tone acoustic signal is generated by such a monomorph with a frequency response dropping off rapidly on either side of the resonate frequency of the monomorph.

In one prior art approach to altering the frequency response of a piezoelectric transducer, such transducer _ transducer was mounted in an enclosure which formed a resonant chamber including an aperture (port) . The dimensions of the enclosure and the port were selected such that the enclosure resonated at the resonant frequency of the piezoelectric transducer and thus the acoustic signal generated at the resonant frequency of the piezoelectric transducer was reinforced or boosted.

Although the amplitude of the signal generated at the resonant frequency of the transducer is increased by this approach, unfortunately, the frequency response remains a single tone or peak.

In some applications, it is desirable to have a piezoelectric electroacoustic transducer apparatus which exhibits a broader frequency response than the substantially single tone frequency response discussed above.

One object of the present invention is to provide a piezoelectric transducer apparatus exhibiting an enhanced or broadened frequency response. Another object of the present invention is to provide a piezoelectric transducer apparatus which exhibits water resistant properties and is substantially unaffected by humidity.

These and other objects of the invention will become apparent to those skilled in the art upon consideration of the following description of the invention.

Brief Summary of the Invention

The present invention is directed to providing an electroacoustic device which exhibits an enhance or broadened frequency response.

In accordance with one embodiment of the invention, an electroacoustic device includes a piezoelectric driver for converting electrical energy into acoustic energy. The driver exhibits a predetermined resonant frequency and includes to opposed major surfaces. A first resonant structure is acoustically coupled to one of the major surfaces and includes at least one aperture. The first resonant structure is dimensioned to resonate at a frequency less than the resonant frequency of the driver. A second resonant structure is acoustically coupled to the remaining major surface of the driver and includes at least one aperature. The second resonant structure is dimensioned to resonate at a frequency greater than the resonant frequency of the driver.

The features of the present invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings.

Description of the Drawings

Pig.- 1 is a cross-section of one embodiment of the electroacoustic device of the present invention. Fig. 2 is a frequenc response graph of the electroacoustic device of Fig. 1.

Detailed Description of the Preferred Embodiment

Fig. 1 illustrates one embodiment of the electro¬ acoustic device of the present invention as loudspeaker 10. Loudspeaker 10 includes an enclosure 20 exhibiting a rectangular geometry in this embodiment although it is understood that other geometries may be employed consistently with the subsequent description of the invention. Rigid materials such as plastic, polyvinyl- chloride, metals, nonmetals and the like may be employed to fabricate enclosure 20. As seen in Fig. 1, enclosure 20 is an essentially hollow structure.

As shown in Fig. 1, enclosure 20 includes protru¬ sions 20 and 24 extending toward each other from opposite sides of enclosure 20. A piezoelectric driver 30, for example a monomorph including a ceramic disc 31 bonded to a metallic backplate 32, is appropriately mounted between protrusions 22 and 24 which form the support for driver 30. Driver 30 includes two major opposed surfaces 30A and 30B. It is understood that electrically conductive leads (not shown) are attached to driver 30 to provide electrical energy thereto so as" to excite driver 30 into mechanical vibration. Thus mounted, driver 30 divides enclosure 20 into two cavities (chambers) 40 and 50, respectively. When electrically excited, driver 30 is induced into, mechanical vibration and generates acoustic signals having the majority of their frequency components at the resonant frequency F_j of driver 30. In one embodiment of the invention discussed in more detail subsequently, the resonant frequency F-_j of driver 30

(here a monomorph) is equal to approximately 940 Hz, for example. By examining Fig. 1, it is seen that the acoustic signals generated at major surface 30A of driver 30 are acoustically coupled into cavity 40 and the acoustic signal generated at driver surface 30B are acoustically coupled into cavity 50.

The portion of enclosure 20 adjacent chamber 40 includes a port (or aperture) 42. The dimensions of cavity 40 and port 42 are selected such that cavity 40 exhibits resonant frequency F2 less than the resonant frequency of F-_j of driver 30. More specifically, it has been found that providing cavity 40 with a volume of 27,661 π_rι3, a port length -_| (see Fig. 1) of 1.5 mm and a port area of 42.3 mm² for port 42 results in cavity 40 exhibiting a resonant frequency F2 approxi¬ mately equal to 728 Hz. Cavity 40 and port 42 cooperate to form a resonant structure or Helmholtz resonator which radiates acoustic energy out port 42 with substantial frequency components at frequency F2_(It is noted that the drawings are not to scale) . The portion of enclosure 20 adjacent to cavity 50 includes a port (or aperture) 52. The dimensions of cavity 50 and port 52 are selected such that cavity 50 exhibits a resonant frequency F3 greater than the resonant frequency F-j of driver 30. More specifically, it has been found that providing cavity 50 with a volume of 5,032 mm³, a port length 2 (see Fig. 1) of 1.5 mm and a port area of 31.1 m ² for port 52 results in cavity 50 exhibiting a resonant frequency F3 approximately equal to 1,560 Hz. Cavity 50 and port 52 cooperate to form a resonant structure or Helmholtz resonator which radiates acoustic energy out port 52 with substantial frequency components at frequency F3.

As seen in Figure 2, which is a graph of frequency versus sound pressure level (dB) of apparatus 10, a device exhibiting a broadened frequency response com¬ pared to the resonant frequency of driver 30 alone (F-_|) is achieved. More specifically, acoustic signals exhibiting a frequency of approximately F-- are generated by driver 30 and travel through cavities 40 and 50 and out of enclosure 20 via ports 42 and 52, respectively. These acoustic signals result in the peak in the frequency response curve of Fig. 2 seen at frequency F-j . The acoustic signals generated at driver surface 30A excite cavity 40 into resonance at a frequency of approximately F2 and such acoustic signals exit enclosure 20 at port 42 resulting in a peak in the frequency response curve of Fig. 2 at F2« The acoustic signals generated at driver surface 30B excite cavity 50 into resonance at a frequency of approximately F3 and such signals exit enclosure 20 via port 52 resulting in a peak in the frequency response curve of Fig. 2 at F3. Thus, as seen in Fig. 2, the electroacoustic apparatus 10 achieves a three-pole type frequency response.

Those skilled in the art will appreciate that the resonant frequencies F2 and F₃, respectively of cavities 40 and 50, may be made closer to or further from driver resonant frequency F-_j by appropriately selecting the dimensions of cavities 40 and 50, namely, cavity volume, port length and port area. Further, the electroacoustic device of the present invention is not limited to the piezoelectric monomorph employed as driver 30 in the example above. Other drivers such as bimorphs and multimorphs may also be employed as driver 30.

The foregoing describes an electroacoustic apparatus exhibiting an enhanced or broadened frequency response. The electroacoustic apparatus of the present invention is desirably water resistent and operable under conditions of relatively high humidity.

While only certain preferred features of the invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the present claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

OMPI

Claims

1. An electroacoustic device comprising: piezoelectric driver means for converting electrical energy into acoustic energy, said driver means exhibiting a predetermined resonant frequency and includ- ing two opposed major surfaces; first resonant structure means, acoustically coupled to one of said major surfaces and including at least one aperture, said first resonant structure means being dimensioned for resonating at a frequency less than the resonant frequency of said driver means; and second resonant structure means acoustically coupled to the remaining major surface of said driver means and including at least one aperture, said second resonant structure being dimensioned for resonating at a frequency greater than the resonant frequency of said driver means.

2. The electroacoustic device of claim 1 wherein said first resonant structure means comprises a Helmholtz resonator.

3. The electroacoustic device of claim 1 wherein said second resonant structure means comprises a

Helmholtz resonator.

4. The electroacoustic device of claim 2 wherein said second resonant structure comprises a Helmholtz resonator.

5. The electroacoustic device of claim 1 wherein said piezoelectric driver means comprises a monomorph.

6. The electroacoustic device of claim 1 wherein said piezoelectric driver means comprises a bimorph.

7. The electroacoustic device of claim 1 wherein said piezoelectric driver means comprises a multimorph.

8. An electroacoustic device comprising: piezoelectric driver means, having opposed major surfaces, for converting electrical signals applied thereto into acoustic energy radiating from each of said major surfaces, said driver means exhibiting a first pre¬ determined resonant frequency; first Helmholtz resonator means, acoustically coupled to one major surface of said driver means, and exhibiting appropriate dimensions for resonating at a said resonant frequency less than said first resonant frequency, and second Helmholtz resonator means, acoustically coupled to the remaining major surface of said driver means, and exhibiting appropriate dimensions for resonating at a third resonant frequency greater than said first resonant frequency.

9. The electroacoustic device of claim 8 wherein said piezoelectric driver means comprises a monomorph.

10. The electroacoustic device of claim 8 wherein said piezoelectric driver means comprises a bimorph.

11. The electroacoustic device of claim 8 wherein said piezoelectric driver means comprises a multimorph.