Acoustic transducer including piezoelectric driving element

[57] ABSTRACT

A disk shaped piezoelectric element constructed to operate in a planar mode so as to define a first overtone nodal ring on one of the major surfaces, a conically shaped diaphragm having a truncated apex defining a generally circular area affixed to a major surface of the element concentric with the nodal ring and spaced radially therefrom so as to reduce the amplitude of the output of the first overtone to approximately the amplitude of the output of the fundamental frequency, and a rubber disk affixed to the opposite major surface of the piezoelectric element to lower the fundamental resonance frequency and damp the peak output of the fundamental and first overtone resonance frequencies to provide a flat response over a desired bandwidth.

Claims, 5 Drawing Figures

IO

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179/11OA

PATENTED JAN 151974

3,786,202

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3,786,202

ACOUSTIC TRANSDUCER INCLUDING PIEZOELECTRIC DRIVING ELEMENT

BACKGROUND OF THE INVENTION

1. Field of the Invention

Acoustic transducers are utilized to convert electrical energy to sound or acoustic energy to electrical energy. In the present embodiment a piezolectric element is utilized for a driver, which piezoelectric element bends or warps in a particular mode in response to electrical energy being applied thereacross or produces electrical energy thereacross in response to bending or warping thereof. While the present invention might be utilized in acoustic transducers for converting mechanical energy to electrical energy, it is especially useful in acoustic transducers, such as speakers and the like, converting electrical energy to acoustic energy or sound. In the speaker art, it is highly desirable to provide a transducer with a flat response over a desired bandwidth, i.e., all frequencies of sound between two desired frequencies are produced at approximately equal amplitudes. Because piezoelectric elements are mechanical vibrating devices, they have specific resonant frequency points, referred to as the fundamental, first overtone, second overtone, etc., at which points the amplitude of the output is substantially increased.

2. Description of the Prior Art

In prior art acoustic transducers, any flattening of the frequency response is accomplished through design of the electronic circuitry driving the transducer or, in some instances and for certain frequencies, may be accomplished to some extent through design of the housing and size of components. In general, these solutions are unsatisfactory because they are limited in scope and extent. Further, for these solutions to have noticeable results the design becomes extremely complicated and expensive and may apply only to a specific transducer.

SUMMARY OF THE INVENTION

The present invention pertains to apparatus providing conversion between electrical and mechanical stimuli including a piezoelectric element constructed to operate in a planar mode and having first overtone nodal lines present on a major surface, a generally conically shaped diaphragm with truncated apex defining a circular area affixed to the major surface of the element approximately centrally within the first overtone nodal line so as to be spaced from the line sufficiently to reduce the amplitude of the output of the first overtone to approximately the amplitude of the output of the fundamental, and a resilient damping member affixed to an opposed major surface of the piezoelectric element to lower the fundamental frequency of the element and damp the fundamental and first overtone peaks.

It is an object of the present invention to provide an improved piezoelectric driven acoustic transducer.

It is a further object of the present invention to provide a piezoelectric transducer having a substantially flat response over a desired bandwidth.

These and other objects of this invention will become apparent to those skilled in the art upon consideration of the accompanying specification, claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings, wherein like characters indicate like parts throughout the figures:

FIG. 1 is an axial sectional view of an acoustic transducer constructed in accordance with the present invention;

FIG. 2 is a view (a) in top plan of a piezoelectric driver illustrating nodal rings, (b) in side elevation of the driver illustrating the fundamental nodal ring, (c) in side elevation of the driver illustrating first overtone nodal rings, and (d) the truncated apex of a diaphragm;

FIG. 3 is a graph illustrating generally the frequency response of a transducer similar to that illustrated in FIG. 1 constructed in accordance with prior art techniques and, in dotted lines, the frequency response of a transducer similar to that illustrated in FIG. 1 constructed in accordance with the present invention;

FIG. 4 is an enlarged sectional view of a piezoelectric driving element and a resilient damping member affixed thereto, portions thereof removed; and

FIG. 5 is a greatly enlarged fragmentary view of the resilient damping member.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the figures the numeral 10 generally designates a piezoelectric transducer utilized for providing conversions between electrical and mechanical stimuli. Applications for transducers of this type are speakers, sound sensors, etc. The transducer 10 includes a housing 11 defining a generally cup shaped cavity 12, a generally conically shaped diaphragm 13 affixed within the cavity 12 by its outermost edges and having a truncated apex to which is attached a piezoelectric driver 15 with a piezoelectric element 16 and damping member 17. The operation of piezoelectric transducers wherein a piezoelectric driver is connected directly to a diaphragm and solely supported thereby is described in detail in my U. S. Pat. No. 3,548,116, entitled “Acoustic Transducer Including Piezoelectric Wafer Solely Supported By A Diaphragm,” and issued Dec. 15, 1970.

In the prior art structure of the above described patent the apex of the conically shaped diaphragm is fixedly attached to the center of a piezoelectric element and the frequency response of the transducer is approximated by the solid line graph in FIG. 3. The frequencies specified in FIG. 3 are exemplary and may vary somewhat with variations in transducers. A first output peak in the response at approximately 1 KC, designated 20, is produced primarily by a center supported resonance of the driver at 1 KC (a resonance caused by mounting the driver at the center with a relatively stiff diaphragm) which is coupled to the diaphragm by a slight axial movement of the diaphragm and driver, at the point of connection therebetween, which axial movement is present because the diaphragm is not rigid. A second output peak at approximately 5 KC, designated 21, is produced primarily by the fundamental resonance of the piezoelectric element 16. A third output peak at approximately 19 KC, designated 22, is produced primarily by the first overtone resonance of the piezoelectric element 16.

Referring to FIG. 2, the piezoelectric element 16 is illustrated in top plan in (a) and in side elevation in (b) and (c). The piezoelectric element 16, as illustrated in

3,786,202

FIG. 2, is disk shaped, but it should be understood that substantially any flat shape might be utilized wherein the element operates in a planar bending mode, i.e., flexing or distorting along more than one axis. For example, the element 16 might be square or even irregularly shaped. However, in the present embodiment and to simplify the explanation, a disk shaped element 16 will be described and the operation thereof explained.

At the fundamental resonance frequency, the element 16 vibrates along each diameter thereof as illustrated in FIG. 2b. During the first half cycle of each vibration the center of the element 16 flexes upwardly while the end portions turn downwardly as indicated by the dotted line 25 and during the second half cycle of each vibration the reverse occurs, as illustrated by the dotted line 26. It will be noted that two nodes are formed along the diameter, at which points no axial movement of the element 16 occurs. Because every diameter of the element 16 reacts, at the fundamental resonance frequency, as illustrated in FIG. 26, the nodes define a nodal ring 27 on the upper and lower major surfaces of the element 16, which ring 27 is a locus of nodes or points having no axial movement at the fundamental resonance frequency.

In a similar fashion FIG. 2c illustrates movement of the element 16 at the first overtone resonance frequency. Because of the first overtone being a higher frequency, two concentric nodal rings 28 and 29 are defined on the major surfaces of the element 16. The nodal ring 29 is concentric with the nodal ring 27 defined by the fundamental frequency and spaced radially inwardly therefrom. It will be noted from a comparison of FIG. 26 and c that the output, or amount of axial movement, of the element 16 at the fundamental resonance frequency is substantially constant throughout the area encircled by the nodal ring 29. Further, since the amount of movement of the element 15 determines the amount of movement of the diaphragm 13, and hence, the output ( or conversion between stimuli), connecting the diaphragm at a point concentric to the nodal rings 27, 28 and 29, as in the prior art, will provide the maximum output of all frequencies and result in the frequency response illustrated in full lines in FIG. 3.

In FIG. 2d a portion of the diaphragm 13 is illustrated with the apex thereof truncated to define a generally circular area having a diameter smaller than the diameter of the nodal ring 29. If the diameter of the truncated apex of diaphragm 13 is equal to the diameter of the nodal ring 29 the first overtone will be substantially eliminated, since the output or axial movement of the element 16 at the nodal ring 29 for the first overtone is zero. Thus, by forming the diaphragm 13 so that the truncated apex has a diameter greater than a point and less than the diameter of the nodal ring 29, the output of the first overtone (peak 22 in FIG. 3) can be diminished to approximately the amplitude of the fundamental (peak 21 in FIG. 3). Since the second overtone is substantially above the first overtone and beyond the desired frequency response in acoustic transducers, it is not necessary to consider overtones beyond the first.

Referring more specifically to FIG. 4, the piezoelectric driver 15 is illustrated in enlarged cross section, with a portion thereof removed. The piezoelectric element 16 includes first and second piezoelectric wafers 40 and 41 having an electrode 42 sandwiched therebetween and electrodes 43 and 44 fixedly engaged in overlying relationship on opposed major surfaces thereof. The operation of the element 16 is well known to those skilled in the art and, as previously mentioned, is described in detail in U. S. Pat. No. 3,548,116. It is, therefore, sufficient to state at this time that the electrodes 42, 43 and 44 drive the element 16 in a planar mode of operation. The element 16 has resilient damping member 17 affixed to the major surface thereof opposite the major surface having the diaphragm 13 affixed thereto. In the present embodiment, since the element 16 is generally disk shaped, the damping member 17 is also disk shaped, and as illustrated, has a slightly larger diameter than the element 16. The damping member 17 damps or loads the movement of the element 16 to reduce the fundamental peak 21 and to further reduce the first overtone peak 22 so that the frequency response of the driver 15 approaches the dotted line curve 50 in FIG. 3.

The damping member 17 is formed of a resilient material, such as rubber (the term rubber being understood to include natural and synthetic materials) or other elastomers. Elastomers generally have a frequency dependent shear modulus which varies directly with the frequency of stresses applied thereto, i.e., the shear modulus increases with the frequency of stresses applied thereto. At static or slowly occurring stresses the elastomeric material operates in a rubbery region in which it appears elastic to the forces operating upon it. However, as higher frequency dynamic stresses are applied the shear modulus is increased and the elastomer goes through a glassy transition region into a glassy region where it appears metallic or hard. At the lower frequencies, around the peaks 20 and 21 of FIG. 3, the damping member 45 is preferably operating in the glassy transition region and introduces hysteresis losses which substantially remove the peaks 20 and 21. However, at the upper frequencies, around the peak 22, the material of the damping member 17 may begin to approach the glassy region and the hysteresis losses are substantially reduced. Thus, the damping effect of the member 17 at the peak 22 is greatly reduced.

To compensate for the reduction in hysteresis losses small particles 46 of a relatively heavy material, such as iron or lead, are intermixed with the resilient material of the damping member 17 during the formation of the disk. These particles 46 add additional weight to the member 17 and introduce a coulomb type damping which is caused by internal friction between the metal particles and their enclosing rubber walls. This internal friction is caused by a relative movement due to a difference in inertia of the heavy particles and the surrounding resilient material of the damping member 17. This frictional or coulomb type damping increases with the number of particles and the size of the particles. It has been found, for example, that lead particles having approximately a 100 mesh size mixed with rubber in a 3 to 1 ratio, by weight, provide a desired amount of damping for the frequency response illustrated in FIG. 3. Small amounts of a lubricant, such as graphite, may also be added to the damping member 17, as illustrated in FIG. 5 by the numeral 47, to increase the relative movement between the heavy particles 46 and the elastomeric or rubber material and, therefore, increase the damping action.

The effect of the reduction in hysteresis losses at the higher frequencies can also be reduced or eliminated by selecting an elastomeric material with a glass transi

3,786,202 tion region above the highest frequency desired for the frequency response of the transducer 10. It has been found for example that neoprene has a relatively high glass transition region and may, in many instances, provide sufficient damping at high frequencies so that the addition of particles 46 is not required. It should be understood that the type of materials utilized and the frequency response desired dictate the ultimate construction of the transducer 10.

In addition to providing the damping function described above, the damping member 17 increases the mass of the driver 15 and, therefore, improves the weight ratio of the driver 15 over the diaphragm 13. This improved weight ratio results in a tighter coupling between the driver 15 and the diaphragm 13 at the lower frequencies. Thus, in some instances, although the material selected for the damping member 17 provides sufficient damping at the higher frequencies, it may be desirable to add relatively heavy particles 46 to the member 17 to increase the mass of the driver 15.

The addition of the damping member 17 to the driver 15 (and the diaphragm 13 to a much smaller extent) lower the fundamental resonance frequency of the driver 15. Referring to FIG. 3, it can be seen that the knee 51 of the flattened response curve 50 (illustrated in dotted lines) occurs slightly below the peak 21 for the fundamental resonance frequency. The diameter and thickness of the damping member 17 should be adjusted to lower the resonance frequency of the combined piezoelectric element 16 and damping member .17 to a point below the fundamental resonance frequency of the piezoelectric element 16 (peak 21) such that the curve 50 falls away sharply at the lower frequencies (as illustrated in FIG. 3). If the resonance frequency of the driver 15 is too high it will add to the peak 21 and produce too high an output at the lower frequencies while not extending the frequency response sufficiently into the lower frequency range. If the resonance frequency of the driver 15 is lowered too much the curve 50 will not fall away sharply at the lower frequencies but will rise at a much lower rate. Thus, through careful selection of the diameter thickness and mass of the damping member 17 the desired flat response of the transducer 10 can be extended somewhat into the lower frequency range. Further, through careful selection of the type and thickness of material and the amount and particle size of particles 46 the degree of damping can be adjusted to provide a substantially flat response over a desired band of frequencies. It should be noted that the damping member 17 might be constructed with an annular configuration, in which case high frequency damping is controlled by adjusting the size of the inside diameter, since high frequency damping is most effective in the center of the driver 15.

The cavity 12 in the housing 11 has a resonant frequency which, in many instances appears in the desired frequency response of the transducer 10. At the cavity resonance there is a tendency to absorb output power from the transducer 10 and, thus, a notch (not shown) will appear in the output curve 50 of FIG. 3. To prevent the power output loss and the resulting distortions, acoustic absorbing material, in the present embodiment an annularly shaped member 55 of foam rubber or the like, is placed in the cavity 12 between the housing 11 and the diaphragm 13. The member 55 alters the cavity resonance, or lowers the Q of the cavity 12, to substantially eliminate the absorbing of power and consequent notch in the output curve 50. It should be understood that the acoustic absorbing material is only utilized when the cavity resonance falls within the desired frequency response and, in some instances, it may be possible to eliminate the material through design of the transducer components.

Thus, an improved piezoelectric transducer is disclosed having an output which is substantially flat throughout a desired band of frequencies. The transducer output has been referred to throughout the specification and it should be understood that this refers to either mechanical or electrical output in response to either electrical or mechanical input, respectively. Further, in addition to a flat response and improved coupling at the lower frequencies, the critically damped element 16 has an improved transient response over known electrodynamic drive systems for transducers. While I have shown and described a specific embodiment of this invention, further modifications and improvements will occur to those skilled in the art. I desire it to be understood, therefore, that this invention is not limited to the particular form shown and I intend in the appended claims to cover all modifications which do not depart from the spirit and scope of this invention.