PIEZO SPEAKER FOR IMPROVED PASSENGER CABIN AUDIO SYSTEMS

Abstract: This invention outlines several applications of piezoelectric vibrators (4) to produce quality flat panel speakers in passenger cabin applications. A system consisting of an audio amplifier (2) and transformer (3) is used to drive the piezo speaker (4). The electronics are packaged (8) so that they fit in small modules that can be attached to a cabin structure to produce a speaker. The invention includes a variety of flat panel speaker designs, including one in which the existing structure is converted into a speaker, and thin membrane and/or panels that are fitted with piezoelectric elements (Figures 5 and 6). A system consisting of cabin quieting and flat panel speakers is also discussed where the mid and high frequency audio is produced by panel speakers (74-79) and the low frequency audio is produced from dynamic loudspeakers (80-84). The cabin systems discussed in this patent are applicable to automobiles, aircraft, trucks and buses.

Description: PBEZO SPEAKER FOR IMPROVED PASSENGER CABIN AUDIO SYSTEMS BACKGROUND ART Conventional loudspeakers while able to reproduce sound well, require a large amount of space and are an inefficient way to convert electrical power into acoustical power Space requirements are not easily reduced because ofthe need for a moving coil to drive the diaphragm Piezoelectric loudspeakers have been proposed as a diaphragm as an alternative to moving coil loudspeakers Such a device was described by Martin in U.S Patent No 4,368,401 and later Takaya in U S Patent No 4,439,640 Both inventions dealt with attaching a disc shaped piezo to a diaphragm Martin's device used a thick glue layer (10 to 50% ofthe carrier plate thickness) between a carrier plate and the piezo ceramic The adhesive layer served to attenuate resonance Takaya accomplishes the same through use of a film with a smaller Q factor than the diaphragm Both inventors specify disc shaped diaphragms and piezoceramic plates Kompanek in U S Patent No. 3,423,543 uses a plurality of ceramic wafers made of piezoelectric materials such as lead zirconate-lead titanate mixtures of various shapes Conductive layers are affixed to both sides ofthe wafer and then glued to a flat plate Kompanek states that the plate is preferably made of a conductive metal such as steel but may be of plastic or paper with a conductive layer thereon forming the surface Another such device discussed by Kumada in U S Patent No 4,352,961 attempts to improve the frequency response further by using various shapes for the diaphragm, such as an ellipse He also claims the ability to form the speaker from transparent piezoceramic materials such as lanthanum doped zirconium titanate so that the speaker can be used in applications such as watch covers and radio dials He also uses a bimorph to drive the diaphragm rather than a single layer of ceramic All ofthe above methods use a flat panel dπven by a piezo ceramic device and make no attempt to use a three dimensional structure to improve the sound quality. The diaphragm must be attached to some type of frame and clamped to the frame. Bage, Takaya and Dietzsch in U.S. Patent No. 4,779,246 all discuss methods of attaching the diaphragm to a support frame. Early efforts used piezo ceramics to drive conical shapes reminiscent of those found in loudspeakers. Such devices can be found in Kompanek, U.S. Patent No. 3,423,543 and Schaffi, U.S. Patent No. 3,548,116 and 3,786,202. Schaffi discusses building a device suitable for use in loudspeakers. This device is of much greater complexity than flat panel speakers and is not suitable for applications where a low profile speaker is needed. In order to constrain the center ofthe diaphragm from moving, Bage, U.S. Patent No. 4,079,213, uses an enclosure with a center post. He claims that this reduces the locus of nodal points to the location ofthe centerpost and therefore improves the frequency response ofthe device. The enclosure is used to support the center post and has openings to provide for pressure relief, and does not improve the acoustic performance. Piezoelectric speakers were discussed by Nakamura in U.S. Patent No. 4,593,160, where a piezoelectric vibrator is connected to a diaphragm by coupling members formed by wires. More pertinent work in thin speakers using piezoelectrics was discussed by Takaya in U.S. Patent No. 4,969, 197. Takaya used two opposed plane foam diaphragms with a pair of recesses that minimize the restriction of motion ofthe piezoelectric driver. Thin speakers were discussed in U.S. Patent No. 5,073,946 by Satoh et al, which included the use of voice coils. Volume noise cancellation techniques have been discussed by Warnaka in U.S. Patent No 4,562,589 for aircraft cabins. Shakers attached to structures for aircraft quieting have been discussed by Fuller in U.S. Patent No 4,7155,559. This invention differs from Warnaka and Fuller in that the intent is to integrate improved audio by the use of flat panel speakers for the mid and high frequency, while relying on the dynamic loudspeakers ofthe noise cancellation system for low frequency audio. Brief Description of the Invention 5 The present invention in one embodiment involves a module that can be placed on the door or ceiling panels of an automobile, truck, aircraft, or other passenger cabin to produce good mid and high (tweeter) range sound quality. Dynamic equalization using additional piezoelectric elements or the electric potential generated by the flexing ofthe piezoelectric element is also included as an additional feature ofthe present invention. i o One advantage ofthe present invention is that the production of sound is close to the passengers ears. Since mid range and high frequency sound are the most readily attenuated by the materials in the automobile (seat cushions, door panels etc.), placing these sound sources close to the listener improved the perceived sound quality. A single low frequency (woofer) dynamic loudspeaker provides all the bass required for high 15 quality audio, since the low frequencies are not readily attenuated by the materials in the automobile (seat cushions, door panels etc.). This type of audio system can also be adapted to a noise reduction system, where the dynamic loudspeakers ofthe noise reduction system are used to provide the low frequency audio. Although the application discussed here is for an automobile, the same approach can be used in aircraft, trucks, 0 recreational vehicles and buses. Description of the Drawings Figure 1 is a block diagram ofthe audio circuit. Figure 2 is a drawing ofthe module that can be applied to a surface to create a piezoelectric speaker system. 5 Figure 3 illustrates one possible flat panel speaker design for the passenger cabin. Figure 4 illustrates another possible flat panel speaker design for the passenger cabin. Figure 5 illustrates a closed volume flat panel speaker which uses the panel designs illustrated in figures 3 and 4. l o Figure 6 illustrates a closed volume flat panel speaker which uses a thin panel fitted with two piezoelectric elements. Figure 7 is a flat panel speaker that utilizes piezoelectric patches bonded to two stretched plastic diaphragms, that are supported by a rigid frame and held in tension by a rigid post. 15 Figure 8 illustrates an approach to equalization. Figure 9 illustrates the audio driver and a possible form of equalization that utilizes the signal generated by displacements in the piezo as a measure of panel resonance. Figure 10 illustrates the locations ofthe flat panel speakers in a passenger cabin, 20 in this case, an automobile. Figure 11 illustrates the integration of flat panel speaker with an active noise reduction system. Figure 12 illustrates the installation of piezoelectric loud speakers in aircraft cabin trim. Detailed Description of the Invention: All speaker systems require some form of amplifier. The present state ofthe invention utilizes a system illustrated in the block diagram of figure 1. The audio signal 1 is fed into a linear amplifier 2 that provides the signal "boost" or amplification. The output ofthe amplifier 2 is fed into a 17-to-l transformer 3 to increase the voltage swing at the piezoelectric element 4. This is necessary since the displacement in the piezoelectric is directly related to the applied electrical potential. Figure 2 illustrates the assembly ofthe piezoelectric speaker module with built in damping material. The piezoelectric element 5 is applied directly to the surface to be excited 6. Damping material 7 is then placed in proximity to the piezoelectric element, in this case a panel diaphragm. Preferably, the piezoelectric element is surrounded by damping material 7. Placing the damping material in proximity to the piezoelectric element has two benefits. It provides a reduction in the structural resonances in the surface the piezoelectric is applied to, and it insulates the high voltage used to drive the piezoelectric from the outside world. This is important to avoid electrical shock due to the high voltages applied to the piezoelectric. The audio amplifier is potted in a box 8 with thermally conductive epoxy. This not only protects the electronics from the environment, but it also provides good distribution ofthe heat load from the audio amplifier, and prevents possible electrical shock. A cover 9 for substantially covering the electronics is placed over the electronics box providing a final seal ofthe unit from the outside world. The positive and negative power terminal 10,11 and the positive and negative audio signal terminals 12,13 are shown extending outside the box. The mass of the lid and the electronics box, mounted to the damping material is basically a load on a spring, which can be tuned to add damping at the fundamental resonance ofthe structure. Figure 3 illustrates one possible flat panel speaker design for the passenger cabin. A piezoelectric patch 14 is bonded to the center of coupling layer in the form of a small, thin plastic elliptical disc 15 that provides a transition to a larger elliptical disc 16 that is bonded to panel 17. This may be a light weight foam plastic panel or a trim or lining panel ofthe cabin. The elliptical shaped discs help reduce the severity of structural resonances in the thin panel speaker and also provide a coupling transition to the panel. The panel should be made from anisotropic materials to further mitigate the effects of structural resonances. An electrical terminal 18 is used to provide the audio signal. Figure 4 illustrates another possible flat panel speaker design for the passenger cabin. A piezoelectric patch 19 is bonded off center to a small, thin plastic elliptical disc 20 that provides a transition to a larger elliptical disc 21 that is bonded to panel 22. This may be a light weight foam plastic panel or a trim or lining panel ofthe cabin. The elliptical shaped discs help reduce severity of structural resonances in the thin panel speaker and also provides a coupling transition to the panel. The placement ofthe piezoelectric patch off center provides additional reduction in strucrure resonances. The panel should be made from anisotropic materials to further mitigate the effects of structural resonances. An electrical terminal 23 is used to provide the audio signal. Figure 5 illustrates a closed volume flat panel speaker which uses the panel designs illustrated in figure 3 and 4. The panel 24 is fitted with the combination of piezoelectric element and transition layers 25 as discussed above. The volume is closed from the back with a box frame means comprising a thin plate 26 that is held together with four screws to a frame. A front view ofthe flat speaker 30 shows the location of the four screws 31, 32, 33, 34 and the combination (in relief) 35 ofthe piezoelectric element and the elliptical transition layers. The panel is only fixed at the corners to provide a high degree of compliance. The four sides ofthe panel are sealed with a flexible cover, (thin plastic sheet or tape). This seal prevents self canceling ofthe pressure waves that wrap around the edges ofthe panel. The cavity is filled with a fiber glass insulation to dampen any cavity resonance. The panel 24 may be part ofthe roof liner or trim ofthe cabin, in which case plate 26 will be the structure (such as the roof). In this case the screw and frame are not needed, but the trim must be acoustically sealed to the structure at the edges so as to form an enclosure or cavity between the panel 24 and the plate 26. Figure 6 illustrates a closed volume flat panel speaker which uses a thin panel 36 fitted with two piezoelectric elements 37, 38. The volume is closed from the back with a thin plate 39 and held together with four screws to a frame 40. A front view ofthe flat speaker 43 shows the location ofthe four screws 46, 47, 48, 49 and the location ofthe piezoelectric elements 44, 45. The element 44 placed near the center excite predominately odd modes of vibration which produce the lower frequency pressures waves. The piezoelectric element 45 placed near the fixed corner will excite both even and odd modes and the combined effect ofthe two elements will result in a flatter frequency response. The panel is only fixed at the corners to provide a high degree of compliance. The four sides ofthe panel are sealed with a flexible cover, (thin plastic sheet or tape). This seal prevents self canceling ofthe pressure waves that wrap around the edges ofthe panel. The cavity is filled with a fiber glass insulation to dampen any cavity resonance. Figure 7 is a flat panel speaker that utilizes piezoelectric patches 50, 51 bonded to two stretched plastic diaphragms 52, 53 that are supported by a rigid frame 54 and held in tension by a rigid post 55. The tension in the diaphragm provides additional acoustic energy when the piezoelectric is excited and also increases the modal density, which helps to flatten the frequency response. The diaphragms are of slightly different size to generate more frequency components and thus a flatter frequency response. A rubber stand off 56 is used to isolate the direct panel vibrations from the ceiling 57 ofthe passenger cabin. Figure 8 illustrates one approach to equalization. A piezoelectric patch 58 is mounted to a structure to be vibrated 59. The piezoelectric element is driven by a transformer 60 and a pair of linear power amplifiers 61, 62 in a "push-pull" mode. A smaller piezoelectric patch 63 is placed on the panel to sense the strong resonant vibrations in the panel. This signal is amplified to an appropriate level by an operational amplifier 64, which is then subtracted from the input audio signal 65 in the input ofthe amplifier. Figure 9 illustrates the audio driver with another possible form of equalization that utilizes the signal generated by displacements in the piezo as a measure ofthe panel resonance. A piezoelectric patch 66 is mounted on the structure 67 to be vibrated. The piezoelectric element is driven by a transformer 68 and a pair of linear power amplifiers 69, 70 in a "push-pull" mode. A differential operation amplifier 71 is used to pick up the signal on the secondary side ofthe transformer (both the driving audio signals and the signals generated by the piezoelectric driven panel resonance). The gain ofthe amplifier 71 is set to a value to scale this combined signal back to the input levels ofthe audio signal. An additional differential operational amplifier 72 is used to subtract the input audio signal 73 so that the remaining signal is composed ofthe electrical signal generated by the piezoelectric element. Any significant signal created by the piezoelectric element are the result of strong panel resonances. This signal is subtracted from the audio drive to reduce the peaks in the frequency response ofthe panel. Figure 10 illustrates the locations ofthe flat panel speakers in a passenger cabin, in this case an automobile. Four mid range panels 74, 75, 76, 77 are placed within, or form part of, the roof liner ofthe automobile, and one possibly in each door 78, 79. Pairs of tweeters 80, 81, 82, 83 are also placed in, or form part of, the roof liner. Tweeters 84 can also be placed on the sides ofthe passenger cabin frame as shown. The advantage of this configuration is that the sound is generated close to the passengers' ears. Since mid range and high frequency sound are the most readily attenuated by the materials in the automobile (seat cushions, door panels etc.), placing these sound sources close to the listener improved the perceived sound quality. A single low frequency (woofer) dynamic loudspeaker provides all the bass required for high quality audio since the low frequencies are not readily attenuated by the materials in the automobile (seat cushions, door panels etc.). In another embodiment, the piezoelectric driven flat speakers are comprised of piezoelectric elements that drive selected areas ofthe trim or liner ofthe passenger cabin Figure 11 illustrates a system for a passenger cabin that would include an active noise reduction (ANR) system. The ANR system 86 would consist of at least one of each, but preferably numerous microphones 87, 88, 89 and low frequency dynamic loudspeakers 90, 91, 92. The audio system 93 would utilize the speaker in the ANR system for low frequency audio and flat panel mid range 94, 95, 96, 97 and flat panel tweeters 98, 99, 100, 101. This system would provide the added benefit of a noise reduαion system with the improved audio performance resulting from better placement ofthe mid range and high frequency sound sources. Figure 12 illustrates the installation of piezoelectric loud speakers in aircraft cabin trim. In this particular application the speakers are used as part ofthe PA system. Piezoelectric elements 102, 103 are placed on the stiff part ofthe trim to produce the high frequency audio. Piezoelectric elements 104, 105 are placed on the thinner more flexible part ofthe trim to produce the low and mid range frequencies so that collectively lower, mid and upper range frequency sounds can be produced during vibration ofthe trim, i.e., when electric potential is applied to the piezoelectric elements. When coupled with a public address system, a crossover network 106 is used to slit the audio into its high and lower frequency components as it is transmitted from the PA System 107. Piezoelectric materials exist in a variety of forms as naturally occurring crystalline minerals, such as quartz, manufactured crystalline and other materials, plastic materials, including films and foams. .All these materials are considered as part of this invention. Furthermore, piezoelectric materials are merely used as illustrative of thin sheet-like or plate-like materials that may appropriately be used to form transducers. Such other transducers may include magneto-strictive transducers, electro-magnetic transducers, electro-static transducers, micro-motors, etc. The forgoing is considered as illustrative only ofthe principles ofthe invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and, accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope ofthe invention.

Claims: WHAT IS CLAIMED IS: 1. A loudspeaker system module comprising: a piezoelectric element subject to displacement by applied electric potential and having a top side and an under side; a panel diaphragm that is driven by the piezoelectric element and to which the under side ofthe piezoelectric element is joined; damping means for reducing the structural resonances in the panel diaphragm and located in proximity to the piezoelectric element; electronic means for receiving an input audio signal and amplifying said signal, said means being electrically connected to said piezoelectric element to apply electric potential thereto, said means being positioned above the top side of said piezoelectric element; and means for substantially covering the electronic means and the top side ofthe piezoelectric element. 2. The module of claim 1 further comprising at least one coupling layer attached to and positioned intermediate the piezoelectric element and the panel diaphragm. 3. The module of claim 2 wherein the at least one coupling layer is in the form of an elliptically shaped disc. 4. The module of claim 1 further comprising two coupling layers, being a first layer positioned on top of a second layer, with both layers positioned intermediate the piezoelectric element and the panel diaphragm. 5. A closed flat panel loudspeaker system module comprising: a piezoelectric element subject to displacement by applied electric potential and having a top side and an under side; a panel diaphragm that is driven by the piezoelectric element, said panel diaphragm having an under side and a top side, with the under side ofthe piezoelectric element being attached to said top side; and box frame means for substantially covering and enclosing the top side ofthe panel diaphragm, the at least one coupling layer, and the piezoelectric element. 6. The module of claim 5 further comprising a second piezoelectric element attached to the top side ofthe panel diaphragm. 7. The module of claim 5 further comprising at least one coupling layer positioned intermediate the piezoelectric element and the panel diaphragm. 8. The module of claim 7 wherein the coupling layers are in the form of elliptically shaped discs. 9. The module of claim 7 wherein the at least one coupling layer are two coupling layers, being a first layer positioned on top of a second layer, with both layers positioned intermediate the piezoelectric element and the panel diaphragm. 10. A loudspeaker system comprising a first piezoelectric element subject to displacement by applied electric potential; a panel diaphragm that is driven by the first piezoelectric element; 5 electronic means for receiving an input audio signal, amplifying said signal, and increasing the voltage swing into the piezoelectric element, said means being electrically connected to said piezoelectric element to apply electric potential thereto; and dynamic equalization means for sensing resonant vibrations in the o panel diaphragm, converting said vibrations to a signal to be amplified and subtracting said amplified signal from said input audio signal. 11. The system of claim 10 wherein the dynamic equalization means includes a second piezoelectric element that is attached to the panel diaphragm and which 5 senses resonant vibrations in the panel diaphragm and converts said vibrations to a signal. 12. The system of claim 10 wherein the dynamic equalization means includes means to detect any electrical signal created by the first piezoelectric element which is o the result of resonant vibrations in the panel diaphragm. 13. The system of claim 10 wherein the electronic means includes a pair of linear power amplifiers operating in a push-pull mode for amplifying the input audio signal. 14. A loudspeaker system for a passenger cabin comprising at least one mid range frequency flat speaker comprising a panel diaphragm driven by a piezoelectric element; at least one high frequency flat speaker comprising a panel diaphragm driven by a piezoelectric element; and means for reducing unwanted background noise in said passenger cabin, said means including a plurality of microphones and low frequency dynamic speakers, wherein said dynamic speakers are also utilized for desired low frequency audio reproduction. 15. A flat panel loudspeaker comprising: a support frame having a top side and an underside; two differently sized diaphragms supported by the frame and capable of producing sound when vibrated, with one diaphragm being attached to the top side ofthe support frame and the other diaphragm being attached to the underside ofthe support frame; two piezoelectric elements for driving the diaphragms, with a separate piezoelectric element being attached to each diaphragm. 16. A method of reproducing sound within a passenger cabin from an audio signal having lower, mid and upper frequency range components, said method comprising (a) covering portions ofthe passenger cabin with trim capable of producing a sound when vibrated, said trim having distinct areas in which the trim is 15 constructed to produce lower, mid or upper range frequency sounds when vibrated; (b) attaching piezoelectric elements for vibrating the trim to selected distinct areas ofthe trim, which distinct areas collectively are capable of producing lower, mid and upper range frequency sounds when vibrated; and (c) applying electric potential to the piezoelectric elements to vibrate the trim attached thereto to produce sounds in accordance with the audio signal. 17. The method of claim 16 further comprising originating the audio signal from a public address signal and utilizing a crossover network located intermediate the public address system and the piezoelectric elements to split the audio signal of the public address system into lower, mid and upper frequency range components. 18. A method of reproducing sound within a passenger cabin from an audio signal having lower, mid and upper frequency range components, said method comprising (a) placing piezoelectric driven flat speakers in areas ofthe passenger cabin close to the ears of seated passengers, said speakers capable of reproducing mid and upper range frequency sounds, and (b) placing at least one low frequency dynamic loudspeaker within the passenger cabin 19. The method of claim 18 wherein only one low frequency dynamic loudspeaker is placed within the passenger cabin, with said one low frequency dynamic loudspeaker being placed away from the ears of seated passengers. 20. The method of claim 18 wherein the piezoelectric driven flat speakers are comprised of piezoelectric elements that drive selected areas ofthe trim or liner ofthe passenger cabin.

Inventors: EATWELL GRAHAM P, MACHACEK STEVEN L, PARRELLA MICHAEL J, EATWELL, GRAHAM, P., MACHACEK, STEVEN, L., PARRELLA, MICHAEL, J.
Applicants: NOISE CANCELLATION TECH, NOISE CANCELLATION TECHNOLOGIES, INC.
Application year: 1996
Main classification: H04R25
IPC codes: H04R7, H04R17