Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS
  • H04R17/00—Piezoelectric transducers; Electrostrictive transducers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS
  • H04R3/00—Circuits for transducers
  • H04R3/002—Damping circuit arrangements for transducers, e.g. motional feedback circuits

Definitions

• This invention relates to a piezoelectric loud ⁇ speaker in which an integrally formed transducer is excited by the motion of the piezoelectric driver to generate a voltage having an amplitude proportional to the acoustic output of the loudspeaker.
• This invention is also directed to an audio system utilizing the voltage generated by the transducer as a feedback signal to control the amplitude of the applied audio signal and provide an acoustic output having a predetermined frequency response.
• a conventional electromagnetic speaker includes a piezoelectric transducer pivotally mounted along two edges to a mounting board which engages the voice coil form. The output of the transducer is degeneratively fed back to the input of an associated amplifier.
• Another object of this invention is to provide an acoustic system utilizing such a loudspeaker wherein the feedback signal developed by the transducer is used to control the gain of an amplifier which drives the loudspeaker to yield an acoustic output having a predetermined frequency response.
• a piezo- electric driver including at least two opposing surfaces with electrodes is mounted to a diaphragm.
• a transducer is formed on one surface of the driver by isolating a portion of the electrode on the surface to form an island electrically insulated from the remainder of the elec- trode.
• a voltage is developed at the transducer relative to the opposing electrode and provides a feedback signal proportional to the acoustic output of the speaker.
• An acoustic system in accordance with an embodiment of the present invention includes a signal source, a shaping circuit and an amplifier having its gain controlled in response to the shaped feedback signal generated by the transducer.
• Figure 1 illustrates an embodiment of an acoustic system in accordance with the present invention.
• Figure 2 is a cross sectional view taken about line 2-2 of figure 1.
• a piezoelectric loud ⁇ speaker 10 includes a diaphragm 12 and a piezoelectric driver 14.
• the illustrative piezoelectric driver 14 is a bimorph consisting of piezo ⁇ electric discs 16 and 18 sandwiched between electrodes 20, 22 and 24.
• a wire connection 26 connects electrodes 20 and 24 together and wire 28 provides electrical connection with the center common electrode 22.
• the driver 14 may be mounted to the diaphragm 12 such as by means of an adhesive 30 which may consist of an epoxy.
• An electrode 32 formed on piezoelectric disc 16 is substantially surrounded by and electrically insulated with respect to electrode 20.
• Electrodes 20 and 32 may be formed separately on disc 16 or electrode 32 may be formed by etching away or otherwise removing a pattern in electrode 20 to define an island-like electrode 32.
• electrode 32 provides a radial finger-like projection extending from the center of the driver to its edge.
• a wire 34 provides an electrical connection to electrode 32.
• a driving voltage is applied to the driver by means of wires 26 and 28. The piezoelectric driver vibrates in response to the application of an electric field and thereby sets the diaphragm 12 in motion to produce an acoustic wave.
• the driver is piezo ⁇ electric, its movement will induce a voltage across the feedback transducer, i.e., electrode 32 with respect to common electrode 22, that is substantially proportional to the magnitude of the vibratory movement of the driver and hence is proportional to the acoustic wave generated by the speaker 10.
• Figure 1 illustrates an audio system for driving speaker 10 which utilizes the voltage developed by electrode 32 as a feedback signal to control the magnitude of the driving signal (voltage) to obtain a predetermined acoustic response.
• the output signal from signal source 36 is filtered by a high pass filter 38 before being applied as an input to control amplifier 40.
• the output voltage ⁇ eveloped at electrode 34 provides an input to a shaping circuit 42 which provides an input for controlling the gain of control amplifier 40.
• the output of controlled gain amplifier 40 is amplified by voltage amplifier 44 which is utilized to provide the driving signal to speaker 10.
• a high pass filter 38 is used to prevent the passage of signals having a frequency lower than the natural cut-off frequency of speaker 10, If a flat frequency response was the desired acoustic output, the gain of amplifier 40 could be increased excessively in an attempt to produce the same acoustic output for a signal having a frequency below the low frequency cut-off frequency of speaker 10.
• a high pass filter 38 is not an indispensable portion of this system since alternative solutions are available, namely: the gain of amplifier 40 could be limited to a maximum or a signal source 36 could be utilized which does not exceed the frequency limits of speaker 10.
• the shaping circuit 42 may consist of a time averaging circuit such as a low pass resistor-capacitor filter to provide a time averaged control signal for controlling amplifier 40.
• the specific time constant of shaping circuit 42 is dependent upon the degree of smoothing desired. If other than a flat frequency response of the acoustic output is desired, the shaping circuit 42 could also include frequency selective circuitry such as a plurality of band pass filters covering adjacent frequency ranges and a variable attenuator associated with each. Each attenuator is adjusted to produce a control signal corresponding to the band pass frequency range of each filter and having a predetermined amount of attenuation. The control signals from each attenuator are summed to provide a gain control input to amplifier 40.
• Amplifier 40 consists of a gain controllable amplifier which is well known in the art.
• Amplifier 44 may comprise a fixed gain amplifier serving to buffer the output of amplifier 40 and provide additional gain for driving speaker 10. It will be apparent that amplifier 44 may not be needed depending upon the magnitude of acoustic output desired.
• the sensed feedback signal at electrode 32 After being smoothed by shaping circuit 42, provides a signal to which the gain of amplifier 40 is inversely proportional.
• the gain of amplifier 40 decreases as the amplitude of the feedback signal increases.
• the effect of such negative feedback is to maintain the signal at electrode 32 at a substantially constant amplitude and hence keeps the acoustic output which is proportional thereto at a substantially constant acoustic output level.
• the voltage generated at electrode 34 in response to movement of the piezoelectric driver provides a feedback signal proportional to the acoustic output of speaker 10 which can be utilized to shape the acoustic output of the speaker by controlling the gain of the drive signal applied to the driver. It is important to note that the acoustic output of speaker 10 can be made substantially flat versus frequency by the present invention without dampening the speaker such as by adding selective mass to the diaphragm or driver. Since mechanical dampening need not be added to achieve a desired frequency response, the sensitivity of the actual speaker itself is maximized.
• This invention further contemplates other potential applications in which a separate isolated electrode on a piezoelectric driver or transducer is utilized. For example, such an electrode permits the simultaneous utilization of a piezoelectric transducer as both a speaker and microphone in which the electrical input to the speaker and the output from the microphone are electrically isolated.

Landscapes

• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Piezo-Electric Transducers For Audible Bands (AREA)
• Circuit For Audible Band Transducer (AREA)

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• 1985
  • 1985-08-01 EP EP19850903960 patent/EP0188609A1/de not_active Withdrawn
  • 1985-08-01 WO PCT/US1985/001469 patent/WO1986001362A1/en not_active Ceased

Non-Patent Citations (1)

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