JP2000333288

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DESCRIPTION JP2000333288
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
piezoelectric speaker, and more particularly to an ultrasonic apparatus having a mobile
telephone, a telephone and a piezoelectric element for generating sound.
[0002]
BACKGROUND OF THE INVENTION Earphones and speakers can be realized using several
operating principles including electrodynamic, electromagnetic, electrostatic and piezoelectric
effects. Piezoelectric transducers are often referred to as ceramic transducers. The most common
operating principle is that of current electrodynamics, which has the advantage of good
frequency response, low distortion tendency and the generation of sound consisting of a wide
range of amplitudes output thereby. Despite these advantages, situations exist where the
piezoelectric element becomes attractive, especially when the device is inexpensive, has low
power consumption, is small and lightweight. However, in general, the performance of the
piezoelectric element is far from achieving the expected quality. Piezoelectric elements are
plagued by both strong coloration and non-linearity of the frequency response. In addition, the
maximum movement of the surface of the piezoelectric element is much smaller than the
maximum movement of the surface of the normal speaker, and as a result, the volume that the
piezoelectric element can output is limited.
[0003]
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In order to solve these problems, serious attempts have been made to use the piezoelectric
element to improve the frequency response of the piezoelectric element to have good
characteristics. Piezoelectric elements, while small and inexpensive, require low input power,
while having at least one unique resonant frequency band, the output of which will be
significantly amplified by resonance. Most of the invention is to improve the frequency response
of the piezoelectric element as a mechanical amplitude damping part. International Patent
Application No. WO 86/01362 exists in which a portion of the surface of a piezoelectric element
is used to perform electrical feedback to an amplifier for driving the piezoelectric element to
perform gain control of the amplifier. By using such a configuration, it is possible to adjust the
output power of the piezoelectric element and adjust the volume according to the correct level.
This solution of linearizing the volume produced by the piezoelectric element is feasible when the
sound consists of a very narrow bandwidth single tone. However, this solution is not suitable for
linearizing the frequency response of more complex sounds such as human voice and
instrumental sounds. The reason is that the amplification of the volume changes over the entire
frequency band, which preserves the colorization caused by the piezoelectric element. Moreover,
due to gain control based on narrow frequency bands, it can easily become unstable. That is, in
the uncontrolled state, the piezoelectric element may generate a loud noise. U.S. Pat. No.
4,451,710 discloses an electricity-to-sound converter that utilizes a piezoelectric element. There
is the problem of variable sensitivity (i.e. sensitivity as a function of temperature, age and
manufacturing process). In order to correct the voltage applied to the drive element, the voltage
generated at the sensing element can be used to cancel the dependence of the sensitivity of the
transducer on the piezoelectric sensitivity of the foil. Thus, the above solution is almost similar to
the one in International Patent Application No. WO 86/01362 and therefore has the same
drawback of colorization of voice or musical sound produced by voice or piezoelectric elements.
[0004]
SUMMARY OF THE INVENTION A novel arrangement has been invented in which a piezoelectric
sensor is mechanically attached to a sound producing piezoelectric element or actuator, for the
output of a negative feedback signal corresponding to the vibration of the actuator. The actuator
is provided with a sound signal generated by actuation of the piezoelectric element. The feedback
signal is electrically filtered to compensate for the mechanical resonance frequency band of the
feedback loop stabilization actuator, and is guided to the sound signal stabilization piezoelectric
element driver together with the incoming audio signal. By selective amplification of frequency,
frequency selective phase number conversion, or a combination of the two, it is possible to
realize electrical filtering for stabilization. The purpose of this filtering is to reduce the amplitude
of the sound signal in the resonant frequency band and to compensate for the increased gain of
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the actuator at that resonant frequency.
[0005]
SUMMARY OF THE INVENTION According to a first aspect of the present invention, a step of
inputting a first electrical signal to a piezoelectric plate, and a second electrical signal
substantially proportional to the movement of said plate Acoustically filtering the second signal
and compensating for the mechanical resonant frequency band of the piezoelectric plate to
stabilize the output signal; Receiving the dynamic audio signal, subtracting the filtered electrical
signal from the electrical audio signal to generate the output signal, and outputting the output
signal to the piezoelectric plate using the piezoelectric plate Generating an audible audio signal
corresponding to the electrical audible signal. A method of generating sound is provided.
[0006]
According to a second aspect of the invention, a piezoelectric plate having a first input for
inputting a first electrical signal and having at least one mechanical resonant frequency band,
and attached to said plate, said plate A piezoelectric acoustic transducer comprising a sensing
element having a first output that outputs a second electrical signal substantially in proportion to
the movement of said mechanical element, for stabilizing said first signal. An electrical filter for
generating a filtered signal from the second electrical signal for the purpose of compensating a
resonant frequency band, and an addition means, the addition means being an electrical
reproduction acoustically reproduced by the piezoelectric plate A first input for receiving a third
electrical signal having an audible signal; a second input for receiving the filtered electrical
signal; and the third electrical signal for receiving the filtered electrical signal. Means for
generating said first electrical signal, and an output functionally connected to said first input, for
producing an acoustic audible signal corresponding to said electrical audible signal by means of a
piezoelectric plate. A piezoelectric acoustic transducer is provided, characterized in that
[0007]
The transducer preferably comprises an amplifier for amplifying the sound signal prior to the
delivery of the sound signal to the actuator.
According to a third aspect of the present invention, there is provided a piezoelectric plate having
a first input for inputting a first electrical signal and having at least one mechanical resonant
frequency band, said plate being attached to said plate, An earphone comprising a sensing
element mounted on a plate and having a first output for outputting a second electrical signal
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substantially in proportion to the movement of the plate, for stabilizing the first signal An
electrical filter for generating a signal filtered from the second electrical signal for the purpose of
compensating the mechanical resonance frequency band, and an adding means, wherein the
adding means is acoustically reproduced by the piezoelectric plate A first input for receiving a
third electrical signal having an electrical audio signal to be transmitted, a second input for
receiving the filtered electrical signal, and the filtered electrical signal Means for subtracting said
signal from said third electrical signal to produce said first electrical signal, and said first input
and function to produce an acoustic audible signal corresponding to said electrical audible signal
by means of a piezoelectric plate And an output connected in a manner as described above.
[0008]
The earphone preferably comprises an amplifier for amplifying the sound signal prior to the
delivery of the sound signal to the actuator.
According to a fourth aspect of the invention, a piezoelectric plate having a first input for
inputting a first electrical signal and having at least one mechanical resonant frequency band,
and attached to said plate, said plate A communication device comprising a sensing element
having a first output outputting a second electrical signal substantially in proportion to the
movement of said mechanical resonance frequency for stabilizing said first signal. An electrical
filter for producing a filtered signal from the second electrical signal for the purpose of band
compensation and summing means, said summing means being an electrical audio signal
acoustically reproduced by the piezoelectric plate A first input for receiving a third electrical
signal, a second input for receiving the filtered electrical signal, and the filtered electrical signal
from the third electrical signal Means for generating said first electrical signal, and an output
functionally connected to said first input, for generating an acoustic audio signal corresponding
to said electrical audio signal by means of a piezoelectric plate There is provided a
communication device characterized by having:
[0009]
The communication device preferably comprises an amplifier for amplifying the sound signal
prior to the delivery of the sound signal to the actuator. According to a fifth aspect of the present
invention, there is provided a piezoelectric plate having a first input for inputting a first electrical
signal and having at least one mechanical resonant frequency band, and attached to said plate,
said plate An ultrasound device comprising a sensing element having a first output that outputs a
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second electrical signal substantially in proportion to the movement of said mechanical
resonance, for stabilization of said first signal. An electrical filter for generating a filtered signal
from the second electrical signal for the purpose of compensating a frequency band, and a
summing means, the summing means being an electrical audible signal acoustically reproduced
by the piezoelectric plate A first input for receiving a third electrical signal having a signal, a
second input for receiving the filtered electrical signal, and whether the filtered electrical signal
is the third electrical signal Means for reducing and producing said first electrical signal, and an
output functionally connected to said first input for producing an audible audible signal
corresponding to said electrical audible signal by means of a piezoelectric plate An ultrasound
apparatus is provided, characterized in that it comprises:
[0010]
The ultrasound device preferably comprises an amplifier for amplifying the sound signal prior to
the delivery of the sound signal to the actuator.
[0011]
DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in detail
below with reference to the accompanying drawings.
A typical piezoelectric speaker comprises a thin circular (or rectangular) piezoelectric disc affixed
to a plate (usually metal). The plate is used as one electrode and the other electrode is deposited
on the other surface of the piezoelectric disc. Typically, the electrodes are printed using silver
paint, and when the piezoelectric material is heated to create polarity, the silver paint forms a
conductive surface. The disk is polarized and the electric field between the electrodes causes
radial stress to be generated. This stress causes the plate to bend as this radial stress is exerted
across only one surface of the plate.
[0012]
The piezoelectric transformation from the electric field to the stress inside the material is
relatively linear, and the non-linearity associated with this transformation is of the third order.
The conversion from a radial stress or force to plate displacement may exhibit significant nonlinearity and may exhibit significant asymmetry. Piezoelectric speakers rely on the bending of a
fairly rigid plate, so the achievable displacement remains small, and improvements in mechanical
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design result in only limited success in improving the linearity. Besides nonlinearity, another
common problem with piezoelectric speakers is the irregularity of frequency response. This
problem arises from the various resonance modes of the system. The axial mode (this mode may
be the source of the problem) can be controlled by proper installation of the driver, but at the
same time without a significant loss of sensitivity, the lowest radial mode (Lowest radial mode: In
practice, mechanical control of mass-spring-type resonance can not be performed.
[0013]
FIG. 1 shows a block diagram of a feedback piezoelectric transducer 1 according to an
embodiment of the present invention. The transducer comprises an actuation element AE and a
sensing element ME. The connection between the actuation element AE and the sensing element
ME is indicated by the connection MC. If the various parts of the system are treated as black
boxes, the mathematical model indicating the type of feedback system itself is quite simple. The
feedback system comprises an inverting summing amplifier SN with a finite gain amplifier with a
transfer function comprising the transfer function A1 of the drive amplifier A1, a converter
(speaker or actuator A2 and sensor A3 with transfer functions A2 and A3) It can also be
described in the simplest way as a sense amplifier A4 with a compensation network (transfer
function A4). Furthermore, there is a transfer function from the speaker drive voltage to the
acoustic response (Ama = pout / Vspkr). Here, Ama is an input voltage of the speaker, that is, a
transfer function for sound output obtained from a driving voltage of the speaker. All these
transfer function coefficients can be complex depending on the frequency.
[0014]
The transfer function of the loop of such a circuit can be described by the following equation:
[0016]
Now the total transfer function is calculated at the output of A4 (or at the negative input of the
summing node).
Therefore, the voltage at the actuator drive point P1 is as follows.
[0018]
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From this equation, the output sound pressure can be written as:
[0020]
When the electromechanical and electroacoustic transfer functions are determined either by
experiment or numerical model, the above equations allow for proper equalization design of the
sound.
The invention will now be illustrated by describing one experimental system. Since the output
impedance of the piezoelectric sensor is high, most of which are purely capacitive, the input
impedance of the sense amplifier determines both the sensitivity and the bandwidth of the
device. The capacitance of the piezoelectric sensor varies in the range of approximately 10 pF to
1 nF because it depends on the sensor structure. At low frequencies, this range implies very high
impedance (eg, 100 pF at 20 Hz means about 77 M ohms). However, since it is not practical to
use the piezoelectric speaker (or the headphones) at the lowest frequency itself, the lower limit
frequency can be selected to be one digit higher. This choice implies that the amplifier input
impedance is practically sufficient with approximately 1 to 10 M ohms: 10 to 20 pF. This
estimate is also supported by the measurement results, and such input impedance is easily
achievable. During the experiments, it has already proved in practice that it is beneficial to limit
the low frequency bandwidth of the signal supplied at the input of the sense amplifier with input
impedance. The aforementioned limited movement of the piezoelectric element limits the low
frequency response. This limitation depends on the size of the element, the mechanical and
electrical properties, and the distance from the element to the viewer. When using headphones or
headsets, the actual frequency band can be extended to low frequencies of approximately 100 Hz
to 200 Hz. Furthermore, for larger speakers that can be heard over long distances, each limit can
be at a much higher frequency.
[0021]
The measurements described below were made by using a loudspeaker consisting of two
separate piezoelectric transducers 24, 25. A sensing transducer or sensor 24 was affixed to the
back plate 23 of the actuating transducer or actuator 25. These transducers were attached to a
sealed enclosure 21 of 0.4 liter size, which was filled with absorbent material. (The capacity of
the enclosure is not so important for a piezoelectric speaker because of the stiff structure of the
speaker and thus there is little effective capacity, and also if feedback is used, Although less
important, this size was chosen to somehow reduce the impact of diffraction during acoustic
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measurements. The edge of the transducer was attached to the enclosure using a visco-elastic
material 22. The edge causing this viscoelastic loss had a significant damping effect on the radial
modes of the transducer.
[0022]
For simplicity, the experimental configuration system 20 was implemented using two separate
split transducers, an actuator 25 and a sensor 24. In commercial practice, cost can be reduced by
using part of the electrode as a sensor. There is a commercially available product available (a
buzzer for positive feedback with three connections). In this product, the piezoelectric material is
sandwiched between two electrodes, and on each side one of the two electrodes is further divided
into two mutually insulated parts. However, with this type of construction, stray capacitance
between the actuator and the sensor electrode can provide a usable amount of feedback at high
frequencies. Similar structures are already used in other applications. For example, in a
piezoelectric buzzer that outputs positive feedback, this positive feedback makes it possible to
sound the buzzer at a mechanical resonance frequency, thereby improving the efficiency.
Further, in ultrasonic imaging, it is possible to prevent overload of the receiving amplifier due to
the transmission pulse by using separate areas for the transmitter and the receiver.
[0023]
FIG. 3 is a block diagram of a measurement arrangement 30 used to test the loudspeaker shown
in FIG. This experimental configuration allows for easy gain adjustment and equalization of both
the input and feedback signal paths, so a small mixing console 32 is used as a summing amplifier
to keep this experimental configuration as simple as possible. It was. A conventional laboratory
measurement amplifier 34 was used as the input amplifier for the sensor 24. Reference numeral
31 denotes a signal source (a kind of signal generator that generates a defined (variable)
frequency used for a test). The frequency was boosted from above 1 kHz using the tone control
of the mixing console, and a maximum boost (about 12 dB) was achieved above 10 kHz. The
mechanical connection between actuation transducer 25 and sensor 24 is shown at connection
33. It is well known in the art that frequency selective analog signal processing may cause a
phase shift to the signal, so there is an alternative way of selectively boosting the signal. In view
of the desire to enhance the subtraction effect of the response signal to the audio input, it is
possible to adjust the amplitude across the entire spectrum while keeping the phase shift
constant over the entire range of the spectrum. Alternatively, it is possible to perform this phase
shift adjustment while keeping the amplitude constant. Alternatively, both of these adjustments
can be made simultaneously. The purpose of negative feedback is to improve the frequency
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response of the piezoelectric actuator, and electrical filtering can be used to stabilize the kinetic
behavior of the control loop.
[0024]
FIG. 4 shows the impulse response of the sensor of FIG. 2 according to the invention with
negative feedback (solid line) and without negative feedback (dotted line). The negative feedback
reduces the impulse decay time. FIG. 5 illustrates the frequency response for both feedback and
non-feedback systems. The reference level of the amplitude scale is arbitrary. The curve with
feedback is about 7 dB smaller in amplitude at a frequency of 6 kHz. This resonance peak is
shifted from less than 6 kHz to approximately 10 kHz using the present invention, and this peak
is also near the maximum amplitude (resonance bandwidth is approximately 2 kHz without
feedback, approximately 10 kHz with feedback) To have a lower gradient, which makes it
possible to significantly enhance the reproduction quality of the sound. This measured value
indicates that control of the feedback radial mode is effective. However, axial modes that cause
irregularities at 1 kHz and 2 kHz are not actually affected.
[0025]
The piezoelectric element according to the present invention can be used in telephones, mobile
phones, wired phones, earphones for wireless phones, portable cassette recorders, earphones for
CD players and DVD players. While the invention is most suitable for lightweight portable
devices, it is also well suited for fixedly attached devices such as sonars and the like. This
specification presents implementations and embodiments of the present invention by way of
illustration. It will be apparent to those skilled in the art that the present invention is not limited
to the details of the above-described embodiment, and that the present invention can be realized
in other embodiments without departing from the features of the present invention. .
Accordingly, these presented embodiments should be considered as non-limiting examples.
Accordingly, the possibilities of realization and use of the present invention are limited only by
the appended claims. Therefore, various modifications of the practice of the invention, including
equivalent devices, as determined by the claims, are also within the scope of the invention.
[0026]
As described above in detail, according to the present invention, the piezoelectric sensor is
mechanically attached to the sound generating piezoelectric element for outputting a negative
feedback signal corresponding to the vibration of the actuator. The sound quality of the device
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can be improved.
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