Piezoelectric audible sound generator

A piezoelectric audible sound generator having a feedback type piezoelectric transducer, an amplifier, and a phase compensator for adjusting the phase shift due to the combination of a bias resistance of the amplifier and equivalent capacitance of the piezoelectric transducer.

BACKGROUND OF THE INVENTION 
The present invention relates to a noise making device using a 
piezoelectric transducer, and more particularly to a piezoelectric audible 
signal generator incorporating a three-electrode piezoelectric transducer, 
an amplifier, a phase shifter and a feedback loop. 
A known, typical piezoelectric noise making device has, as illustrated in 
FIG. 1, a three-electrode piezoelectric transducer X.sub.1 having a 
resilient thin metal plate 1 as a ground electrode, piezoelectric plate 2 
and driving electrode 3 wherein the driving electrode 3 is connected to a 
collector of a transistor Q.sub.1 while a feedback electrode 4 is 
connected through a resistor R.sub.3 to a base of the transistor Q.sub.1. 
However, the known noise making device using the feedback type 
piezoelectric transducer produces a less sound pressure than the expected 
value which is inferred theoretically from the case of a two-electrode 
piezoelectric transducer without a feedback electrode. An attempt has been 
made to overcome the disadvantage by utilizing a transformer to heighten a 
voltage of a power supply, which, however, directs to a large scale of the 
device and does not meet with industrial, commercial requirements. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved piezoelectric 
noise making device which permits to exhibit its maximum noise making 
performance. 
Another object of the present invention is to provide a new piezoelectric 
noise making device which can produce a higher sound pressure at a low 
driving electric voltage than the conventional device can. 
The present invention is based upon a finding from various experiments that 
an unsuitable phase rotation of a signal is produced in the feedback 
circuit from the piezoelectric transducer to the amplifier. Theoretically, 
a driving signal applied to a driving electrode 3 of a feedback type 
piezoelectric transducer X.sub.1 and a sensed signal from its feedback 
electrode 4 is about 180.degree. as far as the transducer itself is 
concerned and a phase difference between a base input voltage and a 
collector output voltage of a transistor Q.sub.1 is 180.degree.. It would 
be understood from the above that a device including the piezoelectric 
transducer, the transistor amplifier, and a feedback circuit connected 
between the feedback electrode and the base of the transistor produces an 
oscillating sound at a frequency in the vicinity of an inherent resonance 
frequency of the feedback type piezoelectric transducer. 
Actually, however, there is produced an unsuitable phase shift of the 
feedback signal by a base bias resistor R.sub.3 of the transistor Q.sub.1 
and an equivalent capacitance of the piezoelectric transducer X.sub.1, 
resulting in that the noise making device oscillates at a frequency 
f.sub.1 which is shifted too much from the inherent resonance frequency 
f.sub.0. Consequently, a practical sound pressure is limited to a value 
much lower than the peak value P.sub.0 of sound pressure at the inherent 
resonance condition. 
Briefly, a piezoelectric audible sound generator according to the present 
invention has a self oscillator having a feed-back type piezoelectric 
transducer and an amplifier, and a phase compensator for adjusting the 
phase shift due to the combination of a bias resistance of the amplifier 
and equivalent capacitance of the piezoelectric transducer. 
Additional objects and features of the present invention will become 
apparent from the detailed description of a preferred embodiment thereof 
which will be made with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
A feedback type piezoelectric transducer applicable to the inventive device 
is known as disclosed in U.S. Pat. No. 3,815,129, and a detailed 
description will not be made for this reason. The piezoelectric transducer 
has characteristics of impendance, phase angle and sound pressure as 
illustrated in FIGS. 4(A), 4(B) and 4(C), respectively. The piezoelectric 
transducer X.sub.1 has a resonance frequency f.sub.r represented by an LC 
series resonance equivalent circuit, and an anti-resonance frequency 
f.sub.a represented by an LC parallel resonance equivalent circuit, 
wherein a minimum value corresponds to a resonance frequency f.sub.r while 
a maximum value corresponds to the anti-resonance frequency f.sub.a in 
respect of an impedance characteristic curve. Besides, in respect of the 
feedback type piezoelectric transducer X.sub.1, a phase difference between 
the driving signal applied to the driving electrode 3 and the sensing 
signal from the feedback electrode 4 is 180.degree. at the anti-resonance 
frequency f.sub.a. The self-oscillation circuit as is shown in FIG. 1 is 
of constant currency to permit the sound pressure P to become maximum at 
the anti-resonance frequency f.sub.a, but the phase difference due to the 
combination between the driving signal applied to the driving electrode 
and the feedback signal fed back to the amplifier imput is not full 
180.degree. due to the combination of the base bias resistance of the 
transistor amplifier and the equivalent capacitance of the transducer. 
Accordingly the phase difference of the driving electrode signal and the 
feedback electrode signal will become 180.degree. plus additional angle, 
and the resultant sound pressure becomes a value P.sub.1 which is much 
less than the maximum value P.sub.o, as shown in FIG. 4(C). 
According to the present invention, a phase compensating circuit 8 is 
provided to adjust the phase difference, namely the aforementioned 
additional angle, so that the noise making device can oscillate at a 
frequency of a maximum sound pressure. A desired, specific structure of 
the inventive device is shown in FIG. 5, in which reference numerals 7 and 
8 designate an amplifier and phase compensator, respectively. 
A collector of the transistor Q.sub.3 is connected through a collector 
resistor R.sub.6 to a power source V.sub.c and also to a driving electrode 
3 of the feedback type piezoelectric transducer X.sub.1. An emitter of the 
transistor Q.sub.3 is grounded, and a base thereof is connected through a 
base biasing resistor R.sub.7 to the collector thereof. A feedback 
electrode 4 of the piezoelectric transducer X.sub.1 is connected to a base 
of a transistor Q.sub.4, while its collector is connected through a 
collector resistor R.sub.9 to the power source V.sub.c and also to a base 
of a transistor Q.sub.5 through capacitors C.sub.1 and C.sub.2. An emitter 
of the transistor Q.sub.4 is grounded through an emitter resistor R.sub.11 
and is also connected between the capacitors C.sub.1 and C.sub.2 by way 
of a semi-fixed (trimmer) resistor R.sub.10. A base of the transistor 
Q.sub.5 is connected through a base biasing resistor R.sub.12 to the power 
source V.sub.c is also grounded by way of a diode D.sub.1. An emitter of 
the transistor Q.sub.5 is grounded while a collector is connected through 
a collector resistor R.sub.13 to the power source and also to the 
transistor Q.sub.3 through a feedback loop 9 and a base resistor R.sub.8. 
An electric signal obtained at the feedback electrode 4 of the 
piezoelectric transducer is fed back so that it becomes 180.degree. out of 
phase with the driving voltage at a base of the transistor Q.sub.3 by way 
of the phase compensator 8 which has a network of resistors and 
capacitors. At this time, the phase is adjusted by the semi-fixed resistor 
R.sub.10 which is connected to the transistor Q.sub.4. This means that 
adjustment of the resistor R.sub.10 permits the phase difference between 
the driving voltage applied to the driving electrode 3 and the signal 
obtained at the feedback electrode lies within the range of from 
170.degree. to 190.degree.. 
FIGS. 6(A) and 6(B) show a phase angle and a sound pressure characteristic 
when the resistance R.sub.10 is varied. When the phase difference is 
negative 180.degree., oscillating frequency will coincide with an 
anti-resonance frequency f.sub.a of the feedback type piezoelectric 
transducer X.sub.1, and the sound pressure becomes a maximum value 
P.sub.o. Within the phase difference range of from 170.degree. to 
190.degree., the oscillating frequency shifts within the range of from 
f.sub.3 to f.sub.4, and it will be understood from the drawing that the 
sound pressure immediately lowers at the outside of the range. In other 
words, a practical, high sound pressure can be obtained within the range 
of from f.sub.3 to f.sub.4, and the semi-fixed resistor R.sub.10 should be 
adjusted so that a phase difference lies within the range of from 
170.degree. to 190.degree.. It will be understood from FIGS. 6(A) and 6(B) 
that it is the most practicable that the semi-fixed resistor be adjusted 
to the phase difference of 180.degree.. 
Comparing the inventive device with the well known device of self 
oscillation type without phase compensating mechanism, it has been found 
that the inventive device produces a sound pressure which is higher by 
about 16 dB than that of the well known device. 
According to the inventive piezoelectric sound generator incorporating a 
phase shifting mechanism, an efficient conversion from electric signal to 
sound can be attained by the adjustment of the phase difference, and the 
piezoelectric transducer can be oscillated at a frequency at which a 
maximum sound pressure can be obtained. 
Though the present invention has been described with reference to the 
preferred embodiment thereof, many modifications and alterations may be 
made within the spirit of the invention.