Communications – electrical: acoustic wave systems and devices – Signal transducers
Patent
1996-11-19
1998-06-02
Eldred, J. Woodrow
Communications, electrical: acoustic wave systems and devices
Signal transducers
367152, 367158, 310322, 310336, 73632, 73644, H04R 1700
Patent
active
057611564
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a piezoelectric ultrasonic transducer according to the kind of art of the first claim, particularly for use in transit time measurements.
Oscillators made of piezoelectric material are conventionally employed to generate ultrasonic sound in which the coupling of the oscillator to the medium into which the ultrasonic sound has to be emitted is of great importance. The coupling is the better the lower is the sound characteristic impedance between the oscillator and said medium. When the ultrasonic sound has to be emitted into air the coupling typically is particularly poor, since the sound characteristic impedances of air and piezoceramics differ by several orders of size. Irrespective of the fundamental problem of said poor matching the sound power emitted is the higher the greater is the sound amplitude Therefore, such transducers are typically operated in resonance when high sound powers have to be obtained. When, for example, in transit time measurements, short sound pulses have to be emitted then either the transducer resonance has to be strongly attenuated or the transducer has to be operated below its lowest resonant frequency. With single-layer thickness oscillators the attainable sound amplitude of the area emitting the sound lies within narrow limits since the excitation voltages required can be very high.
A solution which is nearest to the present invention is described in DE 39 20 663 Al. In said solution the so-called piezoelectric transverse effect (d.sub.31 -effect) is exploited. This paper proposes to manufacture an electroacoustic sound transducer composed of one or a plurality of piezoceramic sheets in which the space between the sheets is filled with a material of lower sound characteristic impedance, such as hollow spheres made of glass or SiO.sub.2 embedded in resin, or the like. The entire transducer element is constituted of a massive sandwich-like body of stacked layers in which the emitting and/or receiving face is a leading face of the body of stacked layers. With such a transducer in-phase surface oscillations with only one predetermined directional characteristic are obtainable. Said transducer is disadvantageous because, on the one hand, it delivers a sufficiently high efficiency only in resonance. On the other hand, the coupled mechanical properties of this layer compound require as concerns frequency, output, and directional characteristic to set up a completely new concept for the entire transducer construction including all its components. Furthermore, a transducer according to DE 39 20 663 Al requires considerable additional attenuation means when it is used for transit time measurements.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a piezoelectric ultrasonic transducer which ensures, independent of a definite selected excitation frequency, an in-phase and two-dimensional pulse emission of low time deformation of the pulse due to initial transient and dying-out transient which follows the electric excitation as closely as possible, and which realizes sufficiently high sound levels at low amplitudes of the excitation voltage. It is a further object of the invention to provide a piezoelectric ultrasonic transducer which, without any additional attenuation means, rapidly returns to the reception state after transmission.
BRIEF DESCRIPTIONS OF THE INVENTION
These objects are realized by the features of the first claim. It is an essential of the invention that it provides a new and substantially decoupled arrangement of at least one sheet made of piezoceramic material operating in d.sub.31 -mode and the operational direction of which, or at least one essential component thereof, is connected via a narrow leading face to a geometrically matched low mass, however, rigid cover plate and cover lamina, respectively, the normal n.sub.2 of which coincides with the abovementioned operation direction and its components, respectively, wherein only the low mass cover plate has the function of a sound-emittin
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Reuter Martin
Voigt Konrad
Weksler Valeri
Eldred J. Woodrow
marco Systemanalyse und
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