Broad beam ultrasonic transducer

Disclosed is a broad beam ultrasonic transducer of sandwich construction, having piezoceramic laminae (2), fitted with electrodes, and plates/films (3, 13) in the shape of a parallelepiped with a width (B) to length (L) ratio of 0.42 and in which the relative thicknesses of the piezoceramic laminae and the plates/films are chosen such that one side surface of the parallelepiped undergoes an in-phase oscillation behavior.

BACKGROUND OF THE INVENTION 
The present invention relates to a broad beam ultrasonic transducer. 
U.S. Pat. No. 4,677,377 discloses a piezoelectric transducer which is 
intended to be used as an emitting transducer or as a receiving transducer 
for ultrasonic waves propagating in air. The use of the transducer 
disclosed by this publication has solved substantial problems which are 
associated with the extreme difference between the acoustic wave impedance 
of the sound-transmitting medium of air and the acoustic wave impedance of 
a solid body emitting or receiving the ultrasonic waves. This acoustic 
wave impedance is also referred to as the acoustic characteristic 
impedance. 
The ultrasonic transducer of the aforementioned publication has an acoustic 
characteristic impedance which in relative terms is substantially closer 
to the value thereof of air. This is achieved by a sandwich construction 
which consists of individual mutually spaced piezoelectric laminae 
disposed in planes parallel to one another, the intermediate spaces, 
corresponding to the spacings, between these laminae being filled with an 
inherently stable material which has a low acoustic characteristic 
impedance value. The material occupying the intermediate spaces forms at 
least one closed surface of this electroacoustic transducer enclosing the 
piezoelectric laminae, namely a surface for the emission and/or for the 
reception of acoustic radiation. In this case, for example, this material 
occupying the intermediate spaces may extend beyond at least a respective 
one of the edge surfaces of the individual laminae, so that these edge 
surfaces of the laminae are covered in relation to the external 
environment by this material occupying the intermediate spaces. 
Such a known transducer may be designed so that this surface of the same 
which is provided for emission and/or reception has relatively large 
dimensions as compared with the wavelength, in air, of the emitted or 
received acoustic radiation. If the individual piezoelectric laminae are 
excited to execute co-phase oscillation, then, originating from this 
surface of the transducer, an acoustic wave with a substantially plane 
phase front is emitted. 
The material employed for the laminae is piezoelectric ceramic, e.g. lead 
zirconate titanate, lead titanate, barium titanate and the like, it being 
possible for these materials to include dopings and/or substitutions, of, 
inter alia, manganese, niobium, neodymium etc. to improve their respective 
properties. The material intended to occupy the intermediate spaces 
between the laminae is, in this known transducer, for example a 
thermoplastic material. By way of example, the entire body consisting of 
this material and the piezoelectric laminae is adhesively bonded together 
while hot. However, the intermediate spaces in this body may also be 
filled with a sealing compound consisting of silicon rubber. 
With regard to further details with respect to the structural configuration 
and the production of such a known transducer, reference is made to the 
aforementioned publication. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an electroacoustic 
transducer with favorable matching of the acoustic characteristic 
impedance to the medium of air, which transducer has as its acoustic 
radiation lobe one which has a relatively small width in the x coordinate 
direction perpendicular to the direction z of the axis of the acoustic 
radiation, i.e. a small beam width, and which has a broad beam in the 
coordinate direction y which is respectively perpendicular to this 
direction x and to the axial direction z, i.e. possesses a broad beam 
width in this direction. The structural configuration of the transducer to 
be provided is intended to be such that, proceeding from a basic type, 
individual types with beam widths in the y direction which differ from one 
another in a predeterminable manner can be obtained by the selection of 
individual dimensions. In particular, it is intended that the broad beam 
width can be selectable within the range from 50.degree. to 100.degree. 
(-6 dB) for the individual type of transducer. 
This object is achieved with a transducer having a transducer body in the 
form of a parallelepiped with the length (L), the width (B) and the 
thickness (D) and one surface of the transducer body as a sound-emitting 
and/or sound-receiving surface (5). The transducer body has on one hand at 
least one lamina provided with electrodes and which consists of 
piezoelectric material and, on the other hand, at least two plates/films 
consisting of a plastic material. These laminae and plate/films are 
alternately connected to one another in succession in the direction of the 
thickness of the parallelepiped. The ratio of the width to the length of 
the parallelepiped is at least approximately 0.42. The long lateral 
surface (thickness.times.length) of the parallelepiped is the 
sound-emitting and/or sound-receiving surface. The plastic material is a 
material having a mechanical oscillation quality factor in the order of 
magnitude of that of the piezoelectric material of the laminae. This 
plastic material has a lower acoustic characteristic impedance than that 
of the piezoelectric material of the laminae, and the Poisson ratio of the 
plastic material is less than 0.3. 
In the transducer the thickness ratio d.sub.p :d.sub.k, with d.sub.p for 
the components of the plates/films consisting of the plastic material and 
with d.sub.k for the components of the lamina consisting of the 
piezoelectric material, is selected so that the particle velocity of the 
transducer is at least approximately half as great as that of the 
piezoelectric material under the same excitation conditions, preferably 
equal voltage, in the case of resonance. The particle velocity is the 
velocity with which the particles (for example, air molecules) move back 
and forth. The plastic material of the plates/films can be a formed glass 
or a coarse-pored sintered glass. The sound-emitting or sound-receiving 
surface of the transducer body can be a closed film region consisting of 
the plastic material. 
Highly directional transducers including, for example, the transducers 
disclosed in references DE-A-2,537,788 and GB-A-1,530,347, have an beam 
width of 5.degree. to 10.degree. (-6 dB). A transducer according to the 
invention having a beam width of, for example, 70.degree. in the direction 
designated above by y and with a highly directional effect in the x 
direction is an extremely broad beam ultrasonic transducer. In a plane 
which is respectively perpendicular to the axial direction z, the cross 
section of the acoustic lobe of such a transducer according to the 
invention is relatively flat in the x direction, but on the other hand 
wide in the y direction, and represents, overall, a surface which, at 
least to an approximation, is similar to an ellipse. With increasing 
spacing (z-z.sub.0) from the surface Zo of the transducer, this cross 
sectional surface area becomes progressively greater, but without losing 
its characteristic form of a transducer having a broad beam in a lateral 
direction y. 
In order to achieve the object specified above, an attempt had been made to 
develop further the transducer disclosed in U.S. Pat. No. 4,677,337. 
However, it became evident that the specific object of the present 
invention could not be achieved in this manner. Difficulties arose, for 
example, if the thickness of the plastic occupying the intermediate spaces 
is substantially greater than the thickness of the piezoelectric laminae. 
In pulsed operation, the excitation of the laminae no longer led to 
co-phase surface deformation, on account of the low acoustic wave velocity 
in the y direction, but permitted interfering surface ripple to take 
place. In the case of resonant excitation of a pulsed transducer with the 
natural oscillation which is necessarily associated therewith, the gain in 
efficiency proved to be relatively limited. In a design and dimensions 
based on the object of high achieving mechanical losses, i.e. a low 
oscillation quality factor, the load capacity of the transducer was 
relatively severely limited on account of the generation of heat. The use 
of the above-mentioned silicon rubber or of a material comparable thereto, 
such materials having relatively large transverse contraction, produced 
excessively severe mode coupling with thickness resonances of the film 
transducer made using sandwich construction, specifically as soon as the 
stack height exceeds a specified value. In the case of pulse transducers, 
this is of advantage per se, since a multimode pulse transducer 
necessarily has a broad band. However, in the case of a single-frequency 
transducer, mode coupling is in most cases associated with an impairment 
of the electromechanical coupling factor and thus of the electroacoustic 
efficiency. In the case of transducers operated using a single frequency, 
as intended or required for the invention, a very comprehensive check of 
the occurrence of natural modes of the piezoelectric laminae contained in 
the transducer is essential with respect to the frequency, the form of the 
oscillation and the electromechanical coupling factor k, specifically for 
the purpose of achieving a defined directional characteristic and optimum 
efficiency. In order to achieve the object according to the invention, it 
would accordingly be necessary to embark upon a fundamentally new path, 
even though a transducer according to the invention in general again 
consists of rectangular piezoelectric laminae and composite material.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the principle of a transducer 1, designed according to the 
invention, and the relative dimensions of which are selected according to 
the invention. In the embodiment shown in FIG. 1, this transducer 1 
consists of a piezoelectric ceramic lamina 2 provided with electrodes (not 
shown in the figure) and of films or plates 3, 13 consisting of a plastic 
material. The length of the represented rectangular composite body of the 
transducer 1 is designated by L. Its width is designated by B. Its overall 
thickness is designated by D, and this is the sum of the thickness 
dimensions d.sub.p of the plates 3, 13 and the thickness dimension d.sub.k 
of the lamina 2. 
That surface of the transducer 1 which is designated by 5 is the emitting 
or sound-receiving surface which is provided or selected in accordance 
with the invention. The emission provided according to the invention is 
indicated by the arrows 6. 
The ceramic lamina 2 and the films or plates 3, 13 are firmly connected to 
one another over their surfaces, as shown. In the case of thermoplastic 
material, the bonding agent (adhesive) may be the material of the films or 
plates 3, 13 itself. 
A transducer according to the invention may also consist of a plurality of 
ceramic laminae and a corresponding number of films or plates. 
For the sake of completeness, reference is made to the IEEE publication, 
Transactions on Sonics and Ultrasonics, Vol. SU 15 (1968) pp. 97/105, 
where numerous forms of resonant oscillation are indicated for a 
rectangular piezoelectric plate, but only for a single active plate alone. 
The material of the plates 3, 13 on both sides of the piezoelectric lamina 
2 is selected with regard to low acoustic characteristic impedance Z and 
with regard to the smallest possible Poisson ration .mu. less than 0.3 and 
with regard to the highest possible oscillation quality factor Q greater 
than 20. A low acoustic characteristic impedance is used to achieve the 
best possible matching to the sound transmission medium of air. A small 
Poisson ratio contributes to the avoidance, as far as possible, of the 
excitation of transverse modes. In fact, these may already occur in 
circumstances in which the thickness D is even smaller than the width B of 
the transducer 1. A high quality factor Q of this material permits the 
achievement of oscillatory deflection in the material of the plates 3, 13, 
which approximates to and preferably exceeds the oscillatory deflection of 
the ceramic lamina 2. An example of such a material is that material which 
is described in reference DE-C-2,537,788 and in reference GB-A-1,530,347, 
and which is an epoxy resin filled with glass or silicon dioxide hollow 
spheres, also known under the trademark Scotch-Ply. Another material is 
polystyrene, a "glass foam", a sintered glass or the like. Where in this 
instance the material of the plates/films 3, 13 . . . is designated as 
plastic material, the mineral substance "glass" in forms as indicated is 
also included within the meaning of the invention. 
According to the invention, the ratio of the two dimensions B and L 
indicated in FIG. 1 is dimensioned as: 
EQU B:L at least approximately=0.42. 
Using this specified dimensioning, according to the invention a mode of 
oscillation of the transducer 1 is ensured in which the surface 5 
oscillates as far as possible approximately in-phase i.e. executes a 
"piston oscillation", and specifically with a coupling factor which is 
high at the same time. 
FIG. 2 shows an embodiment according to the invention with two ceramic 
laminae 2 and with 3 plates, 3, 13, 23. 
FIG. 3 shows an embodiment, likewise according to the invention, with two 
ceramic laminae 2, one plate 3 and two coatings 3.sub.1 and 3.sub.2, which 
are considerably thinner as compared with the thickness of the plate 3 and 
which are situated on the outwardly pointing surfaces of the ceramic 
laminae 2. 
Preferably, an embodiment according to FIG. 1 is selected if the quality 
factor Qp of the material of the plates 3, 13 . . . is smaller than the 
quality less factor Q.sub.k of the piezoceramic of the laminae 2. If 
Q.sub.p is approximately equal to Q.sub.k, the selection of a transducer 
according to FIG. 1 is recommended. If Q.sub.p is greater then Q.sub.k, an 
embodiment according to FIG. 3 is expediently selected, and specifically 
with 1/2 d.sub.p greater than d.sub.p, greater than 1/5 d.sub.p. For the 
purpose of the respective selection, the decisive matter is the specified 
objective of ensuring an amplitude decreasing towards the edge regions 
with an as far as possible (transversely to the laminae) in-phase 
oscillation behavior of the emitting surface 5; this gives rise to a 
directional behavior which has few sidelobes. 
In the case of all embodiments, the plastic material can also cover the 
entire surface 5, as shown by FIG. 4 with the film region 51. 
Optimum acoustic effectiveness for a mode of oscillation arises for a 
transducer according to the invention if the thickness ratio d.sub.p 
:d.sub.k is selected to be optimum. Other modes which have an interfering 
effect are avoided by complying to the above specification, namely that 
the plastic material is so selected or is present in such a form (e.g. 
foam) that its Poisson ratio is less than 0.3. An optimum d.sub.p :d.sub.k 
ratio is applicable if the transducer thus dimensioned has, in the case of 
resonant excitation, an amplitude of oscillation or particle velocity 
which is half as great as is applicable in the case of a transducer 
(having the same external dimensions) which however consists purely of the 
piezoelectric material or is not such a composite transducer. In these 
circumstances, the oscillation energy is apportioned by halves to the two 
material components of the individual transducer. 
The invention is not limited to the particular details of the apparatus 
depicted and other modifications and applications are comtemplated. 
Certain other changes may be made in the above described apparatus without 
departing from the true spirit and scope of the invention herein involved. 
It is intended, therefore, that the subject matter in the above depiction 
shall be interpreted as illustrative and not in a limiting sense.