Pressure pulse source operable according to the traveling wave principle

A pressure pulse source for generating acoustic pressure pulses in an acoustic propagation medium has a foil arrangement formed by a number of electrically contacted piezoelectric foils stacked directly on top of one another with no interstices between the foils, and employs a drive system for driving the individual foils in succession according to the traveling wave principle.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a pressure pulse source for generating 
acoustic pressure pulses in an acoustic propagation medium, and in 
particular to a pressure pulse source operating according to the traveling 
wave principle. 
2. Description of the Prior Art 
Pressure pulse sources can be employed, for example, in medicine for the 
disintegration of calculi (lithotripsy), for treating tumors and for 
treating bone pathologies (osteorestoration). Such pressure pulse sources 
can also be used for non-medical purposes, for example in materials 
testing. For all uses, the pressure source must be acoustically coupled to 
the subject to be acoustically irradiated in a suitable manner, in order 
to ensure a low-loss introduction of the pressure pulses into the subject. 
The pressure pulse source and the subject must also be aligned relative to 
each other so that the region of the subject which is to be acoustically 
irradiated is located in the propagation path of the pressure pulses, or 
is located in the focal zone of the pressure pulses in the case of focused 
pressure pulses. 
A pressure pulse source for medical purposes operating according to the 
traveling wave principle is described in German OS 38 17 996. In this 
pressure pulse source, a plurality of foils are disposed spaced from each 
other by a defined acoustic propagation path in a liquid acoustic 
propagation medium disposed between each of the foils. The foils are 
driven according to the traveling wave principle, which is understood in 
the art and is used herein to mean that the foil farthest from the 
acoustic propagation medium is first, separately driven to generate a 
pressure pulse, and the foil immediately following in the propagation 
direction of the pressure pulse is then separately driven for generating a 
pressure pulse when the pressure pulse generated by the first-driven foil 
reaches that foil, and so on until all of the foils have been driven in 
succession. This results in a superimposition of the pressure pulses 
generated by the individual foils, so that the peak amplitude of the 
wavefront, and thus the pressure associated therewith, is continuously 
increased. 
In the aforementioned German OS 38 17 996, the spacing of the foils, 
separated by a defined liquid propagation path, is intended to prevent the 
foils from mutually influencing each other in terms of their frequency 
behavior. It has been shown that this known pressure pulse source is 
fundamentally functional, but the pressure magnitude obtainable, even with 
the use of a large number of foils, is rather low in comparison to known 
electro-hydraulic sources (of the type described in German OS 23 51 247), 
known electromagnetic sources (of the type described in European 
Application 0 188 750, corresponding to U.S. Pat. No. 4,697,588) and 
piezoelectric pressure pulse sources (of the type described in German OS 
34 25 992), which are not operated according to the traveling wave 
principle. 
As is understood by those skilled in the art, and as used herein, the term 
"foil" means a planar structure having a thickness which does not exceed a 
few millimeters. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a pressure pulse source 
operable according to the traveling wave principle which generates 
pressure pulses exhibiting a higher pressure than have heretofore been 
achieved with that type of pressure pulse source. 
The above object is achieved in accordance with the principles of the 
present invention in a pressure pulse source for generating acoustic 
pressure pulses in an acoustic propagation medium, operable according to 
the traveling wave principle, having a foil arrangement consisting of a 
plurality of electrically contacted, piezoelectric foils stacked directly 
on top of one another with no interstices therebetween, and a drive system 
for driving the foils according to the traveling wave principle for 
generating pressure pulses. The invention is based on the perception that 
liquid propagation paths between the individual piezoelectric foils are 
not required, as has been heretofore believed. Consequently, the acoustic 
attenuation caused by the such liquid propagation paths is eliminated, so 
that pressure pulses having a pressure higher in comparison to those 
obtainable in the structure described in German OS 38 17 996 can be 
generated under operating conditions which are otherwise the same (i.e., 
identical piezoelectric foils with respect to their dimensions and 
electrical contacting, the same number of piezoelectric foils, and driving 
the piezoelectric foils with the same electrical signal). As used herein, 
the phrase "foil arrangement of piezoelectric foils stacked directly on 
top of one another with no interstices therebetween" is intended to 
describe foil arrangements having foils which are "loosely" placed on top 
of one another so that their respective end faces which face toward one 
another press flush against the adjacent face, as well as foil 
arrangements wherein the foils are glued to each other at the end faces 
which face toward each other. In the latter arrangement, the thickness of 
the individual glue layers is small in comparison to the thickness of the 
foils and in comparison to the wavelength of the fundamental oscillation 
of the generated pressure pulses. The electrical contacting of the foils 
can ensue, for example, by metallizing the respective end faces of the 
foils. 
Although German OS 31 19 295, corresponding to U.S. Pat. No. 4,526,168, 
discloses a pressure pulse source driveable according to the traveling 
wave principle, the individual transducers thereof are arranged in 
succession but are not disposed directly on top of one another. 
Piezoelectric ultrasound transducers are described in East German Patent 
283 077 and British Patent 1 250 523 which are composed of individual 
transducers stacked directly on top of one another, however, no drive 
according to the traveling wave principle is used in those systems. On the 
contrary, the individual transducers are simultaneously driven in order to 
give the overall ultrasound transducer a behavior which corresponds to 
that of a single transducer having dimensions coinciding with those of the 
composite transducer. 
In one embodiment of the invention, the foils of the foil arrangement are 
electrically connected so as to form a plurality of layers, with the 
layers being driven according to the traveling wave principle and with 
each layer preferably comprising a plurality of foils. In this embodiment, 
the end faces of the piezoelectric foils can be provided with an electrode 
for electrical contacting, and at least one layer is formed by two 
piezoelectric foils which press against each other with electrodes of the 
same polarity, and the layers press against other layers with electrodes 
of the same polarity. In another version of this embodiment, the 
piezoelectric foils also have their respective end faces provided with an 
electrode for electrical contacting, but at least one layer is formed by a 
bilaminarly folded piezoelectric foil, and the layers press against each 
other at electrodes of the same polarity. In both versions, insulating 
measures can be eliminated both between the individual piezoelectric foils 
and between the layers of the foil arrangement, so that acoustic losses 
caused by such insulating layers, as a consequence of attenuation therein, 
are avoided. Moreover, the wiring outlay is reduced because in 
conventional sources operating according to the traveling wave principle, 
a plurality of electrical lines connecting the foils to the drive circuit 
corresponding in number to twice the number of piezoelectric foils is 
needed. In the acoustic source disclosed herein, only a number of such 
electrical lines equal to the number of piezoelectric foils (or layers), 
plus one is needed. Moreover, no parasitic capacitances of significance 
can arise between the piezoelectric foils or layers which are directly 
adjacent. 
Preferably, all of the piezoelectric foils and/or layers of the foil 
arrangement have the same thickness. This simplifies the drive of 
piezoelectric foils (or layers), because the respective transit times of a 
pressure pulse from foil to foil, or from layer to layer, are the same. If 
such layers or foils of identical thickness are used, in one version of 
this embodiment the end face of the foil arrangement, which is opposite to 
the end face from which the pressure pulses emerge from the foil 
arrangement, is pressed against a backing which is acoustically hard in 
comparison to the piezoelectric foils. Because, when driven, the 
piezoelectric foils emit pressure pulses both in the desired propagation 
direction, toward one end face, an in an opposite direction toward the 
other end face, the use of the backing results in the pressure pulses 
emitted in the direction opposite to the desired propagation direction 
being reflected in-phase, with proper operational sign, at the backing. 
Given continuing drive of the pressure pulse source, these reflected 
pulses are superimposed with subsequently generated pressure pulses, and 
thereby further contribute to the increase in the wavefront amplitude and 
thus further contribute to increased pressure generation. 
Preferably, an electrically contacted piezoceramic transducer is provided 
as the aforementioned backing, which, together with the piezoelectric 
foils and/or layers, is driveable according to the traveling wave 
principle. Although a passive (i.e., non-driven) acoustically hard backing 
can be used, by using an active (i.e., driven) acoustically hard backing 
such as a piezoceramic transducer, and active contribution to the pressure 
increase is also delivered, in addition to the contribution made by the 
reflected pulses. In a preferred version of this embodiment, the end face 
of the acoustically hard backing facing away from the foil arrangement is 
acoustically coupled to, such as by being directly adjacent, an acoustic 
absorber. This prevents pressure pulses having a polarity opposite to the 
pressure pulses generated by the piezoelectric foils (and by the backing 
if it is an active backing) from being introduced into the acoustic 
propagation medium as a consequence of acoustically soft reflection at the 
end face of the backing which faced away from the foil arrangement. 
The piezoelectric foils are preferably piezoelectrically activated polymer 
foils, such as polyvinylidene fluoride (PVDF) foils. 
Lead-zirconate-titanate is particularly suited as material for the backing 
if it is an active backing in the form of a piezoceramic transducer. Brass 
is particularly suitable for use as the acoustic absorber. 
As noted above, the respective end faces of the piezoelectric foils can be 
provided with an electrode for electrical contacting, with the respective 
end faces of adjacent foils or layers having the same polarity being 
pressed against each other. If an active backing is used in the form of a 
piezoceramic transducer, the piezoceramic transducer can also have its end 
faces respectively provided with an electrode for electrical contacting, 
with the electrode of the piezoelectric foil or layer which faces the 
piezoceramic transducer having the same polarity as the electrode of the 
piezoceramic transducer which it faces and is pressed against. As noted 
above, insulating measures between the elements, including the foil or 
layer and the piezoceramic transducer, which are adjacent one another are 
not needed, so that acoustic losses caused by insulating layers as a 
consequence of attenuation are avoided. The same advantage in reducing the 
wiring outlay is also present in embodiments having an active backing, 
with the number of electrical lines from the drive source to the driven 
elements (i.e., the foils or layers, plus the active backing) being equal 
to the number of such driven elements, plus one.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The pressure pulse source constructed in accordance with the principles of 
the present invention as shown in FIG. 1 includes a cylindrical tubular 
housing 1 in which a plano-concave acoustic positive lens 2 and a foil 
arrangement are disposed. The foil arrangement in the exemplary embodiment 
of FIG. 1 is formed by six piezoelectric foils 3a through 3f of identical 
thickness, stacked directly on top of each other with no interstices 
therebetween. A piezoceramic transducer 4 and an acoustic absorber 5 are 
also contained in the housing 1. The housing 1 has an application end 
which is closed by a flexible membrane 6 with the volume defined by the 
flexible membrane 6 and the concave side of the positive lens 2 being 
filled with an acoustic propagation medium, such as water. The entire 
arrangement is substantially rotationally symmetrical relative to a center 
axis M. 
The piezoelectric foils 3a through 3f are respectively polarized in the 
direction of their thickness, and are preferably piezoelectrically 
activated polymer foils, such as PVDF foils. The piezoceramic transducer 4 
consists of a ceramic material which is acoustically hard in comparison to 
the material comprising the piezoelectric foils 3a through 3f, i.e., the 
material of the piezoceramic transducer has a higher acoustic impedance 
than that of the piezoelectric foils 3a through 3f. The piezoceramic 
transducer 4 may consist, for example, of lead-zirconate-titanate. The 
acoustic absorber 5 consists of a material having an acoustic impedance 
roughly corresponding to that of the material of the piezoceramic 
transducer 4, and which has a high acoustic attenuation. If the 
piezoceramic transducer 4 is formed by lead-zirconate-titanate material, 
the acoustic absorber 5 may, for example, consist of brass. 
The thickness of each piezoelectric foil 3a through 3f is in the range, for 
example, 40 .mu.m through 4 mm, and the thickness of the piezoceramic 
transducer 4 is in the range of 2 through 20 mm. 
The pressure pulses exiting from the foil arrangement are planar pressure 
pulses, and the positive lens 2 serves the purpose of focusing these 
planar pressure pulses onto a focal zone lying on the middle axis M of the 
arrangement. The center of this focal zone is referenced F. 
The foil arrangement formed by the piezoelectric foils 3a through 3f has 
one end face which presses against the planar side of the acoustic 
positive lens 2. The other end face of the foil arrangement presses 
against the piezoceramic transducer 4, which is in the form of a wafer 
having end faces in respective parallel planes. The end face of the 
piezoceramic transducer 4 which faces away from the foil arrangement 
presses against one end face, which is a planar end face, of the acoustic 
absorber 5. The opposite end face of the acoustic absorber 5 has a conical 
depression or recess, for reasons described below. 
The positive lens 2, the foil arrangement formed by the piezoelectric foils 
3a through 3f, the piezoceramic transducer 4 and the acoustic absorber 5 
are clamped liquid-tight against a shoulder of the housing 1 by a 
retaining ring 7 and a plurality of screws (only the center lines of two 
such screws being schematically indicated in FIG. 1), so that the end 
faces of each of these components which face other are pressed flush 
against one another. As a result of the end faces being pressed flush 
against each other, good acoustic coupling from component-to-component is 
achieved. It is also possible to glue the respective end faces of adjacent 
components to each other using a thin adhesive layer. This is particularly 
suitable for the piezoelectric foils 3a through 3f since the foil 
arrangement, possibly in combination with the piezoceramic transducer 4 
also glued thereto, then constitutes a unitary structure which is easy to 
manipulate. 
As can be seen in FIG. 2, each piezoelectric foils 3a through 3f has a 
positive electrode, respectively referenced 9a through 9f, and a negative 
electrode, respectively referenced 10a through 10f. The piezoceramic 
transducer 4 has a positive electrode 11 and a negative electrode 12. The 
piezoelectric foils 3a through 3f are stacked within the foil arrangement 
so that piezoelectric foils which are adjacent are pressed against each 
other with electrodes of the same polarity. The piezoceramic transducer 4 
and the piezoelectric foil 3a of the foil arrangement adjacent thereto are 
also pressed against one another with electrodes of the same polarity. 
The electrodes 9a through 9f, 10a through 10f, 11 and 12 are formed by 
metallizing the end faces of the foils 3a through 3f and the piezoelectric 
transducer 4. The thickness of the electrodes, which is shown exaggerated 
in FIG. 2, is at least one order of magnitude smaller than the thickness 
of the piezoelectric foils 3a through 3f, or of the piezoceramic 
transducer 4. 
The positive electrode pairs 9a and 9b, 9c and 9d, and 9e and 9f which 
press against each other as well as the positive electrode 11 of the 
piezoceramic transducer 4, are respectively connected to high-voltage 
pulse generators 21a through 21d, which form a part of a drive circuit 17. 
The high-voltage pulse generators 21a through 21d each have a trigger 
input. These trigger inputs are each connected to a clock generator 25 via 
respective trigger lines 23a through 23d. The clock generator 25 supplies 
a square-wave signal having a constant cycle to each of the pulse 
generators 21a through 21d, thereby causing each of those pulse generators 
to generate one a high-voltage pulse per square-wave clock pulse, for 
example, at the appearance of the leading edge of the square-wave clock 
pulse. The negative electrodes 10a through 10f and 12 are connected in 
common to a reference potential, for example ground potential 22. 
In the above-described structure, two piezoelectric foils, such as the 
foils 3a and 3b, 3c and 3d, and 3e and 3f are driven in common and in the 
same direction to generate a pressure pulse. The piezoelectric foil pairs 
3a and 3b, 3c and 3d, and 3e and 3f which are operated together thus 
behave as a single piezoelectric foil in terms of their frequency 
behavior, and having a thickness corresponding to the combined thickness 
of the foils comprising the pair. The foil arrangement thus has three 
layers A, B and C which can be driven to generate pressure pulses, layer A 
being formed by the piezoelectric foils 3a and 3b, layer B being formed 
by piezoelectric foils 3c and 3d and layer C being formed by piezoelectric 
foils 3e and 3f. 
The cycle of the square-wave signal generated by the clock generator 25 is 
such that it exactly corresponds to the transit time of a pressure pulse 
through one of the layers A through C. Consequently, the high-voltage 
pulse generators 21a through 21d each supply a sequence of high-voltage 
pulses I.sub.1 through I.sub.n. As a consequence of the triggering of all 
high-voltage pulse generators 21a through 21d by the same trigger signal, 
the high-voltage pulses supplied to the layers A through C and to the 
piezoceramic transducer 4 are separated from one another by a 
chronological duration which is equal to the transit time of a pressure 
pulse through one of the layers A through C. The high-voltage pulse 
generators 21a through 21d are thus synchronized, so that all of the 
high-voltage pulse generators 21a through 21d simultaneously deliver a 
high-voltage output pulse in the sequence. Upon the simultaneous 
appearance of these trigger pulses at each of the layers A through C and 
at the piezoceramic transducer 4, each of those layers A through C and the 
piezoceramic transducer 4 is simultaneously excited so as to generate a 
pressure pulse. More precisely, the layers A through C and the 
piezoceramic transducer 4, when excited by a high-voltage pulse, each 
generate a planar pressure pulse propagating in the direction toward the 
positive lens 2 as well as a planar pressure pulse propagating in the 
direction toward the acoustic absorber 5. 
Considering, for example, the pressure pulse emitted by the piezoceramic 
transducer 4 in the direction toward the positive lens 2 given the 
occurrence of a high-voltage pulse I.sub.1 supplied by the high-voltage 
pulse generator 21a, this pressure pulse will emerge from the layer A 
simultaneously with that pressure pulse from the layer A which the layer A 
is caused to generate when it is driven by the high-voltage pulse 
generator 21b by the next high-voltage pulse I.sub.2. The pressure pulse 
generated by the piezoceramic transducer 4 as a consequence of the 
high-voltage pulse I.sub.1 is thus superimposed, in the sense of a 
pressure increase, with the pressure pulse generated by the layer A as a 
consequence of the high-voltage pulse I.sub.2. This pressure pulse formed 
by superimposition, in turn, emerges from the layer B simultaneously with 
that pressure pulse generated by the layer B as a consequence of being 
excited by the next high-voltage pulse I.sub.3, supplied by the 
high-voltage pulse generator 21c. Consequently the pressure pulse 
generated by the layer B is superimposed with the pressure pulse which 
arose by superimposition of the pressure pulse generated by the 
piezoceramic transducer 4 as a consequence of the high-voltage pulse 
I.sub.1 and the pressure pulse generated by the layer A as a consequence 
of the high-voltage pulse I.sub.2. It thus clear that, after the 
appearance of the high-voltage pulse I.sub.4, that a pressure pulse 
emerges from the layer C which has arisen by "addition" of all the 
pressure pulses which were generated by the piezoceramic transducer 4 as a 
consequence of the high-voltage pulse I.sub.1, by the layer A as a 
consequence of the high-voltage pulse I.sub.2, by the layer B as a 
consequence of the high-voltage pulse I.sub.3, and by the layer C as a 
consequence of the high-voltage pulse I.sub.4. This result also arises for 
the sequences of high-voltage pulses I.sub.2 through I.sub.5, I.sub.3 
through I.sub.6, etc., through I.sub.n-3 through I.sub.n. 
Considering the pressure pulses emitted by the layers A through C in the 
direction toward the acoustic absorber 5, it can be seen that as a 
consequence of the piezoceramic transducer 4 being acoustically hard in 
comparison to the material of the piezoelectric foils 3a through 3f, these 
pulses are reflected at the boundary surface between the piezoelectric 
foil 3a (also forming the boundary surface of the layer A) and the 
piezoceramic transducer 4. The degree of reflection (reflectivity) is 
dependent on the relationship of the acoustic impedances of the materials 
of the piezoelectric foils 3a through 3f and the piezoceramic transducer 4 
and becomes higher as the difference between these acoustic impedances 
increases. The pressure pulse emitted by the layer A in the direction 
toward the acoustic absorber 5 as a consequence of the high-voltage pulse 
I.sub.3, for example, following reflection at the boundary surface of the 
piezoceramic transducer 4, emerges from the layer A propagating in the 
direction toward the positive lens 2 precisely at the time when a further 
pressure pulse is generated by driving the layer A with the high-voltage 
pulse I.sub.4. Superimposition of those occurs. Simultaneously with these 
pressure pulses, those pressure pulses respectively emitted by the layer B 
(as a consequence of the high-voltage pulse I.sub.2) and the layer C (as a 
consequence of the high-voltage pulse I.sub.1) in the direction toward the 
acoustic absorber 5 are reflected at the boundary surface of the 
piezoceramic transducer 4 and emerge from the layer A. This sequence 
occurs for each of the groups of high-voltage pulses I.sub.2 through 
I.sub.5, I.sub.3 through I.sub.6, etc. A superimposition of the pressure 
pulses emitted by the piezoelectric foils 3a through 3f, or by the layers 
A through C, in the direction of the acoustic absorber 5 thus also arises 
in the sense of a pressure increase. 
The pressure pulses emitted by the piezoceramic transducer 4 in the 
direction toward the acoustic absorber 5 are not utilized. As a 
consequence of the coinciding acoustic impedances of these two components, 
these pressure pulses proceed into the acoustic absorber 5 essentially 
without the occurrence of reflections and, to the extent these pressure 
pulses are not converted into heat as a consequence of the attenuation of 
the material of the acoustic absorber 5, are reflected at the rear side of 
the acoustic absorber 5. Because this rear side of the acoustic absorber 5 
is adjacent ambient air, which is acoustically softer than the material of 
the acoustic absorber 5, a phase shift occurs upon this reflection, so 
that the components of the pressure pulses reflected at the rear side of 
the acoustic absorber 5 have a polarity opposite that of the pressure 
pulses generated by the pressure pulse source. A significant attenuation 
of the pressure pulses generated by the pressure pulse source in the 
direction toward the acoustic lens 2, due to the pressure pulse components 
reflected at the rear side of the acoustic absorber 5, will not arise 
because the pressure pulse components reflected at the rear of the 
acoustic absorber 5 will diverge due to the conical recess in the rear 
side of the acoustic absorber 5. These reflected components can therefore 
only be imperfectly focused by the positive lens 2. It is clear that those 
parts of the pressure pulses emitted by the piezoelectric layers A through 
C in the direction toward the acoustic absorber 5, which are not reflected 
as a consequence of incomplete reflection at the boundary surface of the 
piezoceramic transducer 4, proceed into the acoustic absorber 5. The same 
circumstances apply to those pressure pulses as was described above for 
the pressure pulses emitted in that direction by the piezoceramic 
transducer 4. 
Minor losses may arise due to the attenuation of the pressure pulses when 
traversing the piezoelectric foils 3a through 3f and as a consequence of 
incomplete reflection at the boundary surface of the piezoceramic 
transducer 4. Leaving these minor losses out of consideration, it is clear 
that the pressure of the pressure pulses emitted by the pressure pulse 
source in the direction toward the positive lens 2 corresponds to twelve 
times the pressure of a pressure pulse emitted by one piezoelectric foil, 
plus the increase in pressure caused by the pressure pulses emitted by the 
piezoceramic transducer 4. In comparison to known acoustic pressure pulse 
sources operating according to the traveling wave principle, a significant 
pressure increase is achieved not only by stacking the piezoelectric foils 
3a through 3f directly on each other with no intervening, attenuating 
liquid propagation paths, but also by using the piezoceramic transducer 4, 
in addition to its function as a backing so as to maximally exploit the 
pressure pulses emitted by the piezoelectric foils 3a through 3f in the 
direction toward the acoustic absorber 5, to actively contribute to 
increasing the pressure by driving the piezoceramic transducer 4 to 
generate pressure pulses. 
As schematically shown in FIG. 2, it is possible to switch the polarity of 
the high-voltage pulses generated by the high-voltage pulse generators 21a 
through 21d by means of a switch 26, so that positive pressure pulses in 
comparison to ambient pressure or negative pressure pulses in comparison 
to ambient pressure can be generated, dependent on the position of the 
switch 26. 
A further embodiment of a layer structure, using layer B as an example, is 
shown in FIG. 3. In this embodiment, layer B is composed of a single 
piezoelectric foil 24, which is folded in a U-shape so that the two legs 
of the layer B press each other at the positive electrode 25. The negative 
electrode 26 covers the two outer end faces of the folded layer B, as well 
as the curved exterior of the fold. Such a structure is referred to herein 
as a bilaminar structure. The same structure can be employed for layers A 
and C. 
It will be understood that it is possible to employ a single high-voltage 
pulse generator for all of the layers A through C and for the 
piezoelectric transducer 4. If such a single generator is used, however, 
it must be assured by appropriate dimensioning that the single generator 
can deliver the high-voltage pulses of the required amplitude with the 
necessary repetition rate. 
It is also theoretically possible to individually drive the piezoelectric 
foils 3a through 3f. If this is done, however, it would be necessary to 
electrically insulate the respective adjacent faces of the foils from 
foil-to-foil, and to electrically insulate the piezoelectric transducer 4 
from the piezoelectric foil 3a. In such an arrangement, however, there 
would be the risk that parasitic capacitances formed by adjacent 
piezoelectric foils and the insulating layers therebetween would 
significantly exceed the capacitances of the individual piezoelectric 
foils, with the result that an optimum functioning of the pressure pulse 
source is no longer ensured. 
The electrical contacting of the layers A through C can ensure, for 
example, by placing a metal foil strip between the electrodes to be 
contacted, for example between electrodes 10d and 10e. If an adhesive is 
present between the electrodes, it must be assured that this adhesive does 
not insulate the metal foil strips from the electrodes. Correspondingly, a 
metal foil strip can be disposed between the acoustic absorber 5 and the 
electrode 11 of the piezoceramic transducer 4, between the electrode 10f 
and the positive lens 2, or between the two legs of a layer, such as the 
layer B, in the embodiment of FIG. 3. 
The pressure pulse source disclosed herein particularly suited for medical 
purposes, for example for treating tumor and stone pathologies. Using a 
known x-ray and/or ultrasound locating system, the flexible membrane 6 of 
such a pressure pulse source is pressed against the body surface of a 
patient to be treated, with the pressure pulse source being positioned so 
that tumor or the calculus to be treated is located in the focal zone of 
the pressure pulse generated by the pressure pulse source. The region to 
be treated is then charged in the required manner with pressure pulses, 
with negative pressure pulses being employed when treating tumor 
pathologies and positive pressure pulses being preferably employed when 
treating stone pathologies. The pressure pulse source disclosed herein, 
however, can also be utilized for non-medical purposes. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventors to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of their contribution to the art.