DE2821573A1 - ACOUSTIC DEVICE - Google Patents

Description

Acoustic device

The invention relates to acoustic devices and, more particularly, relates to devices having a focused Deliver sound beam.

It has recently been found that it is possible to manufacture an acoustic microscope in such a way that a plane sound wave is passed into a water-filled cell through a concave surface in a sapphire crystal which is in contact with the water; the concave surface focuses the sound radiation in the water-filled cell, a sample is scanned in the focal plane, and radiation transmitted through the sample is picked up by a corresponding concave surface in a further sapphire crystal. Such an arrangement was made by RA Lemons and CF. Qua te in the ultrasonics Symposium Proceedings 1973 of the Institute of Electrical and Electronic Engineers (No. 73 CHO 8O7-8SU) in an article "A scanning acoustic microscope ^{" (1 A Scanning Acoustic Microscope 1} ) described.

However, when working with frequencies that are so high that the microscope can provide good resolution The sound attenuation in the water is very high, so that only a short one Path length in the water can be allowed; the focal length of the curved surface must be small and the radius of curvature this area is therefore very small. To create such an area

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pose is difficult.

The invention is therefore based on the object of developing an improved acoustic device which is shown in FIG a bastard sonic microscope or similar devices can be used is.

According to the invention there is an acoustic device comprising a solid body with a practically flat sound transmission surface and acoustic excitation means arranged so that within the body a converging sound beam arises, which impinges on said flat sound transmission surface.

The acoustic excitation means generally have at least one piezoelectric oscillator, the said touches solid body, the transducer having a suitable electrode on one side of a layer of piezoelectric material owns; If a suitable electrical alternating current signal is fed to the electrodes, the desired sound wave is emitted. Such a device can be operated in both directions, so that a sound wave captured by the oscillator is in a AC signal is converted. It can also be a source or a detector for an electrical alternating current signal may be provided} the source or the detector are connected to the (or connected to the) piezoelectric oscillator (s).

Instead of a single electrode on one side of the piezoelectric material, a group of electrodes can be used be provided with which the transmitted or received sound wave can be controlled more precisely.

The solid body is usually made of a material with low acoustic loss, especially when using high sound frequencies should be worked.

In the arrangement, the acoustic excitation means are located opposite one another on a flat or curved surface of the body the flat sound transmission surface and generate a converging sound beam. With another basic form of the The device is the surface of the solid body opposite the practically flat sound transmission surface that is convex shaped, and the acoustic excitation means are arranged so that they generate a plane sound wave from the convex curved surface is internally reflected on the flat sound transmission surface.

In a first embodiment, the surface opposite the practically flat sound transmission surface is the solid body arched, and with the curved surface is a single piezoelectric oscillator in contact.

In a second embodiment, the surface opposite the practically flat sound transmission surface is the arched solid body, and a single piezoelectric oscillator is in contact with the said practically planar Sound transmission surface.

In a third embodiment, the surface opposite the practically flat sound transmission surface is curved, and between the curved and flat surfaces and parallel to the latter there is a single flat piezoelectric Schwinger.

In a fourth embodiment, a group of concentrically arranged circular piezoelectric oscillators in contact with a second of the practically planar ones Sound transmission surface opposite surface of the solid body provided. The second surface can be flat, and a device can also be provided which excites the oscillators in a phase which progressively extends over

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the group of wrestlers changes away. The second surface can also be curved or stepped, if the transducers either all have the same phase or are excited in a phase that extends across the group of transducers progressively changes away. The transducers are as equivalent an optical Fresnel lens - because the arrangements are made in such a way that the excitation of the vibrators generates a convergent sound beam that passes through the solid body through to the practically flat sound transmission surface wanders.

If a coupling liquid, such as water, is present on the practically flat sound transmission surface, the Change of the refractive index in all embodiments Focusing the incident convergent beam in the coupling liquid. When an acoustic Device suitably by an AC power source is excited, it can work as a sound radiation transmitter; when such a device emits sound radiation from a focal point it can work as a receiver for sound radiation.

The practically flat sound transmission surface can either be completely flat or can have a slight curvature have in one direction or the other in order to correct aberrations or to enable a mechanically more favorable construction. However, such a weak curvature does not provide the focusing effect of the device! at the first, the second and third embodiment, the focusing is brought about by the curved surfaces, which is practically face flat surface, and in the fourth embodiment by the arrangement of the electrodes.

According to the invention comprises an acoustic system two erfindungsgeraäSe acoustic devices, their respective practically even sound transmission surfaces are next to each other,

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one connected to the sound exciter means of one device AC power source and an AC current detector, which is connected to the sound exciter of the other device are. Furthermore, a scanning device can be provided which mechanically scans an object through the focal point scans. When the system is in operation, there is a coupling liquid between the sound transmission surfaces, for example Water, and the said surfaces are so far apart that the focal point of the sound energy is that of the Excitation device is generated in such a position relative to the sensor surface that maximum sensitivity and response «· solution is guaranteed. Such an arrangement can be viewed as confocal.

In the drawings, FIG. 1 shows a schematic cross section through M in an acoustic system according to the prior art. The drawings used to explain the invention show the following:

Figs. 2, 3, Jfa, 4b and 5 J are kept schematically Cross-sections through some embodiments of acoustic systems according to the invention J.

6 shows a schematic cross section through a further embodiment of an acoustic system;

7 and 8: representations kept schematically the workability of an acoustic system;

Fi.g 9: a block diagram of a bastard ultrasonic microscope using an acoustic according to the invention Systems]

10 and 11: the use of an inventive acoustic device for non-destructive testing of materials.

In Fig. 1, the above-mentioned arrangement is dargestelltj of Lemons then have two identical Saphirkirstalle 10, 12 each have a polished spherical concave surface Ik, 16 in a

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their side surfaces and attached to the opposite side surface piezoelectric oscillator layers 18, 20, which are therefore opposite the spherical hollow surfaces. The oscillator 18 is connected to an alternating current source 22, which operates at approximately 1 GHz, and the oscillator 20 is connected to a detector 24 which is responsive to a frequency of approximately 1 GHz. Between the crystals there is a drop of water 26, which is held in place by surface tension, and part of a polyethylene terephthalate film 28, to which a sample 30 is attached.

When a signal from the source is fed to the oscillator 18, a plane sound radiation wave is produced in the crystal, which runs to the hollow surface I ^ and in which the water 26 is focused on a point in the middle between the crystals. The arrangement is confocal, so that the sound radiation emanating from the focal point is converted by refraction by the hollow surface 16 into a flat sound radiation surface which passes through the crystal 12, is converted into an alternating current signal by the oscillator 20 and integrated and displayed by the detector Zh will.

When an object 30 is brought into focus, the sound beam is disturbed by scattering and absorption processes and also experiences a phase change. The displayed signal therefore contains information about the in focus located part of the property. If the film 28 is scanned in the manner indicated by the arrows 32, different Parts of the object are "seen" and a picture can be built up will. The system can be used as a scanning sonic microscope.

However, the path length in the water droplet 26 must be so be kept as small as possible because the sound attenuation in the water is extremely strong - about 200 dB / mm at 1 GHz;

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therefore the radius of curvature of the hollow surfaces 14, 16 would have to be about 0.13 mm if the focal length is kept sufficiently small · It is difficult to produce a precisely executed hollow surface of these dimensions in a sapphire crystal.

Fig. 2 indicates an improved system according to the invention. Two blocks 34, 36 of fused quartz or quartz glass or Another suitable material have practically even sound transmission surfaces 38, 40 on each of the two flat surfaces opposite there are two curved surfaces 42, 44, each of which is covered with a thin piezoelectric oscillator 46, 48. The vibrator 46 is connected to an alternating current source 50 and the transducer 48 with an alternating current detector 52 in connection (the electrical connections are only indicated schematically), while the flat surfaces are confocal face each other and hold a drop of water 54 between them.

In this arrangement, a sound beam is generated by supplying a signal of, for example, 1 GHz to the oscillator 46, which converges towards the surface 38. At the solid / liquid interface 38, the jet becomes a focal point broken out, which lies in the middle between the surfaces 38, 40. The radiation emanating from the focal point penetrates the block 36 and arrives at the detector 52. The beam paths are indicated in the schematic drawing.

One advantage of such a system is that there are no concave surfaces with a small radius of curvature in the solid bodies need to be trained. In addition, the blocks can be cut off at the ends with the flat surfaces so that the flat surface only has a small surface area compared to the cross-section of the body, thereby making the position control possible of an object in the focal point of the system. The use of flat sound transmission surfaces also enables

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either a reduction in the path length in the water in the center of the acoustic lens or the placement of one Object larger dioke.

If it is now also known that a piezoelectric transducer can be applied to a curved surface there are certain difficulties with such a construction.

A different system is shown in FIG. Two solid bodies 58, 60 each have a sound _{transmission surface 62, 6 ^ f} each of which is opposite a curved surface 66, 68. A piezoelectric oscillator 70, 72 is arranged on one of the two flat surfaces, one oscillator 70 is connected to a Weohselstromquelle 7 ^, the other oscillator 72 to a Weohselstromdetektor 76. Between the oscillators 70, 72 is a water drop 56.

In this construction, which can only be used in pulse mode, is produced by exciting the oscillator 70 a plane sound wave passing through the body 58 up to the domed Surface 66 runs from which a convergent sound beam is reflected on the planar sound transmission surface 62, which focuses the beam by refraction. The arrangement is confocal, and a ray emanating from the focal point passes through the surface 64- to the curved surface 68 and is from this is reflected as a plane wave to the oscillator 72.

This system has advantages over the Systee after Fig. 2, because the transducers rest on a flat surface on each body, but the area of the sound transmission surfaces must be about the same size as the cross-section of the body (see. Fig. 3), so that the access to the focal point between the two extensive areas that are close to each other, can be restricted.

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Another disadvantage is that the detector 76 receives a strong signal originating from a plane wave, which passes directly from the transducer 70 into the body 60; this signal must pass through the associated circuit the time blanking technique known per se in ultrasonic devices be suppressed. The transfer of unwanted signals into the water can be made more difficult if necessary, by covering the interface areas through which such signals could pass with a sound-absorbing layer 77 * can be covered with wax, as in Pig. 3 indicated by dash-dotted lines.

Fig. K & shows a third system. Each solid body 78, 80 then has a first part with a flat sound transmission surface 82, 8Ψ and a second part with a curved surface 86, 88 which is opposite the flat surface, and a flat transducer 90, 92 is located between the two Parts, touches these parts and lies parallel to the associated flat surface. The oscillator 90 is connected to an alternating current source 9 ^ and the oscillator 92 is connected to a detector 96. The beam paths of the sound wave are indicated by the arrows, and the drop of water is denoted by 85. Pulse operation is required for this system.

In this system, the focal point is just as easily accessible as in the system of FIG. 2, and the transducers are attached to flat surfaces. The technique of time blanking can be used if plane waves received directly are to be eliminated, but in the arrangement according to Fig. Jfa is the size of these false signals significantly compared to the Arrangement according to FIG. 3 reduced because a direct transfer can only take place through the flat surface area 82, which only make a few percent of the area size of the oscillator 90 needs.

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4b and 4c show two further variants shown. According to FIG. 4 b, the oscillator 90 is only partially different from the first one with the flat sound transmission surface 82 provided part 81 of the solid body covered. According to Fig. 4c, only a single solid body 83 is used, the flat sound transmission surface 82 is formed on an extension 87 which projects beyond a flat surface 89; the surface 89 lies opposite the curved surface and carries an annular oscillator 91.

Fig. 5 shows a fourth system. Each solid body 100, 102 then has a small flat sound transmission surface 104, 106 in contact with a water droplet 105 and a large flat surface 108, 110 parallel to the sound transmission surface opposite the large flat surface 108, each has a group 112 , 114 attached by circular, concentrically arranged piezoelectric transducers. An alternating current source 116 is directly connected to the central oscillator of the arrangement 112 and is connected to the other oscillators by means of phase shifters 118, 120 which impart a phase shift fL , 0g to the associated oscillators. The oscillator group 114 is connected to a detector via phase shifters 124, 126, which cause complementary phase shifts 0 1, 0 ₂ .

It is easily possible to calculate the phase shifts that would be required to achieve at least one of the many Causing sound waves emitted by the device to fall onto the surface 104 as a converging beam; the Ray passes through a focal point in the water drop, and A diverging beam passing through body 102 is indicated by detector 122. The course of the rays is indicated in Fig. 5

In order not to obscure the drawing, are

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only three concentric transducers indicated! in the practical Execution of the system requires a larger number, namely about ten pieces. The system has the advantage that the curvature of the sound wave front can be controlled directly; one could, for example, aspherical wavefronts produce.

In a modified construction, the Fresnel chair groups according to FIG. 5 on the embodiments according to Figs. 2, 3 and 4 can be used. With another Variant compared to the construction according to FIG. 5, the flat surface 108 could be replaced by its stepped surface, the one oscillator at each stage would carry the signal could then each oscillator with the same phase position or with Phase differences are fed.

In each embodiment, the object to be examined is in the focal plane between the flat sound transmission surfaces set where it can be mechanically scanned. Usually the object is with water between the sound transmission surfaces coupled to the acoustic devices; but you can also work with other suitable liquids.

The previously drawn and described embodiments according to the invention all relate to the use of two corresponding acoustic devices, one of which is used as a transmitter and the other as a receiver and wherein the sound radiation is directed through the focus. You can also use a single device as a transmitter / receiver, whereby the sound radiation is reflected at the focus. The device would then be per se, as in ultrasound technology known to have to work in pulse echo mode.

In the case of transmission operations, it can in some cases be advantageous to use different types of

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directions to use as transmitter and receiver.

Another variant is shown in FIG. 6, according to which an acoustic device 128 corresponds to the two devices shown in FIG. 2 and is used, one Sound beam through a drop of water I30 onto an object I32 to focus, which can be attached to a thin film 134 · can be scanned in the focal plane. There is a piezoelectric film on the side of the film facing away from the object Vibrator 136 connected to a detector 140 by a flexible connection 138. The transducer / electrode structure is scanned together with the film / object unit, and the sound beam in focus is converted into an alternating current signal. With this type of construction, the length of the path in the water is halved

The arrangement according to FIG. 6 can also easily be used in a non-linear mode of operation, through the harmonic Images can be produced. When the oscillator 136 is at a second or higher harmonic of the operating frequency of the Acoustic device 128 is tuned, an image dependent on the harmonic transformation in the object can be produced will.

A further development of this technique is shown in FIG. 7, according to which an acoustic device according to the invention 14-2, which works with the frequency f ^, an object 144 duroh irradiates a water drop 146 with a focused sound beam. The object 144 is held by a thin film 148, which touches a receiving piezoelectric oscillator 150 that connected to a detector I5I. The receiving transducer is also in contact with a second transmitter oscillator 153 "which operates at a frequency f"; the sound radiation from the Oscillator 153 passes through oscillator 150 and irradiates it Object I * t4. The linearity of the object leads to multiplication

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of the two simultaneously incident irradiating frequencies, and the detector 151 can be tuned to display _{either the sum or the difference of f 1} and f _2. Another (not shown) position for the second oscillator 153 would be the contact with the end of a plate on which the object is attached, sound radiation being sent through this plate onto the object.

In connection with FIG. 3, reference was made to the technique of time blanking in order to switch off the direct coupling between two flat, large-area oscillators. A further possible construction is shown in FIG. 8, according to which the two acoustic devices 180, 182 equipped with large-area flat oscillators 18 ^, 186 are inclined relative to one another by a small angle Q; the angle is chosen so that the difference between the path lengths AA ¹ and BB ^{1 is} an integral multiple of the acoustic wavelengths in the water (not shown) located between the oscillators. This has the consequence that the receiving oscillator 186 is excited asymmetrically, so that the signal received in one half of the oscillator is rendered ineffective by the signal received in the other half.

The diameter of the transducers is on the order of 100 wavelengths, so that the angle of inclination Q is usually small and normally less than one.

The inclination also affects the beam that is reflected off the curved surface 188 of the device 182. This can easily be compensated by making the curvature slightly asymmetrical; Fig. 8 shows this greatly exaggerated. The required asymmetry can easily be calculated by introducing the condition of equal delay time for the rays XX ¹ X * and YYΎ ^Η . In this way you can achieve that no

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Loss of signal sharpness occurs even when the direct signal is largely switched off.

Instead of an asymmetrically curved surface, an expediently asymmetrically designed Fresnel oscillator arrangement can be used use (not shown).

The use of a system according to the invention as a scanning sound microscope is shown in FIG. After that is the transmitter device 1 ^ 8 of the microscope system 150 with a High-frequency oscillator 152, which operates at between ^ 50 MHz and 1050 MHz lying frequency works, via a variable attenuation 15 * 4 and a first tuning conduction adaptation ·. device 156 connected. The receiving device 158 is at a standstill via a second tuning stub adapter 160 with a mixer 162 in connection, which is also connected to a Local oscillator 164- is connected, which is connected to one of the frequency of the oscillator 152 operates at a slightly different frequency; the mixer 162 also supplies a signal the difference frequency, for example 30 MHz, via an amplifier 166 and a variable band filter 168 to an arrangement 170 with storing cathode ray tube; the signal forms the z input for the cathode ray tube} the x and y inputs are from a scanning synchronization and Amplifier circuit 172 supplied, which controls the scanning movement of the scanning device 17 ^ to which an object 176 is between the acoustic lenses of the microscope system 150 is connected. An image is built up point by point on the cathode ray tube.

Operation at frequencies between 1 MHz and 10 GHz is generally expected.

An acoustic device according to the invention can be used, apart from in a hybrid sonic microscope according to FIGS. 2 to 9

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also used for non-destructive testing of materials will.

One such application is in Pig. IO shown, according to which an acoustic device 190, for example a device corresponding to the embodiment according to FIG. K , is coupled to an object 192 to be examined by means of a thin layer of liquid 194 (for example water or oil). The device 190 operates according to the pulse echo method, which is common practice in ultrasound technology; the device 190 is connected to a source / detector assembly 196. If the object 192 is homogeneous, the echo picked up by the device 190 is very weak. However, if there is a small cavity or crack in the object, this flaw will emit approximately spherical waves and the received echo will be much stronger.

It should be noted that while the resolving power of the device, i.e. its ability between two adjacent To distinguish between defects, which is determined by the sound wavelength with which the device works, cracks and cavities can be detected that are orders of magnitude smaller are than the resolving power of the device. For example, with a sound wave, its wavelength is of the order of magnitude 100 ^, voids or cracks of the order of magnitude can be detected by micrometers.

A comparable application is shown in FIG. 11, according to which an acoustic device 200 according to the invention is connected to a object to be tested 202 is applied. Furthermore, there is a surface wave oscillator 20Ί-on the surface of the object, which is connected to a power source 206. The oscillator 204 irradiates the object 202 with Rayleigh waves that pass through the arrows 206 are indicated. The acoustic device 200 operates only as a receiver and is connected to a detector 208

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connected; in this arrangement the device is particularly sensitive to micro-cracks and similar surface defects.

In the arrangements according to both FIGS. 10 and 11, the acoustic device is scanned over the surface of the item.

In all arrangements, the flat sound transmission surface can be covered with a layer of a material that matches the acoustic impedance of the solid part to the liquid in order to maximize the power transfer. This is the acoustic equivalent to the coating of an optical lens. If the solid is fused quartz which is opposed to water as a liquid, gold is a suitable material for the adaptation layer satisfactory, if not perfect, resistance adjustment.

The patent attorney:

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Claims (12)

Expectations 1.} Acoustic device, characterized by a solid body (3 * 0 with practically flat sound transmission surface (38) and acoustic Excitation means (46) which are arranged so that a converging sound beam is created within the body, which on the flat sound transmission surface (38) hits 2. Acoustic device according to claim 1, characterized in that the acoustic excitation means (46) on one of the flat sound transmission surface (38) lie opposite surface (42) of the body (34) and generate a converging sound beam. 3. Acoustic device according to claim 2, characterized in that the plane Sound transmission surface (38) opposite surface (42) of the body (34) is convex and that a single piezoelectric Vibrator (46) rests against the convex surface (42). 4. Acoustic device according to claim 2, characterized in that the acoustic excitation means as a group of piezoelectric oscillators (112) are trained. 809811/035? ORIGINAL INSPECTED 5. Acoustic device according to claim k, characterized in that devices (118, 120) are also provided which make each piezoelectric oscillator of the group (112) oscillate with a phase which changes continuously over the group. 6. Acoustic device according to claim 1, characterized in that the surface facing the practically flat sound transmission surface (62) (66) of the solid body (58) is convex, and that the acoustic excitation means (70) are designed so that they Generate a flat sound wave, which from the convex surface (66) in the direction of the flat sound transmission surface (62) is reflected internally. 7. Acoustic device according to claim 6, characterized in that the acoustic excitation means (70) as a single planar piezoelectric Vibrator (70) are formed, which rests on the flat sound transmission surface (62). 8. Acoustic device according to claim 6, characterized in that the acoustic excitation means as a single piezoelectric oscillator (90) are formed, which rests against the solid body (78), between the flat sound transmission surface (82) and the Convex surface (86) is located and also runs parallel to the flat sound transmission surface (82). 9. Acoustic system, characterized by two acoustic devices according to one of the preceding claims, which devices are arranged in such a way that the respective flat sound transmission surfaces (38, kO) are closely adjacent to one another 809851 / OilS? an alternating current source (50) connected to the acoustic excitation means (6) of one device and an alternating current detector (52) which is connected to the acoustic excitation means (k8) of the other device. 10. Acoustic system, characterized by an acoustic device (128) according to one of claims 1 to 8, by an AC source connected to the acoustic excitation means of the device, by a piezoelectric one arranged next to the flat sound transmission surface of the device Oscillator (I36) and a detector (140) for alternating current connected to the piezoelectric oscillator. 11. Acoustic system, characterized by an acoustic device (1 ^ 2) according to one of claims 1 "to 8, by an alternating current source with a first frequency (f 1) connected to the acoustic excitation means of the device, by a first piezoelectric oscillator (I50) next to the flat sound transmission surface of the device, by a second piezoelectric transducer (153) lying on the first transducer and facing away from the acoustic device, by a second alternating current source with a second frequency (f) connected to the second transducer and by one connected to the first transducer AC current detector (151) responsive to a frequency equal to the sum or difference of the first and second frequencies (f _± + f ₂ or f _x - f ₂ ). 12. Acoustic system, gekennzeichne t by an acoustic device (I90) according to one of claims 1 to 8 and by one Alternating current source with alternating current detector (I96) connected to the acoustic excitation means of the device « 109851 / 06S? 13 · Acoustic system according to one of claims 9 "to 12, characterized by a scanning device (174) for bringing about a relative movement between an adjacent the flat sound transmission surface located object (176) and this sound transmission surface, wherein the movement takes place parallel to the surface. 809861 / 0ISf