Compact wide angle acoustic transducer

Abstract

A transducer is provided for emitting and receiving acoustic waves, a method for operating and a method for producing the same. The transducer comprises a casing that forms a cavity. The casing comprises an excitation surface and an emitting surface, which is arranged opposite to the excitation surface. A transducer element is provided at the excitation surface, and an acoustic diffuser is provided at the emitting surface of the casing, wherein a diffusing structure of the acoustic diffuser faces the cavity, and wherein the diffusing structure is provided by an array of columns.

Claims

1. A transducer for emitting and receiving acoustic waves, the transducer comprising a casing, the casing forming a cavity, the casing comprising: an excitation surface and an emitting surface which is arranged opposite to the excitation surface, a transducer element, the transducer element being provided at the excitation surface, an acoustic diffuser, the acoustic diffuser being provided at the emitting surface of the casing, wherein a diffusing structure of the acoustic diffuser faces the cavity, and wherein the diffusing structure is provided by an array of columns, wherein the transducer comprises a second acoustic diffuser, the second acoustic diffuser being provided at the excitation surface, wherein a diffusing structure of the second acoustic diffuser faces the cavity, and wherein the diffusing structure of the second acoustic diffuser faces the diffusing structure of the first acoustic diffuser.

2. The transducer according to claim 1, wherein the columns of the array of columns have a statistically distributed height.

3. The transducer according to claim 2, wherein a height distribution of the statistically distributed height is generated by a random sequence generator.

4. The transducer according to claim 1, wherein the columns of the array of columns are aligned orthogonal to a base surface of the acoustic diffuser.

5. The transducer according to claim 1, wherein a height range of the columns is between 1 millimeter and 10 millimeters.

6. The transducer according to claim 1, wherein the acoustic diffuser is made from a metallic material.

7. The transducer according to claim 1, wherein the acoustic diffuser is made from a plastic material.

8. The transducer according to claim 1, wherein the cavity is a rectangular cavity.

9. A transducer for emitting and receiving acoustic waves, the transducer comprising a casing, the casing forming a cavity, the casing comprising: an excitation surface and an emitting surface which is arranged opposite to the excitation surface, an acoustic diffuser, the acoustic diffuser being provided at the emitting surface of the casing, wherein a diffusing structure of the acoustic diffuser faces the cavity, the transducer comprising a first transducer element, a second transducer element and a third transducer element, the second transducer element and the third transducer element being provided at the excitation surface and the first transducer element being in contact with the emitting surface, wherein the first transducer element is a plate transducer, the plate transducer being provided at the emitting surface.

10. A transducer for emitting and receiving acoustic waves, the transducer comprising a casing, the casing forming a cavity, the casing comprising: an excitation surface and an emitting surface which is arranged opposite to the excitation surface, an acoustic diffuser, the acoustic diffuser being provided at the emitting surface of the casing, wherein a diffusing structure of the acoustic diffuser faces the cavity, the transducer comprising a first transducer element, a second transducer element and a third transducer element, the second transducer element and the third transducer element being provided at the excitation surface and the first transducer element being in contact with the emitting surface, wherein the first transducer element is arranged in a central position of the emitting surface.

11. The transducer according to claim 10, wherein the first transducer element, the second transducer element and the third transducer element are arranged along a diagonal of the excitation surface.

12. A transducer for emitting and receiving acoustic waves, the transducer comprising a casing, the casing forming a cavity, the casing comprising: an excitation surface and an emitting surface which is arranged opposite to the excitation surface, an acoustic diffuser, the acoustic diffuser being provided at the emitting surface of the casing, wherein a diffusing structure of the acoustic diffuser faces the cavity, the transducer comprising a first transducer element, a second transducer element and a third transducer element, the second transducer element and the third transducer element being provided at the excitation surface and the third transducer element being in contact with the emitting surface, wherein the second transducer element and the third transducer element are arranged symmetrically to the first transducer element.

13. A transducer for emitting and receiving acoustic waves, the transducer comprising a casing, the casing forming a cavity, the casing comprising an excitation surface and an emitting surface which is arranged opposite to the excitation surface, an acoustic diffuser, the acoustic diffuser being provided at the emitting surface of the casing, wherein a diffusing structure of the acoustic diffuser faces the cavity, wherein the transducer further comprises a first transducer element, a second transducer element and a third transducer element, the first transducer element and the second transducer element being provided at the excitation surface and the first transducer element being in contact with the emitting surface, wherein the third transducer element is a plate transducer.

14. The transducer according to claim 13, wherein the first transducer element is arranged in a central position of the emitting surface.

15. The transducer according to claim 13, wherein the second transducer element and the third transducer element are arranged symmetrically to the first transducer element.

16. The transducer according to claim 15, wherein the first transducer element, the second transducer element and the third transducer element are arranged along a diagonal of the excitation surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The subject matter of the present specification is now explained in further detail with respect to the following Figures, in which:

(2) FIG. 1 shows a side view of a first embodiment of a wide angle acoustic transducer with two plate shaped transmitter elements and a column shaped receiver element,

(3) FIG. 2 shows a perspective view of an acoustic diffuser for an acoustic transducer,

(4) FIG. 3 shows a perspective view of a further wide angle acoustic transducer with a single transducer element and two diffusers,

(5) FIG. 4 shows a perspective view of a further wide angle transducer having three transducer elements and two diffusers,

(6) FIG. 5 shows a perspective view of a further wide angle acoustic transducer with one plate transducer element, a needle transducer and two diffusers,

(7) FIG. 6 shows an experimental setup for a pressure distribution measurement of an acoustic transducer,

(8) FIG. 7 shows a pressure distribution of an acoustic transducer with a rectangular cavity,

(9) FIG. 8 shows a reference pressure distribution of an acoustic transducer having a cavity with a diffuser having a small column size,

(10) FIG. 9 shows a pressure distribution of an acoustic transducer having a cavity with a diffuser having a larger column size,

(11) FIG. 10 shows a reference pressure distribution of an acoustic transducer having a rectangular cavity,

(12) FIG. 11 shows a pressure distribution of an acoustic transducer with two diffusers facing each other,

(13) FIG. 12 shows an arrangement of clamp-on transducers for use with the embodiments of FIGS. 1 to 11,

(14) FIG. 13 shows a further arrangement of clamp-on transducers for use with the embodiments of FIGS. 1 to 11 in a V-configuration measurement,

(15) FIG. 14 shows the arrangement of FIG. 13 in a W-configuration measurement,

(16) FIG. 15 shows a further arrangement of clamp-on transducers for use with the embodiments of FIGS. 1 to 11,

(17) FIG. 16 shows a received signal in the arrangement of FIG. 12, and

(18) FIG. 17 shows an arrangement of wet transducers for use with the embodiments of FIGS. 1 to 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(19) In the following description, details are provided to describe the embodiments of the present specification. It shall be apparent to one skilled in the art, however, that the embodiments may be practised without such details.

(20) FIG. 1 shows a cross sectional view of a wide-angle transducer 10 according to a first embodiment. The wide-angle transducer 10 comprises a casing 11, which encloses a cavity 12. A first plate shaped transducer element 13 and a second plate shaped transducer element 14 are provided on an excitation surface 15 of the casing 11. The transducer elements have connections for connecting the transducer elements to an electric power source, which are not shown in FIGS. 1, 3, 4 and 5.

(21) In particular, the transducer elements of this embodiment and of the following embodiments can be provided by piezoelectric ceramics. According to specific embodiment, which was used to obtain the pressure curves, one or more 1 MHz piezoelectric discs with a diameter of 2 cm are glued to the flat back of the acoustic diffuser with a thin layer of epoxy. Silicone gel is applied to the other side of the piezoelectric disc for electrical insulation.

(22) An acoustic diffuser 16 is provided on the inside of an emitter surface 17 of the casing 11, which is opposite the excitation surface 15. The diffuser 16 is shaped such that it provides a diffuse reflection of a soundwave emitted by the transducer elements 13 and 14. Furthermore, the diffuser 16 is shaped such that the acoustic modes of the cavity 12, which would otherwise lead to standing waves within the cavity 12, are suppressed. In the example of FIG. 1, the diffusor 16 is a chamber type diffusor, such as a Schroeder diffuser.

(23) Examples of such diffusers include, among others, a Schroeder diffuser or quadratic residue diffuser (QRD), a cubic residue diffuser (CRD), a primary root diffuser (PRD) and a column type diffuser with statistically distributed heights. In one embodiment, which is particularly suitable for 3D printing, the diffuser is a column type diffuser with rectangular columns that have statistically distributed heights.

(24) In the example of FIG. 1, the cavity 11 has the shape of a rectangular block, which is bounded by side walls 15, 17, 18 of the casing 11. The side walls 15, 17, 18 of the casing 11 comprise the excitation surface 15, the emitter surface 17, a first lateral surface 18, a second lateral surface 19 and third and fourth lateral surfaces which are not shown in FIG. 1.

(25) A needle transducer 20 of the wide angle transducer 10 is coupled to the emitting surface 17 and extends through the cavity 17 and the excitation surface 17 and protrudes from the excitation 15 surface towards the exterior of the casing 11. In the example of FIG. 1, the needle transducer 20 is provided in a central position of the emitting surface 17 and the needle transducer 20 extends through the emitting surface 17.

(26) In other embodiments, the needle transducer 20 can be coupled to the emitting surface 17 in other ways. For example, the needle transducer may be welded or glued to an inner surface of the emitting surface 17. Alternatively, the emitting surface 17 may comprise a reception portion with a suitable shape for taking up the needle transducer 20.

(27) The cavity 12 is filled with an amorphous sound transmitting medium 21 such as a liquid or a gel or epoxy or other relevant material. Preferentially, the speed of sound in the transmitting medium is greater than the speed of sound in air and smaller than the speed of sound in the casing 11. Furthermore, the sound attenuation and the compressibility of the sound transmitting medium are substantially smaller than the sound attenuation and the compressibility of air.

(28) In many materials, the sound attenuation depends on the dynamic viscosity, the bulk viscosity of the material and the sound frequency, whereas the speed of sound depends on the compressibility of the material. However, the sound attenuation of sound transmitting materials, such as the sound transmitting medium 21 may also be characterized by the compressibility of the material. In order to avoid corrosion of the interior space of the wide angle transducer, the transmitting medium can be provided by a non-corrosive fluid such as oil.

(29) FIG. 2 shows a further embodiment of an acoustic diffuser 16′ the acoustic diffuser is provided by an array of columns 25, which have statistically distributed heights with respect to a base portion 26. Each pillar or column 25 has a square cross-section and is connected to the base portion platform 26, which has a rectangular shape. The array of columns is also known as a “Manhattan”, “terrain” or “skyline” structure. By way of example, the statistical height distribution may be generated by a random sequence generator. The range of random numbers is restricted between a minimum and a maximum height, which is sufficient to provide a diffuse reflection.

(30) The acoustic diffuser 16′ of FIG. 2 is a 3d-printed acoustic diffuser constructed with multiple pillars of different length. The acoustic diffusor comprises closely packed metal pillars with a random height. Each pillar has square cross-section, and is connected to a platform of rectangular shape.

(31) The random sequence that determines the height distribution may be a pseudo random sequence generated by an algorithm or it may be a real random sequence generated by a hardware random number generator using a quantum mechanical measurement such as shot noise or some other physical random process such as thermal noise. For example, the random heights could be selected according to a uniform distribution, a Gaussian distribution, a Poisson distribution or some other form of statistical distribution.

(32) In a terrain design of an acoustic diffuser the upper and lower frequency limits are defined by the width and height of the square columns, respectively. The height of the columns defines the lower frequency boundary, while the upper frequency boundary is defined by the width of the column. In specific examples, the height range of the columns or pillars may be chosen between 1 mm and 10 mm, the dimensions of the base surface or “foot print” may be chosen as 3.8 cm×3.8 cm, and the column width may be chosen as 1 mm or 2 mm. Thereby, the diffuser has an array of 38×38=1444 columns or, respectively, of 19×19=361 columns.

(33) The acoustic diffuser 16, 16′ is preferentially made from a metallic material, which provides a good coupling to the emitting surface. In particular, the diffuser 16, 16′ can be made by a 3D metal printing. Suitable processing for 3D metal printing include, among others, metal sintering or melting and in particular selective laser sintering (SHS), direct metal laser sintering (DMLS), selective laser melting (SLM), electron beam melting (EBM), powder bed and inkjet head 3D printing (3DP), Fused deposition modelling (FDM) or Fused Filament Fabrication (FFF), Robocasting or Direct Ink Writing (DIW) and electron beam freeform fabrication (EBF3).

(34) The current 3D printing technology is able to print structure as small as tens of micron in a size suitable for high frequency time reversal acoustics applications. It is suitable to manufacture complex surface structures. For a mass production molds could be used and the structure could be hot pressed or molded. The wide angle diffusor that was used for the experiments may be produced as follows.

(35) First, a volumetric model of the acoustic diffuser is generated with a computational geometry programming language, such as OpenSCAD. By way of example, the wide angle transducer of the pressure measurements has a support surface of 3.8 cm×3.8 cm which is decorated with vertical pillars of square cross-section, which is 1 mm by 1 mm and 2 mm by 2 mm wide. The height of the columns is generated randomly. For the diffuser of the pressure measurement, a uniform distribution with values ranging between 1 mm and 10 mm was used. For printing, the geometry may be exported in a standard file format, such as STL, and is converted into a printable file, which is suitable for printing with a 3D metal printer.

(36) By way of example, the 3D metal printer for generating the diffuser that was used in the pressure measurements utilizes a 100 W infrared fibre laser with a focus diameter of 40 micron to sinter austenitic stainless steel powder of 50 micron.

(37) FIG. 3 shows another embodiment of a wide angle transducer 10′. The wide angle transducer 10′ comprises two acoustic diffusers 16′, 36 facing each other. The acoustic diffusers 16 and 36 are column type diffusers, as shown in the previous FIG. 2. For the sake of clarity, FIGS. 3 and 4 show only a portion of the second diffuser 36. The second diffusor 36 can be seen best in the cross sectional view of the embodiment of FIG. 5. For brevity, some explanations of features which are common to the embodiments are not repeated again in the description of FIGS. 3, 4 and 5.

(38) The first acoustic diffuser 16′ is provided at the emitting surface 17 of the wide angle transducer 10′. The second acoustic diffuser 36 is provided at the excitation surface 36, which is opposite to the emitting surface 17. Different from the embodiment of FIG. 1, the wide angle transducer 10′ comprises only a single plate transducer 20′ which is used both for emitting and for receiving acoustic waves.

(39) In a transmitting mode, the plate transducer 20′ is supplied with an electric signal and converts the electric signal into an acoustic signal, which is transmitted to the excitation surface 15 and to the second diffuser 36. From there, the signal is transmitted into the medium 21 and to the first diffuser 16′. A portion of the signal is emitted at the emitting surface 17 and another portion is reflected back into the medium 21.

(40) The reflected acoustic signal is reflected back and forth between the first diffuser 16′ and the second diffuser 36 until it is dissipated. Every time the first diffuser 16 receives an acoustic signal it emits part of it at the emitting surface 17. The emitting surface 17 is either in direct contact with a fluid or is coupled to it and emits the acoustic wave into the medium. In the context of the present specification, a fluid can be a liquid like oil, water, mixtures thereof or also a gas like oil, petroleum gas or mixture thereof. Generally speaking, a leaky cavity is formed, which is driven by one or several transducer elements, such as piezoelectric ceramics, which are attached to the flat back of one of the two diffusers 16, 36.

(41) In a receiving mode, the plate transducer 20′ receives an acoustic signal, which is transmitted from the emitting surface 17 to the diffuser 16 and into the medium 21. Furthermore, the plate transducer 20′ receives signal portions that have been reflected back and forth between the diffuser 16, the walls of the casing 11 and the second diffuser 36.

(42) FIG. 4 shows a further embodiment of a wide angle transducer 10″ in which three plate transducers 13, 14, 20′ are provided in a row along a diagonal of the excitation surface 15. One of the transducers 20′, which is used as a transmitter, is provided centrally on the excitation surface 15. The two other transducers 13, 14 are provided on either side of the transducer 20′.

(43) FIG. 5 shows a cross sectional view of a wide angle transducer 10, which is similar to the transducer 10 of FIG. 1 in that the centrally arranged transducer element 20 is provided as a needle transducer 20. In FIG. 5, the needle transducer 20, the diffuser 16 and the casing 11 are shown in cross section.

(44) Similar to the embodiments of FIGS. 3 and 4 and different from the embodiment of FIG. 1, two column type diffusers 16, 36 are provided at opposite sides of the cavity. Similar to the embodiment of FIGS. 1 and 4 and different from the embodiments of FIG. 3 there are three transducer elements 13, 14, 20, which are arranged along a diagonal of the excitation surface 15. The first transducer element is not shown in the view of FIG. 5. In principle, the transducer elements 13, 14, 20′ could be arranged in other patterns but the pattern of FIG. 4 allows to achieve a large surface of circular shaped transducer elements and an effective coupling to the excitation surface 15.

(45) FIG. 6 shows a schematic configuration of an experimental setup that is used for determining the pressure distribution of a wide angle transducer 30 according to FIGS. 7 to 11. The wide angle transducer 30 refers to one of the wide angle transducer 31, 32, 33, 34 shown in the insets of FIGS. 7, 8, 9, 10 and 11, respectively. The acoustic diffuser 30 is submerged in water 37 in an upright position.

(46) The water 37 is contained between the boundaries of a container or a duct, which are indicated by the boundary lines 28 and 29. For the purposes of this measurement, the water is at rest and is not flowing relative to the boundaries 28, 29.

(47) A piezoelectric ceramic 20′ is attached to the wide angle transducer 30 and is backed by air 38 in order to have an efficient coupling of the signal into the water 37. A needle hydrophone 39, which is arranged at the boundary 29 in upright position and opposite to the wide angle transducer 37 is used to measure the acoustic pressure. The coordinates are chosen at water surface with the origin placed at the center of the piezoelectric ceramic. The X- and Y axes are lateral to the surface of the transducer and the Z-axis increases with normal distance from the transducer 30. For the purpose of the pressure diagrams of FIGS. 7 to 11, only the X-coordinate, which is parallel to the boundary 29, is used.

(48) The needle hydrophone 39 is moved in steps along the X-coordinate to pick up the sound signal from the wide angle transducer 30. In FIG. 6, the needle hydrophone 39 is shown in a position opposite to the centre of the wide angle transducer 30. As mentioned further below, a distance between the hydrophone and the front side of a plate, which serves as an emitting surface, is 150 mm.

(49) The acoustic diffusors are mounted inside a plastic housing, which allows inserting one or two of them. By way of example, this is shown in FIGS. 3, 4 and 5. The housing is in turn connected to a stainless steel rod and fixed in place at the free surface of a water filled basin made of transparent acrylic, see FIG. 6.

(50) In a technical application, the boundary 28 will usually be provided by a wall, such as a conduit wall. In an open channel configuration, the transducer is provided at a free surface. For the purpose of measuring only one direction of the acoustic signal and for ease of placement of the wide angle transducer it is sufficient to measure in an open channel configuration.

(51) By placing the transducer at the water surface, the piezo element 20′ becomes air-backed which increases the pressure amplitude transmitted into water. The signal is generated with an arbitrary waveform generator, fed to 55 dB RF amplifier (350 MHz bandwidth, ENI) and connected directly to the piezoelectric element 20′.

(52) For measurements of the spatial pressure distribution a single cycle at 1 MHz with a peak-to-peak voltage of 80 Volt is applied to the piezo element 20′. The acoustic signals are measured with a small hydrophone 39 with a circular polyvinylidenefluoride (PVDF) sensor. While the emitter/receiver is at a fixed location the position of the hydrophone 39 is moved with a programmable translation stage. When scanning the sound field the hydrophone is translated in steps of 0.5 mm where y=0, x is varied between −100 mm and 100 mm and z=150 mm.

(53) The measurement is automated. After the positioning of the hydrophone 39, the electric signal is generated from a control computer, uploaded to the arbitrary waveform generator, triggered and then captured with a 14-bit sampling oscilloscope. The signals are then transferred to the computer and stored.

(54) To investigate the impact on the decoration of the plate on the spatial scattering, the spatial pressure distribution of two different terrain structures were studied. A flat plate of thickness 6 mm served as a comparison.

(55) The FIGS. 7, 8 and 9 show the spatial distributions of the maximum positive pressure measured at a distance of 150 mm from the front side of the plate for three distinct configurations: a flat plate in FIG. 7, a diffuser with a terrain structure with 1 mm pillar size in FIG. 8, and diffuser with a terrain structure with 2 mm pillar size in FIG. 9.

(56) FIG. 7 shows a reference pressure distribution created by a wide angle transducer having just a cavity and no diffuser. As can be seen in FIG. 7, the flat plate provides a symmetric and narrow beam with a single distinct peak having a full width at half maximum (FWHM) of 26 mm.

(57) FIG. 8 shows a pressure distribution created by a wide angle transducer having a single column type diffusor with a smaller column base size of 1 mm by 1 mm. In the arrangement of FIG. 8 there is a strong difference on the spatial pressure distribution using the terrain structure of 1 mm pillar size as compared to FIG. 7. Several side lobes are prominently visible. Yet the FWHM of the main peak at 25 mm, has not significantly changed.

(58) FIG. 9 shows a pressure distribution created by a wide angle transducer having a single column type diffusor with a larger column base size of 2 mm by 2 mm. The pressure distribution of FIG. 9, which was obtained with a diffuser having a terrain with 2 mm pillar size is significantly different. In particular, the distribution in the centre region is different, it appears to consist of three peaks of approximately similar amplitude spanning around 80 mm at FWHM. This measurement suggests that the terrain diffuser with the larger pillar size will perform better within the leaky cavity.

(59) For the pressure measurements of FIGS. 10, 11 and 12, a reverberation cavity was constructed. Thereby, the acoustic diffuser can also be used as a receiver.

(60) To obtain a compact design, two parallel diffuser plates are embedded into the cavity with their terrain surface facing each other. An ABS made plastic holder fixes and aligns the plates such that there remains a gap between the tallest pillars. This gap is filled with water. However, in other embodiments the cavity can be filled with any other acoustic transparent material, such as a gel etc.

(61) Care is taken that no gas bubbles become enclosed, which could lead to undesired effects, such as cavitation. Therefore, prior to assembly the diffusers are kept in water under low pressure inside a vacuum chamber and the water is degassed. The diffusers are assembled into the cavity while remaining submerged. The final structure of the transducer, namely the pillar cavity, has a dimension of 4 cm×4 cm×2.5 cm. For reference purposes, the performance is performed with a cavity formed by two undecorated plates of 6 mm thickness each, which is also referred to as “flat cavity”.

(62) During the pressure measurements, the large mismatch in acoustic impedance between steel and water results in a high Q-value of the cavity, which is needed for the desired long reverberation times. In other words, a high density contrast leads to a high reflection rate.

(63) In FIGS. 10 and 11, the pressure distribution of the respective cavities is measured to confirm a wide spatial emission. As before in the measurements of FIGS. 7, 8 and 9, a single cycle 1 MHz signal is used but the scan range is increased to −125 mm≤x≤125 mm and z=150 mm of the hydrophone. The measured distribution of the maximum pressure emitted from the cavities is depicted in FIGS. 10 and 11.

(64) FIG. 10 shows a reference pressure distribution created by a wide angle transducer having a cavity which is bounded by two flat or undecorated plates 40, 41. The FWHM of the flat cavity of FIG. 10 is about 33 mm.

(65) FIG. 11 shows a pressure distribution created by a wide angle transducer similar to the one shown in FIG. 3, which has diffuser plates 16′, 36 that are facing each other. Compared to the FWHM of the pressure distribution of FIG. 10, the terrain cavity of FIG. 11 displays a nearly 5-fold increased FWHM of 145 mm. Thus, the FWHM extension is much larger than the width of the diffuser base, which is 3.8 cm=38 mm. Overall, the cavity transducer with the terrain surfaces greatly enhance the spatial spreading while maintaining a compact form.

(66) An opening angle α of the trapezoid, which is formed between the pressure FWHM as first base line and the plate surface as second base line, may be defined by

(67) $\alpha ={\sin }^{-1}\left(\frac{145\mathrm{mm}-38\mathrm{mm}}{2}*\frac{1}{150\mathrm{mm}}\right)\approx 20.9°$

(68) Generally, the time resolution will be limited by the damping time of the cavity.

(69) FIG. 12 shows an arrangement of two clamp-on transducers 10. The arrangement of FIG. 12 and the arrangements of the following FIGS. 13, 14, 15, 17 can be used with the wide-angle transducers 10, 10′, 10″, 10′″ of FIGS. 1 to 11.

(70) A flow direction is indicated by a horizontal arrow and travel paths of acoustic signals is indicated by two diagonal arrows.

(71) FIG. 13 shows a further arrangement of clamp-on transducers 110 for use with the embodiments of FIGS. 1 to 11 in a V-configuration measurement. The wide-angle transducers 10-10′″ can be integrated into a clamp-on transducer 110 in various ways. For example, the wide-angle transducers 10-10′″ can be arranged such that an excitation surface 15 coincides with an outer surface of the clamp-on transducer 110 and the opposite emitting surface 17 is adjacent to a wedge-shaped part of the clamp-on transducer 110.

(72) FIG. 14 shows the arrangement of FIG. 13 in a W configuration measurement in which a dominant acoustic signal is reflected twice at the boundaries of a conduit before it reaches a second transducer.

(73) FIG. 15 shows a further arrangement of clamp-on transducers for use with the embodiments of FIGS. 1 to 11. In the example of FIG. 15 the wide-angle transducers 10 are fitted in to the clamp on transducers, which are clamped to a conduit by means of cables. In the arrangement of FIG. 15, is one set of four transducers, one intermediate set of two transducers and a further set of four transducers. Lines of sight between the wide-angle transducers 10 are indicated by dashed lines.

(74) FIG. 16 shows a received signal in the arrangement of FIG. 12. The signal FIG. 15 illustrates the focusing properties of a wide-angle transducer in the time domain. A focusing in the time domain can be achieved by applying a time inverted signal at the sending wide-angle transducer 10.

(75) A measuring signal according to the present specification can be modelled by a matched filter. If a sharply peaked impulse is used as a probe or test signal, the received signal at the transducer is the impulse response of the channel. The wide-angle transducers of the present specification can be used with or without time inverted signals. When a time inverted signal is used, an inverted version of a received signal is sent back through the same channel as a measuring signal, either in the reverse direction of a flow in a conduit or in the same direction as the flow.

(76) This results in a signal with a peak at the origin, where the original source was, or in a signal with a peak at the original receiver, respectively. The signal is shaped with respect to space and time. The time focussing property is illustrated in FIG. 16, while the spatial shaping is illustrated in FIGS. 8, 9 and 11.

(77) An ultrasonic flow meter according to the present specification can provide a focusing property by using the abovementioned inverted signal, or a similarly shaped signal, for an ultrasonic flow meter to form a response signal, which is both concentrated in space and time. This in turn leads to a higher amplitude at a receiving piezoelectric element and a better signal to noise ratio.

(78) With an ultrasonic flow meter according to the present specification, focusing and beam forming properties can be obtained under very general conditions. For example, a focusing property is obtained even when only one ultrasound transmitter is excited and even when the inverted signal is reduced to signal that is only coarsely digitized in the amplitude range, if the time resolution of the inverted signal is sufficient. Furthermore, a flow meter according to the present specification can be used with clamp-on transducers, which are easy to position on a pipe and do not require modifications of the pipe.

(79) FIG. 17 shows an arrangement of wet transducers 111 for use with the embodiments of FIGS. 1 to 11. The wet transducers 111, which protrude into the conduit, comprise wide angle transducers 10. In the example of FIG. 17 there are altogether five layers of four transducers. Diagonal lines of sight within the same layer have been indicated by dashed lines. This arrangement is particularly useful for measuring the flow velocity in the different layers separately.

(80) The arrangement can be best used in combination with wide-angle transducers of the present specification and beam shaping procedures. The beam shaping procedures are implemented by electronic components such as a signal generating unit and a signal evaluation unit, which is also known as a signal processing unit. In particular, the beam shaping may comprise the application a time inverted signal to a wide-angle transducer.

(81) Although the above description contains much specificity, these should not be construed as limiting the scope of the embodiments but merely providing illustration of the foreseeable embodiments. Especially the above stated advantages of the embodiments should not be construed as limiting the scope of the embodiments but merely to explain possible achievements if the described embodiments are put into practise. Thus, the scope of the embodiments should be determined by the claims and their equivalents, rather than by the examples given.

REFERENCE

(82) 10 wide angle transducer 11 casing 12 cavity 13 transducer element 14 transducer element 15 excitation surface 16, 16′ acoustic diffuser 17 emitting surface 18 first lateral surface 19 second lateral surface 20 transducer element 20′ needle transducer 21 transmitting medium 25 columns 26 base surface 28 boundary line 29 boundary line 30 wide angle transducer 31 reference transducer 32 wide angle transducer, small column size 33 wide angle transducer, large column size 34 reference transducer 35 wide angle transducer, double sided 36 acoustic diffuser 37 water 38 air 39 needle hydrophone 40 flat plate 41 flat plate 110 clamp-on transducer 111 wet transducer