Acoustic transducer element

In an embodiment, an acoustic transducer includes an element with an acoustic radiative surface having two warped edges at opposing sides. In another embodiment, an acoustic transducer includes first and second elements, each divided into at least two spatially separated portions electrically coupled to each other, the portions configured to interleave. In a further embodiment, an acoustic transducer includes first and second transducer elements. The second element is situated adjacent to the first element and includes a radiative surface with an edge having periodic elongations. In yet another embodiment, an acoustic transducer includes a transducer element with an acoustic radiative surface that has a skewed diamond shape.

BACKGROUND

Side-scan sonar systems are typically configured to detect acoustic radiation within a fan-shaped region that is narrow in a plane parallel to motion of a water vessel and broad in a plane perpendicular to the motion of the vessel. Interferometric sonar measurements using multiple transducer detector elements can be used to determine a direction of an object detected in the water with respect to the water vessel with greater specificity. Phase differences between signals received at multiple acoustic transducers can be used to determine the arrival angle for an acoustic signal detected by the transducers.

However, aliasing can occur when the phase difference between signals received at the multiple transducers exceeds 360° (phase wrapping), corresponding to one complete temporal or spatial cycle of the received acoustic radiation. Aliasing limits the range of arrival angles that can be reliably determined with a given set of transducer elements.

SUMMARY

There is a need for increasing directionality of acoustic transducer elements for sonar systems in order to minimize or prevent aliasing due to phase wrapping in interferometric transducer systems. These systems include side scan sonar, side looking sonar, synthetic aperture sonar, swath sonar, and all systems that can use interferometry. The present disclosure relates to transducer element configurations that can increase directionality of acoustic transducers in interferometric sonars. Advantageous beam radiation patterns may be obtained. In some embodiments, transducer elements may be constructed of polyvinylidene difluoride (PVDF). PVDF can facilitate construction of a variety of transducer configurations that can be used to narrow transducer receiver beam patterns, while allowing the proper interferometric spacing between receivers. Embodiments can enable simultaneous control of element beamwidth to minimize spatial aliasing, radiation side lobe level to reduce echo contamination, and channel-to-channel spacing in an acoustic interferometer.

In one aspect, an acoustic transducer includes a transducer element with an acoustic radiative surface, the radiative surface including two warped edges at opposing sides of the radiative surface. The radiative surface may include two tapered ends, and each warped edge may extend from one of the tapered ends to the other. The transducer element may be a PVDF element, a ceramic element, a capacitive element, a magnetostrictive element, a fiber-optic hydrophone element, or an electrostrictive element.

Each of the two warped edges of the transducer element can have a periodic shape. Each of the two warped edges of the transducer element can be smooth. Each of the two warped edges can have a truncated Gaussian shape. Each of the two warped edges of the transducer element can define at least one 90° angle therein, the two warped edges further defining a second portion of the transducer element shifted from a first portion of the transducer element in a direction perpendicular to a major axis of the transducer element.

The transducer element may be a first transducer element, with the acoustic transducer further including a second transducer element with an acoustic radiative surface, the radiative surface including two warped edges at opposing sides of the radiative surface. The first and second transducer elements may be stacked against each other, with the acoustic radiative surfaces of the first and second transducer elements partially overlapping. The transducer may include an electrical circuit configured to drive (or receive signals by) the first and second transducer elements at the same frequency, and the circuit may be configured to acquire interferometric sonar data based on signals received by the first and second transducer elements.

The acoustic transducer can further include one or more additional transducer elements, each element having a respective acoustic radiative surface including two warped edges at opposing sides of the respective radiative surface.

Each of the first and second transducer elements may be divided into at least two spatially separated element portions electrically coupled to each other, and the element portions of the first transducer may be configured to interleave the element portions of the second transducer.

The first and second transducer elements can occupy a common surface and overlap each other. Further, the first transducer element can include a first plurality of non-contiguous sub-elements electrically coupled to each other, and the second transducer element can include a second plurality of non-contiguous sub-elements electrically coupled to each other. The first plurality of sub-elements can be electrically isolated from the second plurality of sub-elements.

In another aspect, an acoustic transducer includes a first transducer element divided into at least two spatially separated element portions electrically coupled to each other. The transducer also includes a second transducer element divided into at least two spatially separated element portions electrically coupled to each other, with the element portions of the second transducer configured to interleave the element portions of the first transducer. The transducer may also include an electrical drive circuit configured to drive (or receive signals by) the first and second transducer elements at the same frequency. The electrical drive circuit may be further configured to drive (or receive signals by) the first and second transducer elements with mutually distinct phases.

The transducer may include an electrical receive circuit configured to detect a phase difference between acoustic signals intended to be received at the first and second transducer elements. The element portions of the first and second transducer elements may be rectangular in shape and have longer sides oriented parallel to one another and to axes along which the two transducer elements, respectively, are divided. At least one element portion of the second transducer element may be in physical contact with at least one element portion of the first transducer element.

The element portions of the first transducer element may have a first acoustic center and the element portions of the second transducer element may have a second acoustic center. The first and second transducer elements can be PVDF, ceramic, capacitive, magnetostrictive, fiber-optic, or electrostrictive transducer elements. Each of the first and second transducer elements can have an acoustic radiative surface with two warped edges at opposing sides of the radiative surface. Each of the warped edges can have a truncated Gaussian shape. The acoustic transducer can further include one or more additional transducer elements, each additional element divided into at least two spatially separated element portions electrically coupled to each other.

In yet another aspect, an acoustic transducer includes a transducer element having an acoustic radiative surface with an edge having periodic elongations protruding therefrom. The edge with periodic elongations may have a sawtooth or sinusoidal shape. The transducer element can be a PVDF, ceramic, capacitive, magnetostrictive, fiber-optic hydrophone, or electrostrictive transducer element. The edge having periodic elongations protruding therefrom can be a warped edge, the transducer element further including an additional warped edge, the warped edge and additional warped edge at opposing sides of the radiative surface of the transducer element. The additional warped edge can have a truncated Gaussian shape.

The transducer element can be a first transducer element, and the acoustic transducer can further include a second transducer element having periodic elongations protruding therefrom, with the first and second transducer elements situated adjacent to one another. The periodic elongations of the second transducer element may be offset from the periodic elongations of the first transducer element, and the edges of the first and second elements with periodic elongations may fit against each other to form a common edge.

The transducer can also include an electrical drive (or receive) circuit configured to drive (or receive signals by) the first and second transducer elements at the same frequency. The electrical circuit may also be configured to drive (or receive signals by) the first and second transducer elements with mutually distinct phases. Furthermore, the second transducer element may be stacked against the first transducer element, and the radiative surface of the second transducer element may be overlapping with a radiative surface of the first transducer element. The acoustic transducer can also include one or more additional transducer elements, each of the additional transducer elements having an edge with periodic elongations protruding therefrom.

In still another aspect, an acoustic transducer includes a first transducer element divided into first and second spatially separated element portions electrically coupled to each other. The acoustic transducer can also include a second transducer element divided into first and second spatially separated element portions electrically coupled to each other. The second element portions of the first and second transducer elements can be spatially offset from the first element portions of the first and second transducer elements, respectively, in a direction perpendicular to a major axis of the first transducer element.

In yet another aspect, an acoustic transducer includes a transducer element with an acoustic radiative surface having a skewed diamond shaped. The transducer element can be a first transducer element, and the acoustic transducer can also include a second transducer element with an acoustic radiating surface that has a skewed diamond shape. Furthermore, the acoustic transducer can include one or more additional transducer elements with acoustic radiative services having skewed diamond shapes.

The first and second transducer elements can form part of an interferometric transducer array with a long axis and a short axis. Each of the first and second transducer elements can have a respective acoustic center and a respective width along the short axis. A center-to-center spacing between the respective acoustic centers along the direction of the short axis can be smaller than the sum of the respective widths along the short axis. The transducer element can be a PVDF transducer element.

The transducer types can also be crystal or piezo-composite.

DETAILED DESCRIPTION

FIG. 1Aillustrates a prior art acoustic transducer102that includes three transducer elements102a-cthat can be used for interferometric side-scan sonar measurements, for example. In particular, the elements102a-care rectangular, which is a typical geometry used for side-scan imaging systems for ease of manufacture and to produce the appropriate radiation pattern for side-scan imaging, as illustrated further inFIG. 2A.

FIGS. 1B-1Eillustrate various transducers104,106,108, and110, respectively, showing features of embodiment devices. These transducer configurations are useful for narrowing a distribution of radiation produced by the transducer elements in the XZ plane shown inFIG. 1Awhile maintaining both receiver sensitivity and acoustic center-to-center element spacing (illustrated inFIG. 10). In particular, the transducer104includes split (divided) elements104a,104b, and104c, each of the elements divided into two spatially separated element portions electrically coupled to each other. Electrical coupling is further described hereinafter in conjunction withFIG. 12A. With the electrical coupling between the element portions104a, for example, the element portions104acan be driven by the same electrical signal, with the same driving frequency and wavelength to produce radiation as a single transducer element but with a radiation distribution that is narrower than if the element portions104awere spatially coupled to each other.FIG. 1Billustrates that a transducer with element104acan include one or more additional elements (e.g. elements104b-c), each additional element divided into at least two spatially separated element portions. The spatially separated element portions of each additional element can be electrically coupled to each other to be driven or to receive as a single element, as described further hereinafter.

With the element portions104bspatially separated from each other but electrically coupled to each other, the element portions104balso act as a single transducer and can be configured to interleave with the elements104aas shown inFIG. 1B. In particular, one element portion104ais situated between the two element portions104b. The divided elements104a-care further described hereinafter in conjunction withFIGS. 6A-CandFIG. 10A. Moreover, as illustrated inFIG. 12A, divided elements need not be rectangular and can have other shapes in other embodiments. Furthermore, while three sets of divided elements104a-care illustrated inFIG. 1B, in other embodiments, only two sets of split elements are present in a transducer.

FIG. 1Cillustrates a transducer106with warped elements106a-c. The warped elements106a-chave acoustic radiative surfaces182a-c, respectively. Each radiative surface includes two warped edges172aand172bat opposing sides of the respective radiative surfaces182a-c. Furthermore, while additional transducer elements (first warped element106aand additional warped elements106b-c) are shown inFIG. 1C, in other embodiments, only one warped element106ais required (e.g., where interferometric measurements are not required). Warped elements are further illustrated and described in conjunction withFIGS. 7B-C,8A-D,9,12A-C,12E, and13, for example.

The warped elements106a-chave smooth edges, meaning that there are no angles formed in the edges. In other embodiments, as described hereinafter in connection with FIGS.12C and14A, for example, angles may be formed in the edges, such as 90° angles or angles forming a sawtooth pattern in the edges, for example.

FIG. 1Dillustrates a transducer108with stacked planes108a-ccontaining elements. Each plane can contain an element shaped such as warped Gaussian (e.g.182a), or a rectangle (e.g.102a), or sawtooth (e.g.182ainFIG. 1E). There are two views inFIG. 1D: a perspective, exploded view (left), and a side-view (non-exploded) (right). While three stacked element planes108a-care illustrated inFIG. 1D, in other embodiments, only two stacked element planes are used. Stacking elements enables elements to be wider along the X axis, which narrows the radiation distribution in the XZ plane for more directive radiation patterns while maintaining a center-to-center spacing of acoustic centers of the elements108a-c, as further illustrated inFIG. 10B. Stacking works when each transducer element is acoustically transparent, allowing wave penetration to reach each of the stacked planes without substantial loss of energy. This is particularly useful with PVDF transducers. The embodiment transducer108is further illustrated inFIGS. 10B-C. Furthermore, stacked elements are not limited to rectangular shapes, and some examples of non-rectangular, stacked elements are illustrated inFIGS. 12B and 12D-E, for example.

FIG. 1Eillustrates a transducer110with interdigital elements110a-c. The transducer elements110aand110badjacent to each other, and the transducer element110cis adjacent to element110b. Each of the elements110a-chas an acoustic radiative surface182a-c, respectively, with an edge having periodic elongations186a-c, respectively, protruding therefrom. Each of the transducer elements110a-c, therefore, includes an acoustic radiative surface182a-cwith an edge having periodic elongations with a partial sawtooth shaped.

In other embodiments, the transducer can vary from the transducer110illustrated inFIG. 1Eand can include only two transducer elements, for example. Furthermore, in some embodiments, only one of the transducer elements has an edge with periodic elongations. In yet other embodiments, the interdigital elements110a-bdo not fit together to form a common edge184, as shown inFIG. 1E. In still other embodiments, an acoustic transducer includes only the single element110a, an edge of the radiative surface having periodic elongations protruding therefrom, the edge defining a periodic shape. The configuration of the transducer elements110a-calso allows for the radiation distribution in the XZ plane to be narrowed compared to a radiation pattern produced by the elements102a-c, for example, while enabling the same spacing between acoustic centers (not shown). The interdigital elements110a-care also illustrated inFIG. 11A, and other embodiment devices including transducer elements with periodic elongation are illustrated inFIGS. 12C-D, for example.

It should be noted that there is an equivalency between the radiation pattern produced when a transducer is driven to produce acoustic radiation and the pattern indicating the sensitivity of a transducer to radiation received by the transducer when it is operated in detection mode. Therefore, throughout this application, when radiation patterns are described, it should be understood that radiation detection patterns are equivalent to radiation production patterns in terms of strength distribution. Furthermore, while both prior art transducers, such as the transducer102inFIG. 1Aas well as embodiment transducers, such as the transducers104,106,108, and110inFIGS. 1B-1E, can be used to both transmit acoustic radiation and to detect acoustic radiation at different times, in some cases, separate transducers can be used for transmitting acoustic radiation, and the use of embodiment transducers can be confined to detection. All the radiation patterns and embodiments described herein are relevant to detection receiver applications.

FIG. 2Ais a perspective illustration of a typical side-scan sonar imaging application. A towfish212, which can be pulled behind a boat or other water vessel, has the prior art acoustic transducer102amounted thereon. The transducer102aboth produces acoustic radiation and detects reflected radiation over a fan-shaped region214. The towfish212also includes a second prior art transducer (not shown) on the other side of the towfish that produces a second radiation pattern214on the other side of the towfish. These are usually referred to as the port and starboard transducers. Objects anywhere in the detection regions214or at a seabed216can be detected. Transducer102is traditionally long in the Y direction and narrow in the X direction, so that its radiation patterns (for both transmit and receive) are narrow along Y and wide in the XZ plane. The towfish212travels in the Y direction indicated inFIG. 2A, with an X direction perpendicular to the direction of travel and a Z direction perpendicular to both X and Y axes. As described further hereinafter, interferometric transducers can be used to narrow a region within which a detected object can be determined to be situated. WhileFIG. 2Ashows a towfish212with the acoustic transducer102a, the acoustic transducer102acan also be mounted to a boat (illustrated inFIG. 2B) or any other water vessel, for example.

FIGS. 2B-2Cillustrate angles relevant to side-scan sonar imaging. In particular, a boat218having a front section220, a rear section222, and a top section224travels in the Y direction on the water. A dashed line226joins a transducer (not shown) on the boat218and a detection object228(a fish inFIG. 2B). A dashed line230extends from the boat218in the direction of the detection object228but in the X-Y plane.FIG. 2Bincludes a side view of the boat218traveling in a Y direction on water in the X-Y plane. For side-scan imaging, an azimuth angle232between the Y-axis and the line230is well known due to the shape of the radiation pattern illustrated inFIG. 2A. However, the angle θ236, which is equal to 90° minus the angle234from a nadir (−Z axis), is not well known unless interferometric sonar measurements are used.FIG. 2Cis a top view of the boat218and object228.

FIG. 2Dillustrates a typical acoustic transducer configuration including the two rectangular elements102aand102b, which can be used for interferometric side-scan sonar measurements to determine the angle θ (shown inFIG. 2B) with greater accuracy.

FIGS. 2E-Fare side-view and rearview illustrations showing where the transducer elements102aand102bare typically mounted to a boat for side-scan imaging.

FIG. 2Gillustrates typical signals and geometry using a dual transducer configuration such as the configuration illustrated inFIGS. 2D-F. The left side ofFIG. 2Gillustrates a transmission signal238, which can be produced by the transducers102a-bor by a separate transducer (not shown). The transmission signal238and received signals240aand240breceived by the transducers102aand102b, respectively, have wavelength λ. A difference242and target distance measured by the receivers102aand102bcan be used to determine the arrival angle θ236, as understood in the art of interferometric transducers. Namely, a time-of-arrival difference between the signal received at the transducer102aand the signal received at the transducer102bcan be used to determine a path length difference Δ which in turn can be used to determine the arrival angle θ. The target angle from the nadir is the arrival angle θ236plus a mounting angle γ237illustrated inFIG. 2F.

FIGS. 2H-Killustrate a phase wrapping issue that can occur in interferometric sonar measurements when the arrival angle θ is too large.FIG. 2His the same diagram shown on the right side ofFIG. 2G, in which the arrival angle θ236is relatively small. When 0 is small, the path difference Δ for radiation226arriving at the transducers102aand102bis correspondingly small. In contrast,FIG. 2Ishows corresponding values when the arrival angle θ is relatively large. In this case, the path difference for radiation226arriving at the two detectors102aand102bis relatively large.

FIGS. 2J and 2Killustrate further effects of small and large arrival angles, respectively, on interferometric measurements. Here, it should be noted that in practice, a phase difference242between signals received at the transducers, rather than a time difference, is used to determine the angle θ.FIG. 2Jillustrates the phase difference242for the case of the relatively small arrival angle inFIG. 2H. In this case, the phase difference242between the signals240aand240breceived at the transducers102aand102b, respectively, is 90°. Because the difference 90° is smaller than a full cycle of 360°, the arrival angle θ can be determined unambiguously. In contrast, inFIG. 2K, corresponding to the large arrival angle θ inFIG. 2I, the phase difference242for the arriving signals is 450°=360°+90°. Thus, the phase-based measurement inFIG. 2Kyields the same results as the phase-based measurement inFIG. 2Jwhen calculations to determine the angle θ are performed, and the angle θ can be erroneously calculated to be the same for bothFIGS. 2H and 2I. In other words, reflections arriving from steep arrival angles can be processed as arriving from shallow arrival angles. It is desirable, therefore, to have transducer receivers with narrower beams in the XZ plane than the large fan-shaped beams214illustrated inFIG. 2A. A phase difference of 360° is the limit at which phase can be determined unambiguously, and an arrival angle θ corresponding to the phase difference of 360° is called the wrapping angle. When the phase difference242exceeds 360°, as shown inFIG. 2K, this is referred to as phase wrapping.

The desirability for relatively narrower beams in the XZ plane and for close spacing of transducers can also be explained in further reference toFIGS. 1A and 1B, as follows. InFIG. 1A, the three channels (from three acoustic elements102a-c) are at different vertical axis positions, as illustrated in the figure. Note that each transducer (channel)102a,102b, or102calso has a small finite vertical width. The width for each element could be made smaller, making the vertical beam for each element wider. Conversely, the respective vertical widths could be made larger, thereby making the respective vertical beams for each element narrower. This is a consequence of the inverse relationship between wavelength λ, and aperture length L, and the resulting radian beamwidth λ/L.

It is usually preferable for a mapping sonar system to have its transmit, receive, or both apertures designed to ignore echoes from zenith or from nadir, with respect to the sonar's main radiation lobe direction, because these extraneous echoes corrupt the desirable echoes from the intended mapping region in the forward- or side-looking direction. This means that the vertical aperture of a single acoustic channel should have a height extent (to control its own vertical beamwidth), and shape (to control its own vertical side lobes) to reduce energy from directly overhead (zenith) surface scattering or from the directly beneath from bottom scattering, or multiple bounces from both. In a single channel system, without interferometry, this imposes no restrictions other than transducer space. However, in an interferometric system with more than one transducer, the desired single-channel vertical beam width and side lobe control, can conflict with the center-to-center vertical separation of the interferometer channels. Embodiments described herein have the advantage of providing favorable characteristics simultaneously for side lobe control, center-to-center interferometer separation, and main beam width.

FIG. 1Billustrates how the desired vertical separation between interferometer channels can be maintained, while at the same time having some control over each channel's vertical beamwidth. InFIG. 1B, the effective vertical center of channels104a,104b, and104cis maintained at the same position as inFIG. 1A, even though each element is geometrically divided into two pieces that are electrically coupled.

FIGS. 2L and 2Mare used to illustrate far-field phase wrapping conditions for two-element and three-element transducer arrays. InFIG. 2L, which illustrates the two-element case, the two transducer elements102aand102bhave a separation d. In a two-element array, the phase will wrap when the arrival path difference Δ is equal to one wavelength λ (seeFIG. 2G) of the acoustic radiation226received. This relationship can be expressed as d sin θ=λ

θ=sin-1⁡(λd).
Thus, if d>λ, there will be phase wrapping.

FIG. 2Millustrates a common three-element configuration (also illustrated inFIG. 1A). In the three-element case, the same phase wrapping conditions described above for the two-element case, namely, that there is phase wrapping for d>λ, apply for each pair of elements used in the array. Varied spacing between pairs of elements allows greater flexibility in measurements. For example, the elements102aand102b, with the separation of 0.5λ, can be used. In addition, a measurement can be obtained with the pair of elements102band102c, with the separation of λ, or with the elements102aand102c, with a total spacing of 1.5λ. Typically, the outer pair102aand102care usually not placed less than one wavelength apart, because the receivers need to be large enough to provide sufficient sensitivity. Thus, phase wrapping is also an issue in the three-element design ofFIG. 2M. For example, the elements102a-102c, with the largest spacing 1.5λ, have a 42° wrapping angle (for a sonar used in water, with nominal 1500 m/s sound speed). Thus,FIGS. 2L and 2Mfurther illustrate the desirability of having transducer receivers that are more directional in the XZ plane illustrated inFIG. 2Ato avoid phase wrapping.

FIG. 2Nincludes a table that lists examples of two-element interferometer phase wrapping angles. The 10 dB and 20 dB rejection angles for implementations with standard rectangular and circular elements are also shown. For example, if the center-to-center spacing d between the elements is 1.5λ, the interferometer phase wrapping angle is 42°. The largest rectangular elements that can be used are 1.5λ, wide (no gap between) which have 10 dB and 20 dB rejection angles of 29° and 37°, respectively, in water. Similarly, the largest circular elements that can be used are 1.5λ, in diameter, which have 10 dB and 20 dB rejection angles of 35° and 47°, respectively, in water.

The above examples illustrate that for two-element interferometers, if 10 dB rejection is sufficient, standard transducer elements such as elements102a-cinFIG. 1A, will be sufficient. However, if 20 dB rejection is required, for example, disk-shaped elements will not be sufficient, and rectangular-shaped elements barely meet the 20 dB rejection requirement under the best-case scenario of having no gap between the elements. In addition, the rejection angles listed in the table inFIG. 2Nare for the main lobe of radiation. Rectangular elements have a peak side lobe amplitude of approximately −13 dB and thus would fail the 20 dB rejection requirements in many circumstances. Furthermore, these considerations are based on calculations using the widest possible elements. In practice, however, neither standard disk-shaped elements nor standard rectangular elements are likely to work when 20 dB rejection is required.

For the three-element case illustrated inFIG. 2M, the phase wrapping angle in water is 0=42°, assuming that the elements102aand102care used together. The maximum transducer element widths are 0.5λ, the limitation imposed by the adjacent transducer elements102aand102bwith the closest spacing. Both standard disk and standard rectangle elements have very wide receive beams, namely a maximum rejection of 2.8 dB at 90° for a disk shaped element of diameter 0.5λ, and a maximum rejection of 3.9 dB at 90° for a rectangular element of width 0.5λ. Thus, in the three-element design ofFIG. 2M, neither standard element can provide 10 dB of rejection at any angle, and it should also be noted that both standard elements have very little rejection (1 to 2 dB) at 42°. Thus, if more than 10 dB of rejection is required, both standard rectangular and standard disk elements are unacceptable.

For an arbitrary array of standard elements with largest element spacing d, the phase wrapping angle is

θ=sin-1⁡(λd).
Rectangular elements would need to be of width W>0.738d for a 10 dB rejection at the wrapping angle, or W>0.908d for 20 dB rejection at the wrapping angle. Disk-shaped elements would need to be of diameter D>0.869d for 10 dB rejection at the wrapping angle, or D>1.09d for 20 dB rejection at the wrapping angle (physically impossible). For transducer element arrays with three or more elements, meeting all of those requirements simultaneously is impossible. Furthermore, even for two-element arrays, practical spacing requirements can make some or all those requirements unobtainable.

As described in part in conjunction withFIGS. 1B-E, embodiments of the invention can overcome these difficulties by stacking or dividing transducer elements, for example. However, both these and additional benefits can also be obtained by embodiment devices that include warped transducer elements or transducer elements with protrusions, for example. Embodiments can significantly narrow radiation detection patterns in the XZ plane while maintaining small side lobes in radiation detection patterns in the YZ plane, for example.

Reshaping of the aperture (transducer shape) can allow further control of each channel's vertical beamwidth and beam pattern structure (i.e. the main lobe plus all side lobes). The connection of aperture shape, main lobe width, and side lobe locations and strength is known in the context of Fourier transforms. In signal processing, a time domain waveform can be filtered using windowing techniques together with the Fourier transform. In other applications such as radar, acoustics, and optics, a spatial Fourier transform (i.e. a beam pattern) occurs when an aperture sends (or receives) energy from the far field of the aperture. The beam pattern shape may be altered by altering the window shape. Thus, principles used in signal processing can similarly be used in radar, acoustics and optics, because they use similar mathematics.

To avoid conflict between the channels' center-to-center spacings, while also altering the window shape for each channel to affect each channel's beam pattern in a desirable way, taper shapes such as the Gaussian shape illustrated inFIG. 1C, for example, may be used. To prevent each channel from physically interfering with adjacent channels, the shapes can be warped such that each effective channel center vertical position is maintained for the needs of interferometry, while at the same time the shape and warping achieve the desired vertical beam pattern (main lobe width, plus side lobe control).

Embodiment devices that are warped, such as shown inFIG. 1C, have significant benefits in addition to devices that are not so shaped, even beyond the advantages of embodiment devices with rectangular shapes, as shown inFIG. 1B, for example. InFIG. 1B, the vertical aperture has center-to-center spacings to meet interferometer system needs, and it also provides control of the vertical beam pattern shape because it has a taller vertical span, thus allowing a narrower vertical beam. However, for some vertical angles, side lobes in the radiation pattern can increase. Warped configurations such as shown inFIG. 1Calso provide a continuous aperture distribution, without spatial breaks, so that the side lobe radiation pattern structure will decrease. The Gaussian shape of the device inFIG. 1Calso controls the location and level of side lobes in both vertical and horizontal planes. This is an additional benefit not obtained by either prior art devices, such as illustrated inFIG. 1A, nor embodiment devices having only split elements, as illustrated inFIG. 1B.

FIGS. 3A-3E and 5A-5Einclude examples of prior art transducer shapes and their corresponding radiation detection distributions for comparison purposes, whileFIGS. 4A-Eillustrate an example diamond element shape and corresponding radiation detection distributions for comparison purposes.

FIG. 3Aillustrates the element shape for the rectangular element102ashown inFIG. 1A. As illustrated in the graphs inFIG. 3A, the rectangular element shape102agives rise to a radiation detection amplitude distribution344in the X dimension, with another rectangular radiation detection amplitude distribution346in the Y dimension. These rectangular amplitude distributions344and346can also be termed “boxcar” distributions, as shown inFIG. 3B.

FIGS. 3B and 3Cillustrate the relationship between the boxcar or rectangular amplitude distribution of a receiver or source and the far field polar radiation distribution. Namely, the amplitude distribution and far field radiation pattern are related by Fourier transform, as understood in the art of sonar transducer construction. The rectangular “boxcar” distribution348shown inFIG. 3Byields a far-field sinc radiation pattern350, illustrated inFIG. 3C, when a spatial Fourier transform is performed. It should be noted that the amplitude distribution in348is shown as a function of time, and the Fourier transform leads to the temporal frequency distribution350shown inFIG. 3C. However, the relationship between a spatial amplitude distribution (such as the Y distribution346) and the spatial frequency distribution, is the same, so the term spatial Fourier transform is used.

FIGS. 3D-Eare graphs illustrating the X dimension and Y dimension polar radiation distributions, respectively, calculated by spatial Fourier transform and ray tracing for the element shape102a. In particular,FIG. 3Dis a polar graph illustrating a Y dimension polar radiation distribution352in the XZ plane illustrated inFIG. 1A, with 0° being centered on an axis normal to the transducer surface. The contours in the plot are in dB units. As can be seen inFIG. 3AandFIG. 3D, a relatively narrower X dimension amplitude distribution corresponds to a relatively wide radiation distribution352.

FIG. 3Eis a polar graph illustrating a Y dimension polar radiation distribution354in a plane intersecting the Y-axis illustrated inFIG. 2A. The highest side lobe356in the radiation pattern354reaches a level of −13 dB.

FIGS. 4A-Eare similar to theFIGS. 3A-3E, except that theFIGS. 4A-Ecorrespond to an existing diamond-shaped element460illustrated inFIG. 4A. The X amplitude distribution444and Y amplitude distribution446are triangular, andFIGS. 4B-Cillustrate that a triangular amplitude distribution448corresponds to a sinc2frequency distribution450.FIG. 4Dshows the X dimension polar radiation distribution pattern452corresponding to the X dimension amplitude distribution444shown inFIG. 4A.FIG. 4Eillustrates a Y dimension polar radiation distribution pattern454corresponding to the amplitude distribution446shown inFIG. 4A. The highest side lobe456for the Y distribution454reaches a level of −27 dB. While the side lobes ofFIG. 4D(including the highest side lobe456) are smaller than those ofFIG. 3Bfor the Y dimension, the X dimension pattern452is wider than the corresponding pattern inFIG. 3Dfor the rectangular shape. Thus,FIGS. 4A and 4Dillustrate that an arbitrary modification of the element shape102inFIG. 3Awill not lead to helpful results as to narrowing the X dimension radiation pattern to make it more directive as desired.

FIGS. 5A-Eare similar toFIGS. 3A-Eand4A-E, except thatFIGS. 5A-Ecorrespond to a tapered element560illustrated inFIG. 5A. The general shape of the element560has been used to decrease side lobes in the Y dimension, as illustrated inFIG. 5E, as well as narrow the radiation distribution in the X dimension, as illustrated inFIG. 5D. The element560, like the element460inFIG. 4A, has tapered ends562. The tapered ends562are smaller in local width than a center section574of the element. While the ends562come to a point, in other embodiments, tapered ends do not come to a point. Thus, as used herein, “tapered” or “tapered ends” refers to ends of a transducer element having a smaller width than a center section of the transducer element. The element560has an X amplitude distribution544and a Y amplitude distribution546. The Y amplitude distribution546has a largely Gaussian shape. As illustrated inFIGS. 5B-C, the Fourier transform of the Gaussian function548is also Gaussian function550.

FIG. 5Dshows an X dimension polar radiation pattern distribution552corresponding to the amplitude distribution544shown inFIG. 5A.FIG. 5Eshows a Y dimension polar radiation distribution pattern554corresponding to the amplitude distribution546shown inFIG. 5A. It should be noted that, as shown inFIGS. 5B and 5C, a full Gaussian amplitude distribution corresponds to a full Gaussian radiation pattern with no side lobes. However, for any real implementation of an element, a Y amplitude distribution will be truncated, as shown in the Y distribution546inFIG. 5A. Thus, in real implementations, there will be side lobes in a Y radiation distribution, as shown inFIG. 5E, but the side lobes can be made very small, such as the highest side lobe556around −30 dB.

FIGS. 6A-Care used to show how the divided (split) elements104a-ccan be used to narrow X dimension polar radiation distribution patterns according to embodiments of the invention.FIGS. 6A-Calso include shapes and graphs corresponding to the standard rectangular element shape102afromFIG. 1A. The element104ais divided into two spatially separated element portions that are electrically coupled to each other. This is in contrast to the standard element shape102a, which is one, undivided structure. The electrical coupling between the element portions104acan be done via an electrical circuit, for example, as illustrated inFIG. 12A, for example, which drives both element portions together or receives detected signals from the element portions104a, treating them as a single element. Element portions104aandFIG. 6Aare equal in size, but in other embodiments, the element portions104aare not equal in width.FIG. 6Aalso shows X dimension amplitude distributions344and644, corresponding to the standard element102aand divided element104a, respectively. The rectangular element portions104aare rectangular in shape and have longer sides oriented parallel to one another and also parallel to an axis along which the element104ais divided, namely an axis parallel to the y axis and centered at zero along the x axis inFIG. 6A.

FIG. 6Billustrates X dimension polar radiation distribution patterns352and652, corresponding to the amplitude distributions344and644, respectively. As can be seen inFIG. 6B, one advantage of the divided element design is that it achieves some narrowing in the X dimension radiation pattern652compared with the pattern352for the standard element102a. This corresponds to the wider overall X dimension amplitude distribution644achieved by dividing the element104ainto two portions. While the element104ais divided into two portions, and other embodiments, the element is divided into three or more portions. The narrowing of the spectrum and652shown inFIG. 6Bis particularly significant in the range between 40° and 60°.

FIG. 6Cillustrates how three divided elements can be incorporated into a three-element transducer. Namely, in addition to the element portions104a, other divided elements104band104care also used.FIG. 6Cillustrates acoustic centers668a-ccorresponding to each of the standard elements102a-cand each of the divided elements104a-c. The acoustic center668a, for example, is located between the divided element portions104a, with the other acoustic centers668b-clocated between respective pairs of element portions104band104c, respectively.

One advantage of the embodiment split elements104a-cillustrated inFIGS. 6A and 6Cis that separations between acoustics centers of the elements can be maintained, relative to the standard elements102a-c, even while the X amplitude distribution is broadened relative to the standard element case. Thus, the separation667abetween the acoustic centers668aand668bis the same for the standard and divided elements ofFIG. 6C. Similarly, the separation667bbetween the acoustic centers668band668cis the same for both the standard and divided elements ofFIG. 6C. In some embodiments, such as the split-element design shown inFIG. 6C, at least one element portion of one transducer element is in physical contact with at least one element portion of another divided transducer element. InFIG. 6C, this is the case for one element portion of transducer element104aand one portion of transducer element104b.

FIGS. 7A-Care similar to the right sides ofFIGS. 6A-C, respectively. However,FIGS. 7A-Cinclude illustrations corresponding to an embodiment warped element768ashown inFIGS. 7A and 7C.FIGS. 7A-Ccan be compared and contrasted with the left sides ofFIGS. 6A-C(corresponding to the standard rectangular element shape102afromFIG. 1A), respectively.

As used herein, “warped” denotes an element shape that has an overall width greater than any local width of the element, as further illustrated inFIG. 8D. Warped elements can further lack inversion symmetry with respect to the Y axis, as illustrated inFIG. 8D. It should be understood that warping and tapering are two different characteristics. Tapered elements, as described hereinabove in conjunction withFIG. 4A, include elements with ends that are narrower (smaller in width) than a center portion of the element, while warped elements include those with an overall width greater than any local width of the element, as illustrated further inFIG. 8D. A warped element, such as the warped element768a, contrasts with elements that are only tapered but not warped, such as the tapered element560inFIG. 5A, for example. It will be noted that the element560inFIG. 5Ahas inversion symmetry and has an overall width that is the same as the maximum local width. The element768aincludes two warped edges772aand772bat opposing sides772aand772bof the radiative surface of element768a. Each of the warped edges has the same shape, with the shape having the same function parameters. The element768ais not tapered; namely, ends762have widths equal to the width of the center portion774. However, in other embodiments, such as the embodiment shown inFIG. 8A, warped elements are also tapered. Further, in some embodiments, such as106a-c, each tapered end comes to a point, joining the two warped edges at each tapered end. Furthermore, in other embodiments, the edges772aand772bhave different functional shapes. For example, in one embodiment, the edges772aand772bhave shapes described by similar functions but with different functional parameters. In yet other embodiments, one edge has one functional shape, while the other edge has a different functional shape.

An X dimension radiation amplitude distribution744for the element768ais broader than the corresponding X dimension amplitude distribution344for the rectangular element102a. The broader amplitude distribution744leads to a corresponding narrower X dimension polar radiation pattern752inFIG. 7B.FIG. 7Cillustrates how the warped element768acan be used with similar warped elements768b-cfor use in a three-element transducer. Because the elements768a-care warped, the corresponding dashed lines769a-cdo not represent acoustic centers of the elements, as the dashed lines668a-cdo for the standard elements102a-c. Acoustic centers (not shown) for the elements768a-cwould be shifted to the right of the dashed lines768a-c, respectively. However, the separations667aand667bbetween adjacent element768a-band768b-c, respectively, do represent the relative separation between acoustic centers of the adjacent warped elements. Thus, the acoustic centers (not shown) for the warped elements768a-ccan have the same spacing as the standard rectangular elements102a-c.

FIGS. 8A-Dpertain to an embodiment transducer element860, shown inFIG. 8A. The element860is tapered, because it has ends862smaller in width than a center portion874of the element. Two edges872aand872bof the elements both have substantially similar shapes, but the shape of the edge872afollows a shallower curve than the edge872b. The edges872a-bare edges of a radiative surface864of the element860. The element860has a substantially Gaussian Y amplitude distribution846and an X distribution844. X and Y dimension polar radiation distributions852and854, respectively, are illustrated inFIGS. 8B and 8C, respectively. The Gaussian Y amplitude distribution846is truncated, as further described hereinabove in relation toFIG. 5A.

FIG. 8Dillustrates that warped elements are defined, as used herein, as having an overall width of a radiative surface of the element being greater than any local width of the radiating surface of the element. InFIG. 8D, for example, an overall width873is greater than any of the local widths863a-c. Other example transducer elements that are warped and share this characteristic of overall width include the elements768a-c(seeFIGS. 7A and 7C) and elements1202a-c,1203a-c, and1205a-c(described hereinafter in conjunction withFIGS. 12A, 12B, and 12C, respectively).

Warped edges of a transducer element are edges on opposing sides of a radiative surface of a transducer element that define shapes that result in the transducer element being warped. For example, the warped edges872aand872b(illustrated inFIG. 8A) are edges on opposing sides of the element860that define shapes causing the element860to be warped (having an overall width of the radiative surface of the element that is greater than any local width of the radiative surface of the element). Some warped edges define shapes that are not smooth. Furthermore, some warped edges define shapes that are periodic, with warped sawtooth or other warped periodic edges with periodic elongations, for example. One example is described hereinafter in conjunction withFIG. 12C. InFIG. 12C, a sawtooth edge1284and smooth edge1213aare warped edges and define a warped element1205ahaving periodic elongations from the sawtooth edge1284.

FIG. 8Dalso illustrates that warped elements can lack of inversion symmetry. For example, the element860is shown on the left, while a mirror image860′ of the element is shown on the right side of the Y axis illustrated inFIG. 8D. It should be noted that some elements, such as elements110(seeFIG. 1E) and1110a(described hereinafter in conjunction withFIG. 11B), similarly lack inversion symmetry. However, elements110and1110aare not warped because they do not have an overall width of a radiative surface greater than any local width of the same radiative surface of the same element.

FIG. 9illustrates effects of tapering and warping elements on X and Y amplitude distributions and X and Y polar radiation distributions. In particular, non-warped elements102aand560are shown on the right ofFIG. 9, while warped elements902and860are shown on the right ofFIG. 9. The elements102aand902at the top ofFIG. 9are not tapered, while the elements560and860at the bottom ofFIG. 9are tapered.

The upper left ofFIG. 9illustrates the rectangular element shape102a, X and Y amplitude distributions344and346, respectively, and X and Y polar radiation distributions352and354, respectively, as also illustrated inFIGS. 3A, 3D, and 3E. In contrast to the rectangular element102a, the element902in the upper right ofFIG. 9is warped. While a Y amplitude distribution946and Y polar radiation pattern954are not substantially different from those of the standard element102a, an X amplitude distributional944is substantially broader than the X distribution344for the rectangular element102a, and an X polar radiation distribution952is substantially narrower than the distribution352corresponding to the rectangular element.

The tapered element560at the bottom left ofFIG. 9has a broader Y amplitude distribution546and narrower Y axis polar radiation distribution554compared to the Y distribution346and Y radiation distribution354for the standard element shape. However, an X amplitude distribution544corresponding to the tapered element560is overall narrower than the X amplitude distribution344for the standard element102a. Furthermore, an X polar radiation distribution552corresponding to the tapered element560is broader than the X radiation distribution352for the standard element. This illustrates that tapering alone may not necessarily narrow X radiation patterns, as desired for increased directionality in acoustic receivers, as described hereinabove. The element shape860at the lower right ofFIG. 9and is the same element illustrated inFIG. 8A. Notably, the X polar radiation distribution852is even narrower than the corresponding distribution952for the element902that is only warped. Furthermore, the wide polar distribution854has significantly smaller side lobes than the corresponding Y polar radiation distribution954for the element902that is only warped. In particular, the X polar radiation distribution is attenuated to a level of about −20 dB for arrival angles steeper than about +/−60°. Thus, warping elements can provide significant advantages in directivity of sonar transducer elements, and tapering in addition to warping can provide yet further advantages in narrowing the distributions.

FIGS. 10A-Cillustrate how transducer elements can be stacked to increase sensitivity and decrease X axis polar radiation distributions.FIG. 10Aillustrates the rectangular elements102a-cfor comparison with stacked elements108a-cillustrated inFIG. 10B. While rectangular stacked elements are illustrated inFIGS. 10B-C, the stacking principles illustrated can apply to many embodiments described herein that use non-rectangular element shapes. For example,FIG. 12Eillustrates warped elements that have the additional advantages of stacking, as described hereinafter. The coordinates1021aapply to the stacked elements on the left ofFIG. 10B. The right side ofFIG. 10B, to which coordinates1021bapply, show and end-on view of the stacked elements108a-c.FIG. 10C, to which coordinates1021capply, is a rotated and on view of the stacked elements108a-c.

The elements108a-care much wider than the standard elements102a-cofFIG. 10A. As understood by those skilled in the art of acoustic transducer design, widening the transducer elements in the X direction significantly narrows the corresponding radiation pattern in the XZ plane. Furthermore, widening the transducer elements can also increase transducer sensitivity and increase signal-to-noise ratios. However, any widening of the standard elements102a-cis limited by potential physical interference between the elements. However, in accordance with embodiments of the invention, transducer elements can be stacked to allow increased size and overlap between the elements.

WhileFIG. 10Bshows three stacked elements108a-c, in other embodiments there are only two stacked transducer elements. Furthermore, while the stacked elements108a-care rectangular, in other embodiments, stacked transducer elements can have other shapes, as further illustrated inFIGS. 12B, 12D, and 12E, for example. Furthermore, there is flexibility in spacing of acoustic centers of the stacked elements. Acoustic centers1068a-c, shown at the right ofFIG. 10B, correspond to respective stacked elements108a-c. Because the elements108a-care stacked, the spacing (not shown) between the acoustic centers1068a-ccan be the same as the spacings667a-bshown inFIG. 10Afor the standard elements, for example.FIG. 10Balso illustrates acoustic radiative surfaces1082a-ccorresponding to each of the transducer elements108a-c, respectively.

FIG. 10Cillustrates how the example stacked elements108a-ccan form part of stacked polyvinylidene difluoride (PVDF) layers1076a-c, respectively. Inactive portions1080of the PVDF layers1076a-care not configured to produce or detect acoustic radiation. In contrast, the stacked elements108a-c, corresponding to active portions of the PVDF layers1076a-c, are configured to generate and/or detect acoustic radiation. The active portions (stacked elements) of the PVDF layers1076a-ccan be defined, for example, by thin conductive layers deposited onto intended active portions108a-cof the PVDF layers1076a-c. Such deposited conductive layers are not shown as inFIG. 10Cbut are further illustrated inFIGS. 13A-B.

Most sound waves travel through PVDF. PVDF has an acoustic impedance close to that of water. Water has an acoustic impedance of approximately 1.5 MegaRayleighs (MRayls), while PVDF has an acoustic impedance of approximately 1.2 MRayls. This property enables each of stacked PVDF elements to react to incident acoustic radiation. In contrast, lead zirconium titanate (PZT) ceramic transducers reflect a very high percentage of incident sound waves, with an acoustic impedance of approximately 30 MRayls. Thus, PVDF layers, for example, can greatly facilitate maintaining sensitivity of stacked transducer elements. Furthermore, while a certain amount of reflection and absorption loss may be expected at each stacked element, losses can be calibrated and offset by calculations in a processor, for example. This is true even where reflection or absorption loss is different for each layer.

The PVDF layers1076a-cinFIG. 10Care supported by an absorptive backing layer1078. As sound waves pass through the PVDF layers, they will generally be reflected from a backing material and combined with incident sound waves. The impedance of the backing layer determines the polarity of the reflected sound waves, and reflection polarity and half length determine how the reflected and incident sound waves will combine (i.e. constructively or destructively). Furthermore, because path lengths to the backing material can be different for each PVDF layer, sound wave reflections can combine with incident sound waves differently for each PVDF layer. To solve this problem, the backing layer1078can be designed to be absorptive to minimize reflections from the backing. In particular, backing materials with high internal damping (e.g., sorbothane and some types of rubber) and acoustic impedance is close to that of PVDF can be used for the absorptive backing1078.

Acoustic radiative surfaces1082a-ccorresponding to respective transducer elements108a-chave some overlap with each other in the X direction. For example, in overlap1023is indicated inFIG. 10Cbetween acoustic radiated surfaces1082band1082cof elements108band108c, respectively.

FIG. 11Aillustrates the interdigital transducer elements110a-cofFIG. 1E. In the embodiment ofFIG. 11A, the periodic elongations186aof element110aare offset in the Y direction from the periodic elongations186bof the element110b. Furthermore, the periodic elongations186a-bare of equivalent shapes such that the periodically elongated edge of the element110aand the periodically elongated edge of the element110bform a common edge184when the elements110aand110bare fit against each other. While the embodiment transducer ofFIG. 11Aincludes an additional third element110cwith periodic elongations186c, in other embodiments, there are only two transducer elements. An example of this is illustrated further inFIG. 11B, which is described hereinafter. The periodic elongations186a-cof the elements110a-cwiden the x-axis amplitude distributions of the elements, which narrows the X polar radiation distributions of the elements. Furthermore, the spacing between X axis acoustic centers (not shown inFIG. 11A) of the elements110a-ccan be maintained while the X axis amplitude distributions are broadened. An electrical drive circuit (illustrated inFIG. 12A) can be configured to drive the transducer elements110a-cat the same frequency if desired. Furthermore, the electrical circuit can be configured to drive the different transducer elements110a-cwith mutually distinct phases, if desired, to steer the transmitted beam in the XZ axis. In some embodiments, separate transducers (not shown) are used to transmit acoustic radiation, while the elements110a-care used only for detection of acoustic radiation.

FIG. 11Billustrates an embodiment transducer including two transducer elements1110aand1110b. The transducer element1110bis rectangular, while the adjacent element1110ahas sinusoidal periodic elongations1186d. Thus, as illustrated inFIGS. 11A and 11B, acoustic transducers can have acoustic radiative surfaces with periodic elongations having a variety of shapes. For example, periodic elongations can be sawtooth shaped, as illustrated inFIGS. 12C-D.

FIGS. 12A-Eillustrate that embodiments can include transducers having transducer elements with more than one of the characteristics of being warped, divided, stacked, and having periodic elongations.

FIG. 12Aillustrates transducer elements1202a-cthat are both warped and divided (split). The element portions1202a, for example, are spatially separated but electrically coupled to each other. In the embodiment ofFIG. 12A, an electrical circuit1215is configured to provide the same electrical drive signal238to the element portions1202a, thus electrically coupling the element portions1202a. In other embodiments, an electrical circuit can be configured to process signals collected by the element portions1202awhen incident radiation is detected. In other embodiments, there is an electrical junction connecting the two element portions1202a. In each case, an electrical circuit electrically couples the two spatially separated element portions1202atogether, and electrical coupling as used herein should be understood to include using any means to treat spatially divided element portions such as the element portions1202aas a single element for purposes of transmitting or detecting acoustic radiation. The elements1202a-care warped, as described in conjunction withFIGS. 7A, 8A, and8D. Furthermore, the divided elements1202aand1202bare interleaved with one another because one element portion1202bis situated between the element portions1202a.

FIG. 12Billustrates a transducer configuration with transducer elements1203a-cthat are both warped and stacked. The elements1203a-care warped in accordance with the definitions provided inFIGS. 7A, 8A, and 8D. In particular, a lower edge1225indicates that the acoustic radiative surface of the element1203aoverlaps with the acoustic radiated surface of the element1023b. Thus, in the embodiment ofFIG. 12B, the benefits of both warping and stacking, as described herein above, can be obtained.

FIGS. 12C-Dillustrate embodiment transducers having transducer elements with periodic elongations protruding from acoustic radiative surfaces of the elements. Elements1205a-cof the embodiment inFIG. 12Chave sawtooth shaped periodic elongations, including the periodic elongations1286aof the radiative surface of element1205a. But the periodic elongations of elements1205a-bare offset from one another along the X direction such that when the elements1205aand1205bare brought together, they are shaped to form a common edge1284. Furthermore, each of the elements1205a-cincludes a warped edge1213a-c, respectively, on an opposing side of the transducer element from the edge with periodic elongations. Thus, the embodiment ofFIG. 12Ccan be used to obtain the benefits of both warping, as described inFIGS. 7A-Cand8A-D, for example, and also periodic elongations, as described inFIGS. 11A-B, for example.

FIG. 12Dillustrates transducer elements1207a-cthat both have periodic elongations and are stacked to obtain the benefits of both configurations. Periodic elongations1286bof the transducer1207b, for example, are sawtooth shaped. A lower edge1288of the element1207aillustrates how radiative surfaces of the elements1207aand1207bare configured to overlap.

FIG. 12Eillustrates transducer elements1209a-cthat are warped, tapered, and overlapping. In particular, each of the transducer elements1209a-cis similar to the transducer element860illustrated inFIG. 8A, in that the elements1209a-ceach include two warped edges on opposing sides of the transducer. Each of the elements1209a-chas two tapered ends1262that are smaller than center portions (e.g., seeFIG. 8A) of the respective elements1209a-c. The lower edge1288of the element1209aillustrates how the elements129aand129bare configured to overlap. An electrical circuit1215is configured to receive respective acoustic radiation signals1227aand1227bfrom the transducers1209a-band to detect the phase difference between the received signals to perform interferometric sonar measurements, as described in conjunction withFIGS. 2D-M.

Although not shown inFIG. 12E, the electrical circuit1215can also be configured to receive transducer signals from element1209cand or to drive elements1209a-cto produce acoustic radiation. Drive signals (not shown) can be configured to drive the elements1209a-cat the same frequency to produce acoustic radiation. The elements1209a-ccan also be driven with the same electrical phase, or with mutually distinct electrical phases to steer transmitted radiation, as understood in the art of acoustic transducers.

FIG. 12Fis a graphical illustration of an embodiment set of transducers with non-contiguous sub-elements interspersed with each other in the same plane. In particular, a first transducer element1291aincludes a plurality of sub-elements1292athat, together, form the overall shape of the warped element1291a. The sub-elements1292acan be formed by using printed circuit board technology, for example, to electroplate small areas of a PVDF surface, for example. The sub-elements1292acan be non-contiguous with each other such (spatially separated from each other) and yet electrically coupled to each other by using printed circuit board technology or wire boding technology, for example, as will be understood by those skilled in those respective arts. The sub-elements1292acan collectively cover a percentage of the overall area of the element1291a, such as 50%, 40%, 30%, 20%, or 10%, for example

The embodiment ofFIG. 12Falso includes a second transducer element1291boccupying a common surface with the element1291a. In particular, the element1291bis in the same plane as the element1291a. The element1291bincludes a plurality of sub-elements1292bthat, together, form the overall shape of the warped element1291b. The sub-elements1292bare non-contiguous, similar to the sub-elements1292a.

The elements1291aand1291boverlap each other in an overlap region1290, where the sub-elements1292aand1292boccupy the same plane. As described hereinabove, the sub-elements can occupy overall, for example, only 40% of the respective areas of the respective transducer elements, such that the sub-elements1292ado not physically contact the sub-elements1292band are also electrically isolated from the sub-elements1292b.

Thus, the embodiment ofFIG. 12Fillustrates another configuration that can be used to bring transducer elements closer together to avoid aliasing, while avoiding any need for stacking elements, as illustrated inFIG. 1D, for example.

FIGS. 13A-Billustrate an additional advantage of embodiments of the invention, namely that transducer elements can be printed onto a sheet of PVDF1390by depositing a metallic layer, for example.FIGS. 13A-Binclude warped, tapered elements1311a-c. The elements1311a-care similar to the warped elements106a-cillustrated inFIG. 1C, except that the curve functions representing the top and bottom edges on opposing sides of the radiative surfaces of the elements1311a-care shallower and broader than functions corresponding to the warped elements106a-cofFIG. 1C. A ruler1392is shown in the photograph ofFIG. 13Bfor size perspective.

In the embodiment shown inFIG. 13B, the three warped and tapered elements1311a-care plated onto the sheet1390of PVDF on only one side of the sheet, and only in the regions intended to constitute active elements1311a-c. Furthermore, as illustrated inFIGS. 13A-B, multiple transducer elements can be plated onto a single sheet of PVDF, providing efficiency and ease of manufacture. Other characteristics and advantages of PVDF are described hereinabove in the description ofFIG. 10C. One advantage of PVDF elements is that they can form part of PVDF layers that can be easily stacked for embodiment transducers including stacked elements. Stacked elements can include rectangular elements, as shown inFIGS. 10B-C, warped elements, as shown inFIGS. 12B and 12E, and elements with periodic elongations such as sawtooth elements, as illustrated inFIG. 12D, for example. In addition, as described hereinabove, PVDF enables transducer elements to be manufactured more easily and efficiently. While all or most of the transducer and elements described herein above can be ceramic elements, such as piezoelectric transducers (PZTs), or capacitive transducer elements, many of the element shapes described herein above are complex and can be most easily manufactured by printing corresponding transducer patterns on sheets of PVDF, such as that shown inFIGS. 13A and 13B.

In other embodiments, electrostrictive transducer elements, for example, can be used in place of PVDF. For example, the warped elements ofFIGS. 1, 7A, 7C, 8A, 5A, 12B, 12C, and12E, for example, can be more easily fabricated from PVDF sheets than from ceramic, capacitive, or electrostrictive materials. Furthermore, transducer elements with tapered ends, such as those illustrated inFIGS. 4A and 8A, for example, and transducer elements with periodic elongations, such as those ofFIGS. 11A, 12C, and 12D, for example, can be fabricated more easily from PVDF with metallic deposits thereon.

FIG. 13Cis a model illustrating how transducers such as those illustrated inFIGS. 13A-Bcan be configured for side-scan applications with through-hull mounting, for example. A threaded post1313is configured to be inserted through the hull of a watercraft such as the watercraft218illustrated inFIG. 2E, for example. Attached to the post1313is a backing surface1315. A three-element PVDF transducer including the active transducer elements1311a-care mounted. Although not illustrated inFIG. 13C, the backing surface1315and transducer elements1311a-ccan be encapsulated with polyurethane, for example. In other embodiments not illustrated, other mounting configurations may be used.

FIG. 13Dis a graph illustrating measured beam patterns in the width (x) dimension. The measurements are from a prototype device built using the PVDF elements shown inFIGS. 13A and 13B. In particular, measured curves1317a-care measured radiation patterns for the transducer elements1311a-c, respectively. The corresponding modeled beam pattern for this type of element is illustrated by the curve852inFIG. 9. Specifically, curve852is the predicted beam pattern in the width dimension. The measured beam patterns1317a-care similar the predicted curve852but slightly narrower than the model. This is not unexpected because the modeled pattern illustrated at852inFIG. 9does not account for the effects of housing diffraction or the backing of the PVDF.

FIGS. 14A-Billustrate warped transducer elements with edges defining 90° angles therein. In particular,FIG. 14Aillustrates an element1402ahaving a top edge with a 90° angle therein and a bottom edge with a 90° angle therein. A second (shifted) portion1402a2of the element1402acan be referred to as a protrusion of the element1402awith respect to a first portion1402a1of the element. The shifted portion1402a2results in the 90° angles in the top and bottom edges of the element1402a. The element1402ais warped because an overall width1473of the element is greater any local width such as local widths1463a-b. In other embodiments, one or more similar elements (1402b,1402c, etc., not shown) with similar shape can be placed adjacent to each other in the same geometric orientation. In yet other embodiments, there are angles other than 90° in the top and bottom edges of element1402a.

FIG. 14Billustrates two elements1404aand1404b. The element1404ahas two element portions1404a1and1404a2that share a corner, and the element1404bhas two similar element portions1404b1and1404b2. The elements1404a-bare also warped, and they can be placed adjacent to each other and even form a common edge1484as shown. Thus, two or more elements such as elements1404aand1404bcan be used to form a “checkerboard” pattern of elements. In each of the elements1402a,1404a, and1404billustrated inFIGS. 14A and 14B, respectively, the second portion of each element is shifted with respect to the first element portion in a direction perpendicular to a major axis1480of a transducer element to form the 90° angles in the top and bottom edges of the elements.

FIG. 14Cillustrates two elements1406aand1406bthat can form part of an acoustic transducer device. The first transducer element1406ais divided into first and second spatially separated element portions1406a1-2, which are electrically coupled to each other in a matter described hereinabove in conjunction withFIGS. 12A and 12E, for example. The second transducer element1406bis also divided into first and second spatially separated element portions1406b1-2electrically coupled to each other. The second element portions1406a2and1406b2of the first and second transducer elements are spatially offset from the first element portions of the first and second transducer elements, respectively, in a direction perpendicular to a major axis1480of the respective first transducer element.

It should be noted that in the embodiment illustrated inFIG. 14C, the second element portions1406a2and1406b2are also spatially offset from the first element portions1406a1and1406b1, respectively, in a direction parallel to the major axis1480. However, in other embodiments, the second element portions are shifted only in the direction perpendicular to the major axis1480.

FIGS. 15 and 16illustrate warped transducer elements1502and1602, respectively, each of which has a respective radiative surface defining periodic protrusions at opposing sides of the transducer. Element1502has two sawtooth edges with protrusions that are aligned with each other (see an alignment axis1590, for example) instead of being offset from one another. Thus, element1502is warped, because an overall width1573is greater than any local width such as local widths1563a-b. The warped sinusoidal transducer element1602inFIG. 16has two sinusoidal edges with protrusions that are aligned with other (see an alignment axis1690, for example) instead of being offset from each other. Element1602is similarly warped because an overall width1673is greater than any local width such as local widths1663a-b.

FIG. 17Ais a diagram illustrating an acoustic transducer element1750with an acoustic radiative surface1752that has a skewed diamond shape. As used herein, “skewed diamond shape” is used to describe a transducer element that has four straight sides that are not all equilateral. In particular, the four different sides have respective side lengths of at least two different values. Furthermore, the skewed diamond shapes illustrated herein have at least three different angle sizes for angles between respective sides of the transducer elements.

For example, the transducer element1750has sides1754a-bthat share a common side length1756a. The element1750also has sides1754c-dthat share a common side length1756b, but the side length1756bis longer than the side length1756a. In contrast to the skewed diamond shape of the element1750, the element460inFIG. 4Ais diamond-shaped but not skewed. In particular, the diamond shape460inFIG. 4Ahas four sides that are all equilateral.

Transducer elements with skewed diamond shapes as illustrated inFIG. 17Acan be made from any of the transducer types described hereinabove. However, it is particularly advantageous to make these transducer elements from PVDF material. As described hereinabove in relation toFIGS. 13A-B, printed circuit board-related technology can be used to define elements on a sheet of PVDF by printing the electrodes in the desired shape. This technology can enable multiple transducer elements to be placed on a single sheet. This has benefits associated with ease of manufacturing and also of placement of multiple transducer elements in proximity with each other for interferometric transducer designs, as described further hereinafter in connection withFIGS. 17B-C.

FIG. 17Billustrates two skewed diamond-shaped transducer elements1750placed in close proximity with each other. This configuration illustrates a significant advantage of skewed diamond-shaped transducer elements. In particular, these elements are advantageous for interferometric transducers because they can be placed with their acoustic centers closer together than for the non-skewed diamond shapes. The advantages of having acoustic centers relatively closer together for interferometric designs have been described hereinabove.

InFIG. 17B, the two skewed diamond transducers1750each have a width1764in the direction of a short axis1760of the transducers. It will be understood that, if a two-element transducer were hypothetically formed of two non-skewed diamond-shaped elements, such as the element460illustrated inFIG. 4A, a separation between acoustic centers of the respective, non-skewed diamonds would need to be greater than a width along a short axis of the diamond element460to prevent physical and electrical interference between the two elements. However, as illustrated inFIG. 17B, the two skewed diamond elements1750can be placed in close proximity with opposite orientations along the long axis1762of the elements. This enables a center-to-center separation1766between the acoustic centers1758to be smaller than the width1764of the transducer elements along the short axis1760.

FIG. 17Cillustrates two groupings1766aand1766bof skewed diamond-shaped transducer elements. These groupings of transducer elements illustrate that transducers may comprise more than two skewed diamond-shaped transducer elements. Furthermore, the grouping1766ahas smaller widths of transducer elements along the short axis of the elements, enabling higher-frequency acoustic radiation to be output and received by the transducer than for the transducers of the grouping1766b. As will be understood by those skilled in the art, different diamond sizes can facilitate sending and receiving different, respective acoustic frequencies. Both of the groupings1766a-bcan be used in a single transducer, thus enabling dual-frequency operation.

FIG. 17Dis a graph of dots1772used for purposes of modeling Y-direction radiation patterns for transducer elements of rectangular and skewed diamond shapes. In particular, the dots1772represent discrete parts (individual, small portions) of respective transducers. It will be noted that the scales X and Y axis scales are very different inFIG. 17D. The Y axis is scaled inFIG. 17Dsuch that the skewed diamond shape can be appreciated more easily.

Radiation patterns were calculated for a rectangular transducer element, as defined by the rectangle outline1768inFIG. 17Dby using all of the discrete parts represented by dots located within the rectangle1768. In complementary calculations, the Y-direction radiation distribution for a skewed diamond-shaped transducer element, as defined by the skewed diamond outline1770inFIG. 17D, was performed using only the dots1772located within the skewed-diamond outline1770. The results of these calculations are illustrated inFIGS. 17E-F.

FIG. 17Eis a graph illustrating a Y-direction polar acoustic radiation pattern calculated for both the rectangle transducer shape1768and the skewed diamond transducer shape1770illustrated inFIG. 17D. In particular, a Y radiation distribution1774is illustrated for the rectangle case, compared with a Y radiation distribution1776calculated for the skewed diamond shape1770inFIG. 17D. As seen inFIG. 17E, side lobes for the skewed diamond distribution1776are generally lower than for the rectangular case.

FIG. 17Fis a more detailed view of the graph inFIG. 17E, showing only the region between about −20° and +20°. A first side lobe1778is shown in the calculated spectrum1774for the rectangular transducer shape.FIG. 17Falso shows a first side lobe1784for the skewed diamond transducer shape.

As seen inFIG. 17F, the strength of the first side lobe1780for the skewed diamond case is at a level1782of about −16 dB. The strength of this first side lobe for skewed diamond cases varies depending on the degree of skew. In particular, as the lengths of the sides (see, e.g.,FIG. 17A) become closer to equal, the strength of the first side lobe advantageously decreases, but the minimum spacing along the short axis illustrated inFIG. 17Bincreases.

In the extreme case, where the diamond shape becomes equilateral and non-skewed, as illustrated for the element460inFIG. 4A, the strength of the first side lobe decreases to about −27 dB, as illustrated inFIG. 4D, which is about the theoretical limit for a diamond aperture. However, as noted hereinabove, for the unilateral diamond shape460inFIG. 4A, separation between elements in an interferometric transducer cannot be decreased for the benefits described in connection withFIG. 17Band elsewhere hereinabove. Thus, there can be an engineering tradeoff between the desire for low strength of the first side lobe and the goal of small separation1766between the acoustic centers of two skewed diamond transducers, such as those illustrated inFIG. 17B.