Patent Publication Number: US-2023132566-A1

Title: Underwater acoustic receiver apparatus and method of monitoring a target portion of a water column

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase under 35 U.S.C. § 371 of International Application No. PCT/GB2021/050447, which has an international filing date of Feb. 24, 2021, and which claims priority to United Kingdom Patent Application No. 2002593.8, filed Feb. 24, 2020, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates to an underwater acoustic receiver apparatus of the type that, for example, comprises an acoustic focal point device to receive an acoustic beam pattern from a water column. The present invention also relates to a method of monitoring a target portion of a water column, the method being of the type that, for example, comprises receiving an acoustic beam pattern from the target portion of the water column. 
     BACKGROUND 
     A so-called current profiler finds numerous applications in the field of underwater acoustics. The current profiler, for example of the kind described in U.S. Pat. No. 5,208,785, measures backscatter intensities and water velocity over various ranges of distance relative to the measurement instrument. Different current profilers, more accurately referred to as Acoustic Doppler Current Profilers (ADCPs), are designed to measure water currents over different ranges. Furthermore, ADCPs can employ different frequencies depending upon the range of profile sought. This can lead to performance and cost penalties when compared to instruments that are comparable in some technical respects. 
     For example, existing low-frequency, long range, ADCPs typically operating at frequencies between about 45 kHz and 100 kHz employ large transducers in order to provide beam widths comparable to those of higher frequency ADCPs, which results in an increase in cost of manufacturing the instrument. The increased transducer size can also hinder performance owing to, for example, the presence of a large surface in an underwater environment that can be prone to collisions with debris. Existing ADCPs are “fragile” and can be damaged during deployment, for example using a winch. Such damage can be to seals, leading to water ingress. In the absence of physical damage during deployment, water ingress into a transducer head is nevertheless also a risk. Indeed, so-called pressure cycling can occur when the instrument is repeatedly removed from the water and redeployed, which can lead to seal failure and the consequential water ingress into the transducer head mentioned above. 
     US 2008/0080313 describes a low-frequency ADCP comprising a single phased-array transducer head that employs acoustic beam steering. However, such an instrument has increased complexity over traditional ADCP designs, which is accompanied by a corresponding manufacturing complexity owing to the need for additional multiple manufacturing stages as compared with ADCPs that do not employ beam steering. Such an instrument is therefore more costly to manufacture than simpler designs of ADCPs. Furthermore, the increase in complexity invites an increased risk of instrument failure. The design of this ADCP is also susceptible to mechanical damage, which can lead to water ingress and consequential damage to the instrument&#39;s electronic circuits. 
     Another design is embodied in the Signature55™ current profiler by Nortek AS, which comprises a monolithic transducer head arrangement of, for example, three sometimes upwardly facing angularly spaced transducer heads. However, this design is also susceptible to mechanical damage due to the large size of the transducers, which increases the likelihood of the transducer being struck, for example during deployment. Such harm can lead to subsequent ingress of water and hence damage that is inevitably fatal to the instrument. 
     SUMMARY 
     According to a first aspect of the present invention, there is provided an underwater acoustic receiver apparatus comprising: an acoustic reflector; and an acoustic device aimed at the acoustic reflector; wherein the acoustic reflector is disposed at a predetermined distance and orientation relative to the acoustic device. 
     The apparatus may further comprise: a spacer; wherein the acoustic device may be disposed at a first anchoring point of the spacer; and the acoustic reflector may be disposed at a second anchoring point of the spacer; the second anchoring point may be distal from the first anchoring point. 
     The spacer may have a longitudinal axis; the acoustic reflector may be oriented away from the longitudinal axis. 
     The apparatus may further comprise a stand; the stand may constitute the spacer. 
     The acoustic reflector may be a parabolic reflector. 
     The parabolic reflector may comprise a primary axis of symmetry; wherein the axis of symmetry may be inclined at an angle relative to the longitudinal axis and away from the longitudinal axis. 
     The parabolic reflector may define a section of a paraboloid; the acoustic device may be aimed at the section of the paraboloid. A secondary axis of symmetry of the section of the paraboloid may be off-axis relative to a primary axis of symmetry of the acoustic reflector. 
     The primary axis of symmetry of the acoustic reflector may be a central axis of symmetry. 
     The apparatus may further comprise: a line of sight from the acoustic device to an internal surface of the acoustic reflector; wherein the line of sight may be unimpeded. 
     The longitudinal axis may be a central longitudinal axis; and the acoustic reflector may be laterally offset relative to the acoustic device and the central longitudinal axis. 
     The acoustic reflector may be configured to extend laterally beyond the acoustic device. 
     The apparatus may further comprise: a deployed orientation; and a first end and a second end relative to the deployed orientation; wherein the acoustic device may be disposed towards the first end; and the acoustic device may be oriented downwardly towards the second end. 
     The apparatus may further comprise: another acoustic device. 
     The another acoustic device may be aimed at the acoustic reflector; the another acoustic device may be disposed at another predetermined distance from the acoustic reflector. 
     The apparatus may further comprising: another acoustic reflector; wherein the another acoustic device may be aimed at the another acoustic reflector; the another acoustic device may be disposed at another predetermined distance from the another acoustic reflector. 
     The acoustic device may be an acoustic projector. 
     The acoustic device may be an acoustic receiver. 
     The acoustic projector may be configured to generate an acoustic beam pattern having a primary lobe and the acoustic reflector may be located relative to the acoustic projector so as to intersect the primary lobe. 
     The acoustic reflector may define an f-number associated therewith; the f-number may be set so as to minimise reflection of side lobes of the acoustic beam pattern. 
     The acoustic projector may be configured to emit an acoustic beam pattern; the acoustic pattern may comprise a beam portion of interest that is emitted towards the acoustic reflector. The acoustic reflector may be configured to reflect the beam portion of interest, when in use, into a target portion of a water column, thereby ensonifying the target portion of the water column. The apparatus may comprise a structural configuration; the structural configuration may be arranged to prevent a shadow being cast, when in use, on the acoustic reflector in respect of the emitted beam portion of interest and on the target portion of the water column to be ensonified by the reflection of the portion of interest. 
     The acoustic device may be an acoustic transducer. The acoustic projector may be an acoustic transducer. 
     The acoustic transducer may be a single crystal ceramic transducer. 
     According to a second aspect of the present invention, there is provided an Acoustic Doppler Current Profiler apparatus comprising the apparatus as set forth above in relation to the first aspect of the invention. 
     According to a third aspect of the present invention, there is provided an Acoustic Doppler Current Profiler apparatus comprising: a central stand; a first acoustic projector operably coupled at a first end of the central stand and aimed at a first acoustic reflector operably coupled to a second end of the central stand, the first acoustic projector being spaced from the first acoustic reflector by a first predetermined distance; a second acoustic projector operably coupled at the first end of the central stand and aimed at a second acoustic reflector operably coupled to the second end of the central stand, the second acoustic projector being spaced from the second acoustic reflector by a second predetermined distance; a third acoustic projector operably coupled at the first end of the central stand and aimed at a third acoustic reflector operably coupled to the second end of the central stand, the third acoustic projector being spaced from the third acoustic reflector by a third predetermined distance; and a fourth acoustic projector operably coupled at the first end of the central stand and aimed at a fourth acoustic reflector operably coupled to the second end of the central stand, the fourth acoustic projector being spaced from the fourth acoustic reflector by a fourth predetermined distance; 
     wherein the first, second, third and fourth acoustic projectors are disposed about the central stand at the first end thereof; the first, second, third and fourth acoustic reflectors are disposed about the central stand at the second end thereof; and each of the first, second, third and fourth acoustic reflectors is orientation away from each other and away from the central stand. 
     According to a fourth aspect of the invention, there is provided a method of monitoring a target portion of a water column, the method comprising: an acoustic device receiving an acoustic beam from an acoustic reflector from a predetermined distance to the acoustic device; and the acoustic reflector reflecting the acoustic beam from the target portion of the water column, the acoustic reflector being oriented relative to the acoustic projector. 
     It is thus possible to provide an apparatus and method that support a simpler design to existing current profilers and also permits the use of a commonplace, for example single crystal, ceramic transducer, which is considerably smaller, for example by about a factor of ten, than transducers used in other designs. Such transducers are manufactured using well-established manufacturing techniques and benefit from the economies of mass production. The transducers are also physically more robust than transducers used in other designs. The presence of the acoustic reflectors serves as a protective barrier of sorts for the acoustic projectors. In this respect, the acoustic projectors are shielded by the reflectors from mechanical traumas from beneath the acoustic projectors, the reflectors being struck in preference to the acoustic projectors. Indeed, irrespective of the orientation of the apparatus, the reflectors provide a degree of protection from traumas. Furthermore, when the acoustic projectors are downwardly facing, the downward facing inclination of the acoustic projectors and their associated seating within the structure of the housing also serves to protect the acoustic projectors from mechanical traumas from above the apparatus. The arrangement also provides the same or a comparable beam width to exiting apparatus whilst employing a smaller transducer size. Damage to the reflector has negligible effect on the reflector surface accuracy and shape and hence the acoustic performance of the reflector, which is in contrast to the known solutions, where damage to the transducer head results, at best, in degraded acoustic performance and, in the worst case, to ingress of water and ensuing fatal damage to the head and associated electronics. Therefore, the apparatus benefits from a superior lifetime as compared with existing current profilers. Furthermore, in the event of failure, the cost of replacing an acoustic projector is less than for other known designs owing to the use by the apparatus of low-cost commonplace transducers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG.  1    is a schematic perspective view of an underwater ensonification apparatus from a first perspective and constituting an embodiment of the invention; 
         FIG.  2    is a schematic perspective view of the underwater ensonification apparatus of  FIG.  1    from a second perspective; 
         FIGS.  3    is a schematic plan view of the apparatus of  FIG.  1   ; 
         FIG.  4    is a schematic side elevation of the apparatus of  FIG.  1   ; 
         FIG.  5    is a schematic diagram of an acoustic reflector of the apparatus of  FIG.  1   ; and 
         FIG.  6    is a flow diagram of a method ensonifying a target portion of a water column constituting another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS 
     Throughout the following description, identical reference numerals will be used to identify like parts. 
     Referring to  FIGS.  1  to  4   , an underwater ensonification apparatus  100 , for example an Acoustic Doppler Current Profiler (ADCP) apparatus, comprises a plurality of acoustic reflectors, for example a first so-called acoustic mirror  102 , a second acoustic mirror  104 , a third acoustic mirror  106  and a fourth acoustic mirror  108 . The acoustic mirrors  102 ,  104 ,  106 ,  108  are each, in this example, between about 3 mm and about 10 mm thick, such as about 5 mm thick, and are sufficiently thick to provide mechanical robustness, but thin enough to avoid unwanted resonances and unnecessary additional weight. In this example, the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  are formed of a metal alloy, such as using a casting process. One example alloy is a Nickel Aluminium Bronze alloy CC333G, which conforms to the standard BS EN 1982:2008 relating to control copper and copper alloys. Alternatively, the acoustic mirrors can be formed from stamped mild steel, although other suitable materials can be employed. 
     The apparatus  100  also comprises a plurality of acoustic projectors, for example a first acoustic transducer  112 , a second acoustic transducer  114 , a third acoustic transducer  116  and a fourth acoustic transducer  118 . The first acoustic transducer  112  is aimed at the first acoustic mirror  102 , the second acoustic transducer  114  is aimed at the second acoustic mirror  104 , the third acoustic transducer  116  is aimed at the third acoustic mirror  106 , and the fourth acoustic transducer  118  is aimed at the fourth acoustic mirror  108 . Further details of the orientation of the acoustic transducers  112 ,  114 ,  116 ,  118  respectively relative to the acoustic mirrors  102 ,  104 ,  106 ,  108  will be described later herein. In this example, each of the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118  is a single crystal acoustic transducer, for example a Lead Zirconium Titanate (PZT) ceramic transducer available from Sparkler Ceramics Pvt. Ltd., India, or CeramTec UK Limited. However, the skilled person will appreciate that any other suitable transducer type can be employed. 
     The acoustic reflectors  102 ,  104 ,  106 ,  108  are each respectively disposed at a predetermined distance from the acoustic transducers  112 ,  114 ,  116 ,  118  by a spacer  120  having a central longitudinal axis. An annular mounting bracket  110  having a central axial aperture is disposed at a first end of the spacer  120 , the mounting bracket  110  comprising in this example, a first aperture in which the first acoustic transducer  112  is mounted, a second aperture in which the second acoustic transducer  114  is mounted, a third aperture in which the third acoustic transducer  116  is mounted, and a fourth aperture in which the fourth acoustic transducer  118  is mounted. The annular mounting bracket  110  also serves as a baffle. A float  119  is also coupled at a first end thereof to the first end of the spacer  120  and an acoustic modem  121  is coupled to a second end of the float  119 . However, the skilled person should appreciate that the float  119  and the acoustic modem  121  are optional, and can be contingent upon the manner of deployment of the apparatus  100 , for example they are not necessarily required when the apparatus  100  is deployed on the underside of a vessel or a buoy. 
     In this example, the spacer  120  is a cylindrical housing. The cylindrical housing is a pressure housing that contains, inter alia, a power source, for example electrical cells and electronic circuitry to drive and control the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118 , as well as record and optionally process measurements made using the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118 . 
     The mounting bracket  110  retains the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118  at respective first anchoring points. In this example, the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118  are circumferentially spaced about the spacer  120  and electrically coupled to the electronic circuitry contained within the spacer  120 . 
     A first flat spacing bar  122 , a second flat spacing bar  124 , a third flat spacing bar  126 , and a fourth flat spacing bar  128  are respectively attached at one end thereof to a second end of the spacer  120  and extend radially outwards with an equal angular spacing therebetween. The first acoustic mirror  102  is attached to a second end of the first flat spacing bar  122 , the second acoustic mirror  104  is attached to a second end of the second flat spacing bar  124 , the third acoustic mirror  106  is attached to a second end of the third flat spacing bar  126 , and the fourth acoustic mirror  108  is attached to a second end of the fourth flat spacing bar  128 . The first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  are therefore respectively disposed at a second anchoring point distal from the first anchoring point. The apparatus  100  has an in-use orientation as shown in  FIGS.  1  to  4   , the apparatus  100  having an upper end  130  and a lower end  132  relative to the in-use orientation, i.e. the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  are disposed, in this example, closest to the seabed. However, as intimated above, placing the apparatus  100  on the seabed is just one manner of deployment of the apparatus  100 . The first, second, third, and fourth acoustic transducers  112 ,  114 ,  116 ,  118  are disposed towards the upper end  130  and the first, second, third, and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  are disposed towards the lower end  132 . 
     In this example, the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  are each oriented away from the longitudinal axis of the spacer  120 . Indeed, in other embodiments, the acoustic reflectors are oriented outwardly from and away from the structure of the apparatus  100 . Furthermore, the first acoustic mirror  102  is laterally offset relative to the first acoustic transducer  112  and the central longitudinal axis of the spacer  120 , the second acoustic mirror  104  is laterally offset relative to the second acoustic transducer  114  and the central longitudinal axis of the spacer  120 , the third acoustic mirror  106  is laterally offset relative to the third acoustic transducer  116  and the central longitudinal axis of the spacer  120 , and the fourth acoustic mirror  108  is laterally offset relative to the fourth acoustic transducer  118  and the central longitudinal axis of the spacer  120 . In this regard, and in this example, the first acoustic mirror  102  extends laterally beyond a first outermost part of the periphery of the first acoustic transducer  112 , the second acoustic mirror  104  extends laterally beyond a second outermost part of the periphery of the second acoustic transducer  114 , the third acoustic mirror  106  extends laterally beyond a third outermost part of the periphery of the third acoustic transducer  116 , and the fourth acoustic mirror  108  extends laterally beyond a fourth outermost part of the periphery of the fourth acoustic transducer  118 . 
     In this example, the spacer  120  comprises the first, second, third and fourth flat spacing bars  122 ,  124 ,  126 ,  128  and constitutes a stand. 
     Each of the first, second, third, and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  is a parabolic reflector. For the sake of clarity and conciseness of description, only the first acoustic mirror  102  and the first acoustic transducer  112  will now be described in further detail, although the skilled person should appreciate that the description of the first acoustic mirror  102  applies also to each of the second, third and fourth acoustic mirrors  104 ,  106 ,  108  and the description of the first acoustic transducer  112  applies also to each of the second, third and fourth acoustic transducers  114 ,  116 ,  118 . 
     Referring to  FIG.  5   , the first acoustic mirror  102  comprises a central axis of symmetry  134  that is inclined at an angle  136  relative to a notional longitudinal axis  138  parallel with the longitudinal axis of the spacer  120 , the orientation being away from the notional longitudinal axis  138 . In this example, the surface curvature of the first acoustic mirror  102  is defined as being a portion of a much larger parent paraboloid shape, and is used for reflection of acoustic signals. In this regard, the first acoustic mirror  102  is a 0.4 m diameter section of the parent paraboloid. The 0.4 m section of the parent paraboloid has a child central axis and the parent paraboloid has a parent central axis, and the child central axis of the 0.4 m region is located 0.2 m off-axis (in a straight line) with respect to the parent central axis, i.e. the distance between the parent central axis and the child central axis is 0.2 m. As mentioned above, the first acoustic transducer  112  is aimed at the first acoustic mirror  102 . 
     In this example, a segment slant focal length extends from a phase centre of the first acoustic transducer  112  to an acoustic centre point of the 0.4 m child section (the first acoustic mirror  102 ) of the parent paraboloid. The parent paraboloid has a focal length is 0.3 m, resulting (in conjunction with the above design parameters) in the segment slant focal length being about 0.327 m. The ratio of the segment slant focal length to the diameter of the first acoustic mirror  102  yields an effective f-number for the acoustic mirror  102  of 0.8175. The above dimensions are determined in order to minimise reflection of side lobes of an incident acoustic beam pattern, when in use. 
     In this example, a line of sight exists from the first acoustic transducer  112  to the first acoustic mirror  102 , the line of sight being a clear, unimpeded, line of sight to an internal surface  144  of the first acoustic mirror  102 . Indeed, a structural configuration of the apparatus  100 , for example comprising the spacer  120 , the mounting bracket  110  and/or the first, second, third and fourth flat spacing bars  122 ,  124 ,  126 ,  128 , is arranged to prevent one or more shadows being cast, when in use, on the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108 . 
     In operation ( FIG.  6   ), the apparatus  100 , which can be in a sleep mode, is powered up and immersed (Step  200 ) in a water column. The apparatus  100  is then woken up, for example using an acoustic command communicated to the acoustic modem  121 , to place the apparatus in a measurement mode (Step  202 ). 
     In response to being placed in the measurement mode, the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118  generate (Step  204 ) respective first, second, third and fourth acoustic beam patterns, each having a primary lobe and secondary lobes. The radially extending portion of each of the first, second, third and fourth acoustic beam patterns respectively intersect the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108 . In this regard, the primary lobes of the first, second, third and fourth acoustic beam patterns are respectively aimed at the first, second, third, and fourth acoustic mirrors  102 ,  104 ,  106 ,  108 . As some of the respective secondary lobes of the acoustic beam patterns emitted by the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118  can respectively intersect the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  and result in corruption of measurements attributable to the primary lobes, the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118  respectively are configured to under-illuminate the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  so that the secondary, side, lobes do not extend beyond the peripheries of the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108 , respectively. In this regard, it should be appreciated that the amount by which the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  are under-illuminated by the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118 , respectively, is optimised by numerical simulation and can vary between implementations. 
     The first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  respectively reflect (Step  206 ) the first, second, third and fourth acoustic beam patterns to a first, second, third and fourth target portion of the water column, respectively. The first, second, third and fourth target portions of the water column are therefore ensonified (Step  208 ). As mentioned above, the configuration of the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108  is such that reflection of side lobes of the first, second, third and fourth acoustic beam patterns is minimised. In this regard, the primary lobes of the beam patterns constitutes respective beam portions of interest emitted by the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118 , respectively. Furthermore, the structural configuration of the apparatus  100  prevents a shadow being cast respectively on the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108 , when the respective beam portion of interest is emitted by the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118 , as well as being respectively cast on the respective target portions of the water column to be ensonified by the respective reflections of the beam portion of interest. 
     Features and/or properties of the water column, for example in the first, second, third and fourth target portions of the water column, reflect or backscatter (Step  210 ) the acoustic waves ensonifying the first, second, third and fourth target portions of the water column to varying degrees and the reflected acoustic waves are respectively incident (Step  212 ) upon the first, second, third and fourth acoustic mirrors  102 ,  104 ,  106 ,  108 , which redirect (Step  214 ) the reflected acoustic waves to the first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118 . The first, second, third and fourth acoustic transducers  112 ,  114 ,  116 ,  118  each receive (Step  216 ) and translate the received acoustic waves to the electrical domain for further processing before storage of measurement data and/or communication via the acoustic modem  121  (Step  218 ) to, for example, a topside vessel (not shown). This process is repeated (Steps  204  to  218 ) until the apparatus  100  is no longer required to make measurements in respect of the water column and is placed in a sleep mode and/or recovered. 
     The skilled person should appreciate that the above-described implementations are merely examples of the various implementations that are conceivable within the scope of the appended claims. Indeed, it should be appreciated that although the above examples comprise four pairs of acoustic projectors and acoustic reflectors, this number of pairs of projectors and reflectors can be varied and it is contemplated that the apparatus  100  can comprise, in other implementations, fewer pairs of acoustic projectors and acoustic reflectors, for example three pairs of acoustic projectors and acoustic reflectors. Similarly, the apparatus  100  can have, in other implementations, five or more pairs of acoustic projectors and acoustic reflectors. 
     However, providing more than three pairs of acoustic projectors and acoustic reflectors, for example where each acoustic reflector is substantially equidistantly spaced from each other about the spacer  120 , facilitates a level of redundancy in the event of failure of an acoustic projector. 
     It should be appreciated that the acoustic projectors described above are merely examples of acoustic devices. In the examples described herein, the acoustic devices both transmit and then receive acoustic signals, and so are transceivers in such examples. However, the skilled person should appreciate that in some examples the acoustic device can be configured only to transmit, for example an acoustic transmitter, or only to receive, for example an acoustic receiver. In other examples, the acoustic device can be configured to receive and then transmit, as in the case of an acoustic transponder. In some examples, one or more of the acoustic devices can be configured to transmit acoustic signals and one or more of the acoustic devices can be configured to receive acoustic signals. Indeed, in other examples, the acoustic devices can comprise a mixture of two or more of: transmitters, receivers, transceivers and transponders. 
     Although in the above examples, the acoustic projectors have been assumed to project downwardly towards the acoustic reflectors, it should be appreciated that such a relative location of projectors and reflectors is relative to an orientation of the apparatus. Furthermore, in the orientation described herein, the locations of the acoustic projectors and acoustic reflectors can be swapped, either for all of the acoustic projectors and acoustic reflectors or simply some pairs of acoustic projectors and acoustic reflectors. In this regard, the above possible alternative envisages the possibility of a fewer or greater number of acoustic reflectors. 
     The above examples employ parabolic reflectors, which are child reflectors derived from larger parent paraboloids. However, in another example, one or more of the acoustic reflectors can be of a different shape, for example one or more of the acoustic reflectors can be derived from a parabolic trough shape. 
     In other embodiments, the apparatus  100  can comprise fewer acoustic reflectors than acoustic projectors and two or more acoustic projectors can share, for example be aimed at, the same acoustic reflector, such as two acoustic projectors sharing the same acoustic reflector.