Abstract:
An acoustic detector provides a planar arrangement of microphones. A windshield is arranged over the microphones, with a view to reducing the impact of environmental effects. A void is therefore defined over the microphones, and the interior surface of the windshield, which defines the void, is shaped suitably to reduce the potential for acoustic waves propagating within the void to reflect back onto the microphones.

Description:
FIELD 
       [0001]    Embodiments described herein concern acoustic detectors. 
       BACKGROUND 
       [0002]    Acoustic detectors are used in a variety of environments. One particular approach is to use an omnidirectional detector, mounted on a pole, which can then be itself attached either to a vehicle or set in the ground, for stability. However, the form factor of most such designs is badly suited to environments where rugged use may be required. Existing technologies have resulted in large and fragile solutions that do not combine sufficient sensitivity and reliability with an acceptable shape. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0003]      FIG. 1  comprises a side elevation of an acoustic detector in accordance with an embodiment described herein; 
           [0004]      FIG. 2  comprises a cross-sectional view of the acoustic detector illustrated in  FIG. 1 , 
           [0005]      FIG. 3  comprises a plan view of the acoustic detector illustrated in  FIG. 1 ; 
           [0006]      FIG. 4  comprises a plan view of a sensor plate of the acoustic detector illustrated in  FIG. 1 ; 
           [0007]      FIG. 5  comprises the cross-sectional view shown in  FIG. 2 , to demonstrate use of the detector. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0008]    An acoustic detector  10  is illustrated in  FIG. 1 . The components of the detector  10  visible in  FIG. 1  will now be described. The detector  10  comprises a protective cage  20 , enclosing a windshield  30  of a corresponding shape. The shape of the protective cage  20  is described in detail below. A base plate  40  attaches to four mounting feet  50 . The mounting feet  50  can themselves attach to another unit, as required, to provide a stable mounting for the acoustic detector  10 . 
         [0009]    The protective cage  20 , which in this example is constructed of steel wire, defines a generally cylindrical shape, enclosed at one end of the cylinder by a domed end portion. The profile of the domed end portion is, as illustrated, ellipsoidal. The reader will appreciate that other forms of curvature of the domed end portion may be useful, such as paraboloidal, or hemispherical shapes. 
         [0010]    Thus, the protective cage  20  comprises radial and annular wires  22 ,  24 . The radial wires  22  define the outline of the domed shape, and cross at a crossing point coincident with the rotational axis of the protective cage  20 . The annular wires  24  surround the radial wires  22 . At crossing points of the radial and annular wires  22 ,  24 , and at the crossing point of the radial wires  22 , wires are welded to provide a sound structure. 
         [0011]    The cross-sectional diameter of the wires is selected, along with the steel material, to provide sufficient structural strength for the protective cage  20  dependent on the required application. 
         [0012]    The structure and material of the protective cage is merely exemplary, and a suitable component could equally be provided from other materials, such as plastics or ceramics materials. 
         [0013]    A fixing lug  26  is affixed to the end of each radial wire  22 , remote from the aforementioned domed portion. As is illustrated in  FIG. 3 , each lug  26  comprises a rectangular plate, oriented in a plane perpendicular to the axis of the radial wire  22  to which it is affixed. Each lug  26  defines a circular fixing through-hole  28 . 
         [0014]    The aforementioned windshield  30 , as previously noted, is of a shape which cooperates with the interior space defined by the form of the protective cage  20 . The windshield  30  comprises a layer of acoustic foam. The foam of the windshield  30  has acoustic properties such that the windshield  30  allows the transmission of sound, while slowing the velocity of incident wind to zero, without very fast spatial velocity gradients which would generate turbulence and thus noise. Such foams are generally in use and are well known in the field. A typical foam is of an open cell form. The windshield  30  has a substantially consistent thickness throughout its form, thereby defining an interior space which is similar to that defined by the protective cage  20 . The thickness of the windshield  30  will depend upon the application to which the acoustic detector  10  is to be put, the acoustic properties of the windshield foam, and any environmental factors to be taken into account. For instance, if it is known that the acoustic detector  10  is to be placed in an environment susceptible to high winds, different design decisions may need to be taken than if the detector  10  is to be used in more benign conditions. 
         [0015]    Formation of the shape of the windshield  30  may be achieved either by “sewing” a flat sheet of acoustic foam material into the desired shape or, alternatively, milling the shape from a solid block of the acoustic foam material. The latter may have advantages, in not introducing seams or other imperfections into the windshield  30 , which might have an impact on the acoustic properties of the windshield  30 . In certain embodiments, the windshield  30  could be moulded into the required shape from, for instance, liquid components. Various techniques now exist to form foam components 
         [0016]    Additionally, the acoustic foam of the windshield  30  is treated to impart hydrophobic properties. This is achieved by adulterating the acoustic foam with a suitable material. Typically, neoprene can be used for this purpose. As the reader will appreciate, it is important not to impart so much neoprene that the windshield  30  forms an acoustic suspended mass, which would affect the acoustic performance of the detector  10 . The exact level of neoprene adulteration to be imparted will depend on the exact design employed, and thus can be determined by experimentation. 
         [0017]    Alternative hydrophobic treatments could also be used, such as nano-coatings. Nano-coatings, suitable for acoustic foams, already exist in the market, such as Aridion™ produced by P2i Limited of Abingdon, Oxfordshire, UK. 
         [0018]    The interior surface of the windshield  30  is treated further with a waterproofing layer of neoprene. This can be sprayed onto the interior surface to form a thin layer (at least, thin relative to the thickness of the windshield  30  itself), thereby preventing ingress of water, incident on the exterior of the windshield  30 , into the interior space defined by the windshield  30 . This waterproofing treatment is useful, particularly for implementations of the acoustic detector  10  which are intended for use in environments where precipitation can be expected. Of course, for an acoustic detector  10  only for use in interior situations, or in extremely dry environments, this waterproofing treatment may not be necessary. 
         [0019]    Note that the waterproofing treatment of the windshield is advantageously on the interior surface thereof. 
         [0020]    Other materials could be used for the windshield  30 , such as acoustic fur. Acoustic fur is already used to shield microphones for use in outside broadcasting environments. 
         [0021]    The base plate  40  is generally circular, and is dimensioned so as to encompass the four lugs  26  at its circular edge. Four circumferentially spaced and threaded through holes  42 , corresponding with the positions of the through-holes  28  allow connection of the cage  20  with the base plate  40 , using fixing screws  44 . 
         [0022]    The mounting feet  50  are affixed to the base plate  40 , in a manner to correspond with the positions of the lugs  26 . There is no particular significance to this correspondence, and, in other embodiments, the number of mounting feet and the number of lugs need not be equal. The mounting feet  50  are of moulded silicone gel mounts, to act as mounting springs, so as to reduce the possibility of vibration of the acoustic detector  10 , with reference to the unit to which it is affixed, having an operational effect on the acoustic detector  10 . Through holes  52  are formed in the mounting feet  50 , as illustrated in  FIG. 2 , to enable installation of the device on a platform. For instance, the device could be installed on a motor vehicle, in us, though other installation configurations could readily be contemplated by the reader. 
         [0023]      FIG. 2  illustrates the interior construction of the acoustic detector  10 , with particular reference to the thickness of the windshield  30  and the space it defines with the base plate  40 . The space can be considered to comprises two parts. A cylindrical part of the space is taken up by a sensor enclosure  60 , which is itself a cylindrical construction with one closed end. The closed end is substantially adjacent the base plate  40 , but spaced therefrom to define a thin cylindrical cavity, within which anti-vibration mountings  45  support the sensor enclosure  60  on the base plate  40 . The opposite end of the enclosure  60  is closed by a sensor plate  62 . The sensor plate  62  has five through holes  64  defined therein. As further illustrated in  FIG. 4 , the through holes  64  are arranged in a cruciform formation, centred with the disk. Each through-hole  64  has a microphone  70  mounted therein, oriented towards the domed portion of the interior space of the acoustic detector  10 , that is, upwards towards the windshield  30  and the cage  20  as illustrated in  FIG. 2 . 
         [0024]    The anti-vibration mountings  45  are, in this embodiment, made of silicone gel, tuned, together with the mass of the acoustic sensor enclosure  60  to as low a frequency as possible, commensurate with adequate strength to keep the senor enclosure  60  restrained under shocks delivered through any structure or vehicle to which the detector  10  may be attached. 
         [0025]    As will be appreciated by the reader, the five microphones  70  are themselves connected to electronic processing components, designed, for a particular application, to detect and process electronic signals arising from acoustic energy incident on the microphones  70 . The particular detail of the electronic processing components is not necessary for an understanding of the present embodiment. While the particular electronic processing capabilities of the device is not part of this disclosure, an accelerometer  80  is provided, mounted on the base plate  40 . The accelerometer  80  can be used to provide a phase reference for interfering vibrations, permitting their cancellation from acoustic sensor outputs. 
         [0026]    It will be appreciated by the reader that various numbers and arrangements of microphones may be used. In most cases, the microphones  70  will be arranged in a coplanar configuration, such as arranged in a circle, concentric circles or as a cruciform pattern. The present embodiment includes five microphones as an example only. 
         [0027]    In some cases, a configuration may include at least one microphone positioned out of a plane defined by the other microphones. This may aid in three dimensional wave detection. For instance, the illustrated embodiment could be modified such that one of the illustrated microphones is positioned above the plane of the sensor plate  62 . 
         [0028]    The shape of the void created between the sensor plate  62  and the windshield  30  has an operational effect on the acoustic detector  10 . While various specific profiles may achieve the same, or similar, end results, it is useful here to describe the intended properties to be achieved, so that the skilled reader can select a shape suitable to fit the circumstances of any particular implementation. 
         [0029]    The acoustic detector  10  is intended to be used in circumstances presently fulfilled, to an extent, by omnidirectional acoustic detectors. However, it is observed here that the majority of acoustic signals which will be of interest to a user, will emanate from sources roughly at the same altitude as the observation position—i.e. the position of the acoustic detector. Hence, omnidirectionality is not a key requirement for such detectors. The present arrangement, therefore, focuses on effective detection of acoustic waves emanating in directions roughly parallel with ground level. 
         [0030]    This working assumption has two main impacts. 
         [0031]    Firstly, the general direction of propagation of acoustic waves will be roughly parallel to the sensor plate  62 . Secondly, reflected waves will also emanate from the source of any acoustic waves—a particular mode of reflection will be a direct ground reflection between the source and the detector, but other modes may also exist. The time of arrival of these reflected waves at the detector may be a time delay after the arrival of the primary wave, but the time delay may be very short, especially for distant sources close to ground level. 
         [0032]    The design of the acoustic detector  10  therefore very much governs how sensitive the detector will be to such acoustic waves, and particularly to reflected acoustic waves. Previous approaches have relied upon omnidirectionality as being a key component of the sensitivity to reflective acoustic waves, but the present device does not enable this approach. Instead, the internal curvature of the windshield  30  plays a key element in this. The dome-shaped interior void provides a reflection profile which is suited to the particular application. Especially, the curvature should be such that reflections from the internal surface of the windshield  30  do not reflect back onto the microphones  70 . 
         [0033]    Thus, the exact shape of the dome of the windshield  30  (and the consequent shape of other components) will be determined by experimentation. Certain properties have been found to have an ameliorative impact. In particular, it has been found beneficial for the height of the dome (i.e. the distance between the interior surface of the windshield  30  and the closest surface thereto of the sensor plate  62 ) to be substantially the same as the axis-to-axis distance between microphones  70 . 
         [0034]    The curvature of the dome can be ascertained by application of fundamental theory relating to curved reflectors. In particular, by using a substantially paraboloidal approach (with suitable approximation), the passage of an acoustic wave from beneath the plane of the sensor plate  62  can be managed such that it does not become incident on a microphone.  FIG. 5  illustrates this in simple terms. 
         [0035]    Such an acoustic wave will propagate through the acoustic detector  10  in several ways. Firstly, a part of the wave will, inevitably, diffract at the edge of the sensor plate  62 , and will then propagate substantially in the plane of that sensor plate. This will then be detected at the microphones  70 . This is labelled “1” in  FIG. 5 . Another part of the wave (labelled “2”) will continue onwards through the void defined above the sensor plate  62 , to the interior surface of the windshield  30 . The interior surface of the windshield  30  will cause a further segmentation of the propagation of the acoustic wave. Part of the wave (labelled “3”) will, again, propagate directly through the windshield  30  and out of the detector  10 . Another part (labelled “4”) will be reflected by the internal surface of the windshield  30 . This may, in part, be abetted by the neoprene waterproofing coated to the internal surface. However, the curvature of the windshield  30  means that the resultant acoustic reflection will not reflect back onto the microphones  70 —it will, instead, reflect down beyond the outer extent of the microphones  70  and out of the detector  10 . 
         [0036]    Thus, as set out above, the detector  10  offers an opportunity for incident sound waves to be presented cleanly to the microphones  70 , in a manner whereby internal reflections within the detector  10  are minimised if not eliminated. Moreover, the spatial properties of an incident sound wave are maintained on entry to the detector, ensuring that the sound waves incident on the microphones are not distorted in time or space. In essence, the detector&#39;s acoustic transmissive properties allow its use as if the shielding  30  were not in place, but with the advantageous noise reduction and water resistant properties of the device  10 . 
         [0037]    As a result, the sound waves are incident on the microphones  70  in a manner such that timing information can be determined from the moment of incidence of a sound wave on each microphone in turn. By this timing information, it can be seen that electrical signals generated at the microphones  70  can be processed straightforwardly to deduce the bearing of an incident sound wave, and thus the origin of the sound emission producing the sound wave. 
         [0038]    The exact curvature to be employed, will therefore be subject to design selection. It has been found that a paraboloidal shape is effective, but other shapes may also be effective and so the present disclosure is not limited to that particular shape. 
         [0039]    While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel devices described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the sprit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.