Abstract:
A directional listening device includes a reflector having a central axis, a microphone pointing toward the reflector and positioned along the central axis, and a collimator or shield surrounding the microphone and aligned with the reflector for improved directionality. The device also includes a laser pointing away from the reflector and a fitting for receiving a sighting device for aiming the listening device. Handles attached to the listening device have a resilient cover for noise reduction.

Description:
BACKGROUND  
       [0001]     This application relates to a listening device that gathers sound for a microphone and, in particular, to a directional listening system in which sound is collimated.  
         [0002]     Listening devices using a curved reflector to gather sound for one or more microphones are also well known in the art, e.g. see U.S. Pat. No. 4,037,052 (Doi). It is known also to try to track a sound, i.e. point a microphone in the direction of the source of the sound; e.g. see U.S. Pat. No. 5,452,364 (Bonham). For many applications, accurate direction finding can be critical, e.g. search and rescue. In such applications, the listening device must also be rugged because, once a victim is found, care for the device is secondary to rescue.  
         [0003]     A problem with systems of the prior art is the reliance on a reflector, typically parabolic but occasionally hemispherical. Such a reflector has a wide acceptance angle, making it difficult to locate the source of a sound. The sound at any point in space is a complex combination of the original sound and reflections from many objects. A parabolic reflector affects sounds differently at different frequencies. With plural microphones, the problem is more complicated but not resolved. Sounds from behind a curved reflector can be coupled to the reflector by nearby buildings, for example.  
         [0004]     Another problem with systems of the prior art is that, even if the listening device is pointing in the proper direction, the user may not realize exactly what that direction is, particularly with hand-held reflectors. For example, U.S. Pat. No. 5,526,433 (Zakarauskas et al.) attempts to overcome this problem by providing a platform and a gimbal mount for holding a reflector. This merely transfers the problem to another element. The platform must be calibrated for the direction indicated by the gimbal mount in order to have meaning.  
         [0005]     Yet another problem with systems of the prior art is that one may be listening for a faint sound. Sounds mechanically coupled to the microphone can be louder than the faint sound that one is trying to locate. In such case, a faint sound of interest may be overlooked. In search and rescue operations, this can be critical, whether the sound is a human voice or the sound of a support cracking. Even for more mundane operations, such as listening for termites or carpenter ants, mechanically coupled sounds can be at least an inconvenience, if not a source of error.  
         [0006]     In view of the foregoing, it is therefore an object of the invention to provide a directional listening device having a narrower acceptance angle than listening devices of the prior art.  
         [0007]     Another object of the invention is to provide a directional listening device that accurately indicates the direction to a source of sound.  
         [0008]     A further object of the invention is to provide a directional listening device that accurately indicates the direction to a source of sound without prior calibration or alignment.  
         [0009]     Another object of the invention is to provide a listening device that can detect faint sounds without interference from mechanically coupled sounds.  
       SUMMARY OF THE INVENTION  
       [0010]     The foregoing objects are achieved by this invention in which a directional listening device includes a reflector having a central axis, a microphone pointing toward the reflector and positioned along the central axis, and a collimator or shield surrounding the microphone and aligned with the reflector for improved directionality. The device also includes a laser pointing away from the reflector and a fitting for receiving a sighting device for aiming the listening device. Handles attached to the listening device have a resilient cover for noise reduction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0012]      FIG. 1  is a perspective front view of a directional listening device constructed in accordance with a preferred embodiment of the invention;  
         [0013]      FIG. 2  illustrates the geometry of a listening device constructed in accordance with the invention;  
         [0014]      FIG. 3  is a perspective rear view of a directional listening device constructed in accordance with a preferred embodiment of the invention; and  
         [0015]      FIG. 4  is a block diagram of the electronics used in a directional listening device constructed in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     In  FIG. 1 , directional listening device  10  includes curved reflector  11  and collimator  12 . Housing  13  is spaced from the inner surface of reflector  11  by posts  14 ,  15 , and  16 , which, with housing  13 , form a pedestal or mounting for a microphone (not shown in  FIG. 1 ) and a laser (not shown in  FIG. 1 ). The microphone faces the inner or concave surface of reflector  11  and the laser point outward, substantially along the axis of reflector  11 . The lengths of posts  14 ,  15 , and  16  are substantially equal for holding housing  13  on the central axis of reflector  11  and substantially parallel with the central axis of reflector  11 . The posts are attached to disk  19 , which is preferably fastened to reflector  11  by an adhesive. Reflector  11  and collimator  12  are preferably molded from a suitable plastic, such as polycarbonate (such as Lexan® brand polycarbonate).  
         [0017]      FIG. 2  is a diagram illustrating the geometric relationship of reflector  11  and collimator  12 . Reflector  11  is preferably a paraboloid, having central axis  21  and focus  22  intersecting the central axis. A microphone (not shown in  FIG. 2 ) is preferably located at the focal point. In a preferred embodiment of the invention, The depth of reflector  11  is approximately equal to the focal length. That is, reflector  11  has a diameter d approximately equal to the length of a chord through focal point  22  along line  24 .  
         [0018]     Reflector  11  does not require geometrical precision for operation, even if the parabola itself has been known and studied for thousands of years. One is dealing with competing interests, such as portability, manufacturability, and cost, in addition to functionality. Thus, reflector  11  can be approximately parabolic, have a focal length of two to four inches, and have a diameter d of six to twelve inches and still be relatively portable and light enough to be aimed or scanned for long periods of time, if need be. The focal length does not dictate the diameter or depth of reflector  11 , nor vice-versa.  
         [0019]     Reflector  11  is preferably a paraboloid, although this is not critical. Hyperboloids, ellipsoids and spheroids can be used instead for reflector  11 . A surface of revolution is a preferred reflecting surface to avoid asymmetries in response if directional listening device  10  is not held in its expected orientation.  
         [0020]     Whatever directionality, if any, a reflector provides, it is not as good as desired for rescue work. In accordance with one aspect of the invention, the addition of collimator  12  significantly improves directionality, particularly for sounds coming from behind reflector  11 , despite the small height of the collimator relative to the diameter of the reflector. In a preferred embodiment of the invention, collimator  12  has a height h approximately equal to the focal length of reflector  11 ; that is, collimator  12  approximately doubles the depth of directional listening device  10 . As with reflector  11 , collimator  12  is a trade-off among competing interests, including functionality. Collimator  12  provides a substantial improvement without making directional listening device  10  ungainly to use.  
         [0021]     As illustrated in  FIG. 2 . collimator  12  increases in diameter with increasing height; that is, collimator  12  is illustrated as a conic section, not a cylinder. A conic section simplifies manufacturing; specifically, simplifies release from a mold. The angle chosen for collimator  12  is approximately seven degrees. Other shallow angles are suitable.  
         [0022]     In a preferred embodiment of the invention, reflector  11  and collimator  12  are molded as a single piece, which means that collimator  12  is as much a reflector as reflector  11 . It has not been found necessary to treat the inner surface of collimator  12  to reduce reflection, although this could certainly be done, if desired; e.g. roughening the inner surface of collimator  12  or adding a sound absorbing layer. A unitary structure provides greater strength and reduced stress along the joint between the two components.  
         [0023]     In accordance with another aspect of the invention and referring to  FIG. 1 , sighting device  31  is attached to the outside wall of collimator  12  by adapter  32 , which offsets the taper in collimator  12 . In one embodiment of the invention, sighting device  31  is what is known as a red dot sight. These sights project a red dot onto a sight image, indicating where directional listening device  10  is pointing. Red dot sights are commonly used for target shooting with pistols and have a long “eye relief,” the distance from the sight to the eye, making the sights convenient to use with directional listening device  10 . Sighting device  31  couples to adapter  32  by a suitably sturdy and stable connection, such as a sliding dovetail joint, that will hold sighting device  31  in alignment with reflector  11  and the laser in housing  13 .  
         [0024]     Handle  41  is described in conjunction with  FIG. 3 .  
         [0025]     In  FIG. 3 , handle  41  is part of bracket  40 , which includes handle  41 , handle  42 , and hand grip  43 . Push button switch  45  turns the laser on and off and is accessible from handle  41 , handle  42 , or hand grip  43 . A laser pointer is useful for confirming point of aim but may not be visible in direct sunlight. Sighting device  31  is useful under almost any lighting conditions but may suffer from problems of parallax at close range or with large diameter reflectors. (Parallax results from sighting device  31  not being on the central axis of reflector  11 . If sighting device  31  were aligned parallel to the central axis of reflector  11 , it would point to a spot above the central axis by approximately one half the diameter of the reflector.) For most applications, the problem of parallax is insignificant.  
         [0026]     Bracket  40  holds handle  41 , handle  42 , and grip  43  in spaced apart relationship and the assembly is attached to the rear surface of reflector  11  by four screws. Disk  19  ( FIG. 1 ) is at the front portion of the upper end of grip  43  and fits within a closely matched hole in reflector  11 . A suitable adhesive between disk  19  and reflector  11  secures the two and, with the four pins, provides a stable, rugged, and substantially self-aligned support for housing  13 .  
         [0027]     In accordance with another aspect of the invention, handles  41  and  42  each include a resilient cover, like the cushioned grips on the handlebars of a bicycle. Handle  41  includes cover  48  and handle  42  includes cover  49 . Hand grip  43  could include a cover if desired but serves more as a carrying handle than a handle for carefully scanning an area, which is more steadily done with two hands. The covers absorb mechanical vibration and block or attenuate the vibration to avoid mechanically coupling noise to the microphone in housing  13  ( FIG. 1 ). Bracket  40  is further acoustically isolated from reflector  11  by stand-off insulators  51 ,  52 ,  53 , and  54  that attach bracket  40  to the outside or convex side of reflector  11 . The insulators are also resilient and further isolate the microphone from mechanically coupled noise.  
         [0028]      FIG. 4  is a block diagram of the electronics used with a directional listening device constructed in accordance with the invention. The electronics, except for laser  61 , microphone  62 , and headphones  63 , is preferably contained within bracket  40  and grip  43 . As described above, laser  61  and microphone  62  are located in housing  13  ( FIG. 1 ). Laser  61  is of the type used for battery powered pointers. Microphone  62  is preferably an omnidirectional electret microphone. Laser  61  and microphone  62  point in opposite directions along the central axis of reflector  11 , with microphone  62  pointed at reflector  11 .  
         [0029]     Laser  61  is powered by driver  65  and is preferably operated intermittently. A user depresses momentary contact, push button switch  45  to couple driver  65  to power source  67 , illustrated as a battery. Laser  61  and driver  65  are usually available commercially as a single unit.  
         [0030]     Audio processing circuit  68  is coupled to microphone  62  and provides a suitable output signal for headphones  63 . Audio processing circuit  68  includes variable gain, controlled by a user, and automatic gain control, to prevent unexpected loud noises from overloading the circuitry or damaging a user&#39;s hearing. Additional signal processing, such as spectrum filtering, frequency selective gain, noise cancellation, and echo cancellation, can be included as needed or desired for a particular application. The signal processing can be analog or digital.  
         [0031]     The invention thus provides a directional listening device having a narrower acceptance angle than listening devices of the prior art and accurately indicates the direction to a source of sound. Except for initial alignment during manufacture, the directional listening device accurately indicates the direction to a source of sound without further calibration or alignment. The directional listening device detects faint sounds without interference from mechanically coupled sounds through the handles for holding the device.  
         [0032]     Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, collimator  12  could be cylindrical if wall thickness were tapered to facilitate release from a mold. The type of sight is not critical. Red dot sights have little or no magnification. If desired, a standard mount, such as a Weaver rail, can be attached to collimator  12 . In this way, any sight that a person happens to use or to prefer can be attached to the directional listening device by means of the rail. While illustrated as a hand-held device, a directional listening device constructed in accordance with the invention can be fitted with a tripod mount or other mounting system suitable for a specific application, particularly if the directional listening device is scaled to a significantly larger size; e.g., a diameter of more than eighteen inches. A fitting for receiving sighting device  31  could be mounted on bracket  40 , handle  41 , handle  42 , or grip  43 . For smaller diameters, e.g. less than eight inches, handle  41  and handle  42  can be eliminated. Microphones other than omnidirectional electret microphones can be used for microphone  62 .