Abstract:
The invention is an improved piezoelectric noise-making and audible signaling device which produces a distinctive 2 KHz tone that is more pronounced and significantly easier to perceive, especially in a noisy environment. The volume of the audible tone is amplified by using two or three sound-amplifying chambers or cavities.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an improved noise-making device used to provide audible alarms in a wide variety of devices including, for example, automobiles and trucks, industrial equipment, medical devices, traffic signals, appliances and the like. Such devices can use a piezoelectric transducer and associated circuitry to produce sound at a given frequency. The transducer flexes in response to an applied voltage. If an oscillating voltage is applied to the transducer at an appropriate rate, the flexing of the transducer produces an audible sound of substantial volume. Prior devices produce an audible sound at 3 KHz.  
           [0002]    In the invention, the sound produced is not 3 KHz. Instead, the sound produced is at a lower frequency, 2 KHz, that is more easily heard and distinguished, especially in a noisy environment. A sound at 2 KHz is also more likely to be perceived by more persons than is a sound at 3 KHz, due to the loss of hearing as the normal person ages. In another feature of the invention, multiple sound cavities amplify in stages the sound produced by the transducer, making it considerably louder and easier to hear.  
           [0003]    U.S. Pat. No. 5,990,784 “Schmitt Trigger Loud Alarm With Feedback,” is incorporated by reference herein and describes an alarm device using a piezoelectric transducer. This patent is owned by the assignee of the present invention.  
         SUMMARY OF THE INVENTION  
         [0004]    According to the invention, there is provided a piezoelectric transducer and associated electrical circuitry to cause the transducer to oscillate at a resonant audible frequency. The transducer is mounted to a proximal tubular housing which is hollow, thus providing a first cavity. A second or distal tubular housing forms a second cavity adjoining the first cavity, and is of larger diameter than the first cavity. A third cavity adjoining the second cavity may optionally be employed. Sound is produced by the transducer and passes through the first cavity, second cavity and, if used, the third cavity. The sound is through a grill on the last cavity. The invention provides an audible sound at 2 KHz by means of an amplifier circuit including feedback from the transducer to the amplifier. The transducer resonates, producing an audible sound at 2 KHz. The cavities cause the sound to be greatly amplified when compared to similar devices not using multiple cavities. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a cross-section of the noise-making device.  
         [0006]    [0006]FIG. 2 is another cross-section of the noise-making device including dimensions that have been determined to optimize the amplification.  
         [0007]    [0007]FIG. 3 is a graph showing the increase in sound produced by the invention compared to a devices with different configurations, and all using the same electrical circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]    Referring now to FIGS. 1 and 2, there is shown a piezoelectric transducer  18 . Transducer  18  is mounted at its nodal diameter to a knife-edge  17  at an end of a housing insert  16 . Adhesive  19  binds the transducer  18  to the knife-edge  17 . Knife-edge  17  supports the transducer  18  while at the same time allowing the transducer to flex when a voltage is applied to it. Mounting the transducer at its nodal diameter minimizes interference with flexing of transducer  18 .  
         [0009]    Housing insert  16  is cylindrical in cross-section and hollow, forming a sound-amplifying cavity  15  next to the transducer  18 . One suitable material for housing insert  16  is 6/6 nylon or “ABS. ” A source for 6/6 nylon is Zytel  101  available from Pro Tech Plastic Inc., 1295 West Helena Drive, West Chicago, Ill., 60185. The length “A” of housing  16  is adjusted to maximize the amplification.  
         [0010]    A main housing  11  is cylindrical in cross-section and hollow. Main housing  11  is attached to an end of housing insert  16 . A flange  21  on main housing  11  engages and is secured by any convenient means to a flange  22  on insert  16 . Main housing  11  is hollow, and has two cylindrical sections with different diameters. One cylindrical section forms a sound-amplifying cavity  13 , and a second larger cylindrical section forms another sound-amplifying cavity  14 . The diameters of cavities  13  and  15  are typically about the same, whereas the diameter “B” of cavity  14  is larger. A grill  10  may be attached to the end of housing  11  away from the transducer  18 , and allows sound produced by the transducer, and amplified in the cavities, to be emitted and heard.  
         [0011]    [0011]FIG. 2 shows the invention with dimensions that have been found to produce a sound increase of about 10 to 15 dbA compared to devices using the same transducer and circuitry, but lacking the housing insert  16  and therefore having only one cavity. Dimension “A” is 0.438 inches. Dimension “B” is 1.460 inches. Dimension “C” is 0.088 inches. Dimension “D” is 0.492 inches. The diameters of housing  11  and housing insert  16  are 0.875 inches, approximately the same as the nodal diameter of transducer  18 .  
         [0012]    [0012]FIG. 3 shows the sound levels produced by devices which all use the circuitry shown in the &#39;784 patent, and the following different variations in structure:  
         [0013]    #1—Housing insert  16  is omitted, and the transducer  18  is mounted on a knife-edge  12  on main housing  11 .  
         [0014]    #2—Housing insert  16  and main housing  11  are used, with transducer  18  mounted on knife-edge  17  as shown in FIG. 1.  
         [0015]    #3—The same as #2, except that the length of housing insert  16  was reduced by 0.025 inches from the dimension shown in FIG. 2, 0.438 inches.  
         [0016]    #4—The same as #2, except that the angle of knife-edge  12  has been increased slightly, from about 40 degrees to about 20 degrees.  
         [0017]    #5—The same as #2, except that the diameter “B” of sound-amplifying cavity  14  was decreased by 0.100 inches from the dimension shown in FIG. 2, 1.460 inches.  
         [0018]    Use of housing insert  16  causes a large increase in sound produced, from 89.9 dbA to 98.9 dbA. and As can be seen, elimination of the housing insert causes a very significant drop in emitted sound from approximately 97 to 99 dbA to approximately 90 dbA. The maximum increase in sound is achieved by employing three sound-amplifying chambers or cavities, by choosing dimension “A” to be about one-half of the nodal diameter of transducer  18 , and by choosing dimension “B” to be roughly one to two times the nodal diameter. Optimum dimensions are readily determined by measuring the sound output of different configurations.