Patent Publication Number: US-6905001-B1

Title: Sounders for fire alarm systems

Description:
FIELD OF THE INVENTION 
     This invention relates to sounders for fire alarm systems. 
     BACKGROUND TO THE INVENTION 
     Some fire alarm systems have sounders built into the bases which receive the detectors. These are referred to in the industry as “behind detector” sounders. Such sounders are as a consequence usually on the ceiling of the protected area. The specifications of other fire alarm systems require the use of wall mounted sounders. Such sounders are “stand alone” units and do not incorporate detectors. 
     Most sounders are powered directly off the communication lines and hence the power available is small. As a consequence such sounders must be efficient, and the present invention seeks to provide a sounder which produces acceptable all round sound levels using the power available on the communication lines. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to the present invention there is provided a sounder comprising walling defining a first air column leading to a first sound aperture through which the first column opens to atmosphere, walling defining a second air column leading to a second sound aperture through which the second column opens to atmosphere, a diaphragm between said columns, and means for causing said diaphragm to vibrate. 
     Preferably said sounder comprises an elongate cavity extending centrally of said sounder, said diaphragm dividing said cavity into first and second parts, said first part, at the end thereof remote from the diaphragm, leading into a re-entrant hom, said first sound aperture being in the form of a circumferentially extending aperture. 
     In the preferred form said re-entrant horn preferably has a first hairpin bend therein adjacent said end of said elongate cavity, said bend placing said cavity in communication with a reflex section lying adjacent said elongate cavity, a second hairpin bend, the second bend placing the end of said section remote from the first hairpin bend in communication with an outlet section of said hom which section itself leads to said circumferentially extending sound aperture. 
     The end of the second part of the central cavity remote from the diaphragm preferably splits into a plurality of passages, there being a right angled bend in each passage whereby the passages extend radially outwardly from said elongate cavity to sound apertures which open radially outwardly. There are, in the preferred form, six passages which are equally spaced circumferentially of the elongate cavity. 
     To optimize sound output the difference between the length of the first column and the length of the second column is substantially equal to half a wavelength at the frequency at which the diaphragm vibrates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: 
         FIG. 1  is an axial section through a wall mounted sounder; 
         FIG. 2  is a view of a base part of the sounder; and 
         FIG. 3  is a graph illustrating the configuration of a hom. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The sounder illustrated comprises a base part  10  with means (not shown) for fixing it to a wall and a horn part  12 . Between the parts  10  and  12  there is a unit  14  containing a diaphragm  16 . An elongate central cavity  18  extends from the base part  10  to the underside of a space  20  which receives a lamp  22 . The space  20  is closed by a transparent cover  24 . 
     The cavity  18  joins, just underneath the space  20 , a tortuous passage  26  which constitutes a reflex hom. The hom is circular in front elevation. 
     The passage  26  has a first hairpin bend  28  therein, the bend  28  reversing the direction of the passage  26  thereby providing an intermediate reflex passage section  30  which extends in the direction away from the space  20 . A second hairpin bend  32  reverses the direction of the passage  28  again. The bends  28  and  32  form the ends of the section  30 . 
     Beyond the bend  32  there is a sound aperture  34  which opens to atmosphere in the direction away from the base part  10 . The sound aperture  34 , when the sounder is viewed in the direction of arrow A, is essentially circular in configuration and is bounded by an inner circular wall  36  and an outer circular wall  38 . 
     The line along which the parts  10  and  12  join along their outer peripheries is designated L. The part  10  comprises a web  40  of disc-like form with a flange  42  around the periphery thereof and a web  44  of dish-like form with a flange  46  around the periphery thereof. 
     The unit  14  comprises a circular casing having a base wall  48 , a short hollow sleeve  50  protruding from the base wall, and a peripheral wall  52  protruding from the base wall  48  in the opposite direction to the sleeve. A lid  54  closes off the space which receives the diaphragm  16 . The lid has an opening in it which communicates with the part of the cavity  18  to the right of the unit  14  as the sounder as illustrated in FIG.  1 . The periphery of the diaphragm  16  is between a circumferential rib  56  which stands proud of the base wall  48  and an O-ring  58  which is placed in the unit  14  before the lid  54  is pressed into place. 
     The end of the part of the central cavity  18  to the left of the unit  14  splits into six passages. Two of the passages, designated  60 , are shown in FIG.  1 . Each passage  60  has a right angled bend  62  therein and opens to atmosphere generally radially of the sounder, that is, parallel to the wall on which the sounder is mounted. 
     The sound aperture at the end of each passage  60  is designated  64 . 
     It will be noted that the diaphragm  16  divides the airways of the sounder into two separate sections. The impedance of the columns of air between the diaphragm  16  and the outlets  34  and  64  respectively are optimized acoustically to provide the requisite sound distribution between the reflex horn and the passages  60 . This is achieved by making the difference in length between the column of air extending from the diaphragm  16  to the outlet  34  and the column of air extending from the diaphragm  16  to the outlet  64  equal to one half a wavelength at the frequency at which the diaphragm vibrates. If this relationship is not adhered to the sound levels decrease due to destructive interference between the sound sources represented by the outlets  34  and  64 . 
     Each horn passage  26  and each hom passage  60  is bounded by a shown in FIG.  3 . This plots the length of the horn area of the horn. It will be seen that the area of the horn with distance from the diaphragm  16 .