Abstract:
A moving-armature transducer assembly suitable for use as an alerter in a portable telephone. The assembly includes a transducer housed in an enclosure including first and second acoustical chambers. A first sound emitted from a front hole on a front side of the transducer is propagated through the first acoustical chamber and emitted from a first port of the assembly. A second sound emitted from a rear hole on a rear side of the transducer is phase-shifted by the second acoustical chamber acting in combination with a second port or ports on the assembly to have a phase coinciding with the phase of the first sound. The second sound then combines with the first sound, reinforcing the first sound and producing a combined sound having an increased level and bandwidth.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to improvements in portable telephones and the like. More specifically, the present invention relates to improvements in the acoustic output of narrowband magnetic transducers used in alerters, for such phones and devices, flowing from the use of a phase inverting acoustical enclosure. 
     BACKGROUND OF THE INVENTION 
     Magnetic transducers, devices which convert electrical energy into mechanical energy in the form of sound waves, are typically based on a moving-coil or a moving-armature design. Due to their small size and low cost, moving-armature magnetic transducers often find use in portable cordless or cellular phones as alerters which may also be referred to as ringers or buzzers. Typically, a moving-armature transducer includes a diaphragm which produces sound, the sound being emitted from front and rear holes in the transducer. Unlike moving-coil (dynamic) magnetic transducers found in high fidelity speakers and telephone earpiece receivers, smaller moving-armature magnetic transducers having much stiffer diaphragms are narrowband frequency response devices which typically only operate in the 1800 Hz to 2800 Hz range, rendering them unsuitable for use in speech reproduction. In contrast, a moving-coil magnetic transducer can functions from approximately 300 Hz through 3300 Hz and higher, the frequency range typically used to reproduce the human voice for telephone communications. 
     Most designers of telephone sets use narrowband magnetic transducers as alerters by merely placing an acoustical output hole in the transducer close to a port in a housing of a telephone handset. This design is haphazard because acoustical leaks can greatly affect the output volume, not only lowering the output volume, but causing great variability in the output volume among individual telephone sets. Sound from the front output hole can leak into the telephone housing so that less sound gets through the telephone housing port and to the listener. Sound output from the back of the diaphragm also escapes from a rear hole in the transducer and, through destructive interference, can cancel sound from the front hole, either within the telephone housing or in the listening space. 
     A more sophisticated mounting scheme uses a gasket, which is typically soft rubber or closed cell foam, to seal around the front of the narrowband magnetic transducer and prevent the sound from the front hole from leaking into the housing or being canceled by sound from the rear hole. But even in this scheme, the sound from the rear holes is lost in the telephone set or leaks out of openings in the set and partially cancels sound from the front hole within the listening space. 
     U.S. Pat. No. 5,655,017 discloses a portable telephone with a detachable speaker suitable for voice communication having a moving-coil magnetic transducer based on a bass reflex design. The bass reflex speaker increases the acoustic response of the wideband moving-coil magnetic transducer in the frequency range for voice reproduction in hi-fidelity products and telephone communications. For example, a typical moving-coil loudspeaker, 25 mm in diameter and thus approximately 500 square mm in area, might typically have a resonance frequency around 700 Hz. A successful bass reflex design to extend the response to even lower frequencies would require a rear acoustical enclosure in excess of 50 cubic centimeters (cc). In contrast, a miniature moving-armature transducer, such as might be utilized by ever smaller portable telephone and communicator alerters needs to take up less than half that area and be coupled to a far smaller rear enclosure having a volume of approximately 1 to 10 cc. In combination, the resulting lower mass and lower compliance of the moving-armature transducer&#39;s diaphragm and the enclosure&#39;s acoustical compliance produce resonance frequencies in the neighborhood of 2000 Hz. Thus, these magnetic transducers are typically used in very different applications from those in which moving-coil transducers are used. Existing moving-armature alerter designs suffer from having a low acoustical output level due to their small size, as well as narrowband response at higher frequencies. Because of their inherent low compliance and narrowband response, it was not immediately apparent that a moving-armature mechano-acoustic system could be made to function satisfactorily in a phase-inverting mode, particularly with a miniaturized rear acoustical enclosure of the size allowable given typical design constraints in space restricted applications such as portable phones. 
     SUMMARY OF THE INVENTION 
     The present invention provides improved acoustical alerting output of a narrowband moving-armature transducer which may be advantageously contained within a telephone housing. As addressed above, presently, sound from the front hole of the transducer is typically directed outside of the housing, providing an audible alerting signal, while sound from the rear holes of the transducer is typically directed into the housing and attenuated or lost. While moving-armature magnetic transducers are reasonably high in output sound pressure level over a narrow frequency band, they could be even more efficient if the sound directed into the housing could be redirected out of housing, in the correct phase, so as to reinforce the sound generated by the front of the diaphragm and associated front port. When used as the alerter in cordless telephones, the primary complaint against moving-armature magnetic transducers is their low acoustic level. Therefore, improvements in the audible acoustic output of these devices would be extremely advantageous. 
     The present invention provides methods and apparatus for increasing the audible output of narrowband magnetic transducers. As discussed above, the sound output from the rear hole of the narrowband magnetic transducer may be lost in the telephone set or leak out of the housing and partially cancel the sound emitted from the front hole of the transducer. A more efficient implementation of a narrowband magnetic transducer would minimize this interference and use the sound from the rear hole to reinforce the sound emitted from the front hole. 
     The present invention advantageously utilizes a phase inverting acoustical enclosure contained within the telephone handset to augment the sound output of the front hole of a narrowband magnetic transducer. With the phase inverting acoustical enclosure tuned to a frequency below the diaphragm&#39;s resonance frequency, the front hole output is generally reinforced in the frequency band from below the diaphragm resonance to up through the diaphragm resonance. Thus, the acoustical output increases within a frequency bandwidth that is more advantageous for customer alerting. In addition to the higher output sound pressure level, the widened frequency response is extremely useful to: (1) provide a more pleasant lower-frequency alerting signal, (2) provide an alerting signal not as readily attenuated within a room environment in which a portable telephone may be subject to use, (3) provide an alerting signal more likely to be heard by certain listeners with a particular frequency of hearing loss, and (4) provide an alerting signal comprised of multiple frequency components both to avoid being masked by room noise and to provide for distinctive alerting. Utilizing the present invention, these advantages can be enjoyed without the need to deliver additional power to the magnetic transducer, or use a larger or more expensive magnetic transducer. 
     In addition to cordless telephone handsets, the present invention&#39;s applicability extends to other devices, such as cellular or wireless mobile phones, or other devices that use a narrowband magnetic transducer in a small volume for providing an alerting signal. 
     A more complete understanding of the present invention, as well as further features and advantages, will be apparent from the following Detailed Description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a dross sectional drawing of a typical art cordless telephone handset; 
     FIG. 2 is a cross sectional drawing of a cylindrical narrowband moving-armature magnetic transducer; 
     FIG. 3 is a cross sectional drawing of a first mounting scheme for a narrowband moving-armature magnetic transducer in accordance with the present invention; 
     FIG. 4 is a cross sectional drawing of a second mounting scheme for a narrowband moving-armature magnetic transducer in accordance with the present invention; 
     FIG. 5 is a graph showing the frequency response of a narrowband moving-armature magnetic transducer for various mounting schemes in accordance with the present invention; and 
     FIG. 6 is a flowchart of a process in accordance with a present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides methods and apparatus for increasing the output of narrowband acoustical alerters by utilizing a phase inverting acoustical enclosure contained within the telephone handset to augment the sound level output. The present invention now will be described more fully with reference to the accompanying drawings, in which several presently preferred embodiments of the invention are shown. This invention may, however, be embodied in various forms and should not be construed as limited to the embodiments set forth herein; rather, applicants provide these embodiments so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     FIG. 1 shows a cross sectional view of a typical prior art cordless telephone handset  100 . An antenna  114  and a keypad  104  connect to the exterior of a housing  110 . A microphone  106  is contained within the housing  110 . A moving-coil magnetic transducer  102  is mounted inside the housing  110  and functions as the earpiece. Power for the handset  100  is provided by a battery  112 . A narrowband moving-armature magnetic transducer  108  provides an audible alerting signal. 
     FIG. 2 is a cross sectional drawing of an exemplary cylindrical narrowband moving-armature magnetic transducer  200  of diameter 16 mm and depth 8.5 mm suitable for use in accordance with the teachings of the present invention. This transducer  200  may be utilized with the enclosure  300  of FIG. 3 to replace transducer  108  in handset  100  of FIG. 1 as discussed further below. A circular diaphragm  206  connects to a cylindrical permanent magnet  204 . A circular armature  220  is bonded to the circular diaphragm  206 . A pole  214  is positioned within the magnet  204 , leaving a working air gap  222  between the center of the armature  220  and the pole  214 . A coil  218  winds around the pole  214 . A printed wiring board  233  connects to a case  202  and pole  214 . The case  202  encloses the diaphragm  206 , armature  220 , magnet  204 , pole  214 , working air gap  222 , coil  218  and printed wiring board  233 , while connecting to the magnet  204 . The case  202  includes a front hole  212 . The pole  214  and printed wiring board  233  are formed to provide rear holes  208 . A direct sound pressure  216  is emitted from the front hole  212 , while a phase inverted sound pressure  210  (compared to direct sound pressure  216 ), from the rear side of the diaphragm  206 , is emitted from the rear holes  208 . 
     The attraction of permanent magnet  204  mechanically biases the magnetically conducting diaphragm  206  so that a static distance, called a working air gap  222 , between the diaphragm  206  and the pole  214  is created. The magnetically conducting armature  220  serves to direct magnetic flux across the working air gap  222  in an efficient manner to allow transduction. When a signal current flows through the coil  218 , the magnetic attraction between the diaphragm  206  and pole  214  within the working air gap  222  is varied and the diaphragm  206  moves, creating a sound pressure level that varies with the magnitude of current applied. While direct sound  216  is emitted from the front hole  212  of the case  202 , phase inverted sound  210  is emitted from the rear holes  208 . When the direct sound  216  collides with the phase inverted sound wave  210 , destructive interference between the opposing phase of the two waves causes a reduction in the sound level heard by a listener. 
     FIG. 3 is a cross sectional drawing of an exemplary acoustical enclosure based mounting arrangement  300  for the narrowband moving-armature magnetic transducer  200 , described above in FIG. 2, in accordance with the present invention. A presently preferred transducer for use as the transducer for use as the narrowband moving-armature magnetic transducer  200  is the KB-12G, a 16 ohm resistance device that may be obtained from SWC Electronics Ltd. Unless otherwise noted, the dimensions given herein are for a design hereinafter referred to as Design I. The magnetic transducer  200  connects to a cylindrical gasket  330  which is typically composed of a soft rubber, foam or glue. The magnetic transducer  200  contains the front hole  212  and rear holes  208 . The cylindrical gasket  330  connects to a housing wall  332  of thickness 2.5 mm containing a front port  336 , 3.0 mm in diameter, which is positioned alongside, but not immediately adjacent to, the front hole  212 . A front acoustic cavity  342  of volume 0.08 cc is thus formed. A cylindrical acoustical enclosure  306  abuts the housing wall  332  and encloses the magnetic transducer  200  and the gasket  330 , forming a phase inverting rear acoustical cavity  338  having a volume of 1.6 cc. The housing wall  332  also contains two rear ports  334 , 2.0 mm in diameter, which are positioned outside the gasket  330 , but inside the acoustical enclosure  306 . The direct sound pressure  216  emitted from the front hole  212  propagates through the front port  336 . The phase inverted sound pressure  210  emitted from the rear holes  208  passes through the phase inverting acoustical cavity  338  and rear ports  334  before being emitted from the rear ports  334  as a rear sound component  310 . It should be appreciated that variations on this design may be readily employed to achieve a variety of design objectives. For example, the design may be varied depending upon the electrical drive signal to be employed or the resonant frequencies of operation desired. As alternative design, which may be referred to as Design II, varies from Design I in that the front port  336  is 0.9 mm in diameter and the rear ports  334  are 2.8 mm in diameter. 
     When a current passes through the coil  218  of magnetic transducer  200 , the sound emitted from the front hole  212  is passed through the front port  336 , with the gasket  330  preventing or substantially reducing sound leaks into the interior of the phone. The phase inverted sound  210  from the rear holes  208  passes through the phase inverting acoustical cavity  338  and rear ports  334 , which are tuned for Design I to a frequency advantageously below the diaphragm  206  resonance frequency of the magnetic transducer  200 . Likewise, for Design II, the resonance frequency associated with the phase inverting cavity  338  and rear ports  334  is advantageously below the diaphragm  206  resonance frequency of the magnetic transducer  200 . Thus, for both Design I and Design II, the sound from the rear ports  334  reinforces the direct sound  216  emitted from the front port  336 , resulting in an increased sound pressure level and wider frequency response. The rear sound  310  emitted from the rear ports  334  is now in phase with the direct sound  216 . The resonance frequency of the rear ports  334  is inversely proportional to the square root of the product of the compliance of the acoustical cavity  338  and the acoustic mass of the rear ports  334 . The acoustic mass may be adjusted higher by reducing the port diameter and/or increasing the port thickness. The acoustic mass may be adjusted lower by increasing the port diameter and/or reducing the port thickness. It is noted that the transducer case  202  conveniently provides a portion of the boundary of acoustical cavity  338 . 
     FIG. 4 is a cross sectional drawing of a second exemplary acoustical enclosure based mounting arrangement  400  for the narrowband moving-armature magnetic transducer  200  of FIG. 2, in accordance with the present invention. Again, the narrowband moving-armature magnetic transducer  200  may suitably be the KB-12G, a 16 ohm resistance device, which may be obtained from SWC Electronics Ltd. The magnetic transducer  200  connects to a housing wall  432 , such as a wall of the telephone handset  100  of FIG.  1 . Unless otherwise noted, the dimensions given are those for a design referred to as Design III. The housing wall  432  of thickness 2.5 mm contains a front port  436 , 3.0 mm in diameter, and rear ports  434 , 2.0 mm in diameter. A cylindrical acoustical enclosure  406  connects to the magnetic transducer  200  and the housing wall  432  forming a front acoustical cavity  442  having a volume of 0.08 cc, and a rear phase inverting acoustical cavity  440  having a volume of 1.6 cc. The direct sound pressure  216  emitted from the front hole  212  propagates through the front acoustical cavity  440  before being emitted from the front port  436  as direct sound  416 . The phase inverted sound pressure  210  emitted from the rear holes  208  propagates through the phase inverting acoustical cavity  440  and rear ports  434  before being emitted from the rear ports  434  as a rear sound  410 . Design IV varies from Design III in that the front port  436  is 0.9 mm in diameter and the rear ports  434  are 2.8 mm in diameter. 
     When a current passes through the coil  218  of magnetic transducer  200 , the direct sound  216  emitted from the front hole  212  passes through the front acoustical cavity  442  and front port  436 , becoming direct sound  416 . The phase inverted sound  210  from the rear holes  208  passes through the phase inverting acoustical cavity  440  and rear port  434 , which is tuned for Design III to a frequency advantageously below the magnetic tranducer&#39;s diaphragm  206  resonance frequency of the magnetic transducer  200 . Likewise, Design IV, the resonance frequency associated with the phase inverting cavity  440  and rear ports  434  is advantageously below the diaphragm  206  resonance frequency of the magnetic transducer  200 . The rear sound  410  emitted from the rear port  434  is now in phase with the direct sound  416 . Thus, for both Design III and Design IV, the sound from the rear port  434  reinforces the direct sound  416  emitted from the front port  436 , resulting in an increased sound pressure level and wider frequency response. The resonance frequency of the rear port  434  is inversely proportional to the square root of the product of the compliance of the acoustical cavity  440  and the acoustic mass of the rear port  434 . The acoustic mass may be adjusted higher by reducing the port diameter and/or increasing the port thickness. The acoustic mass may be adjusted lower by increasing the port diameter and/or reducing the port thickness. 
     This arrangement allows the energy associated with all resonances to combine constructively and to produce a high output and enhanced bandwidth. The enhanced alerting response can be at lower frequencies than prior designs have readily allowed. Thus, the present invention allows for alerting signals composed of multiple frequencies (distinctive ringing) that are more pleasant and not as easily masked by noise. This aspect is particularly useful for those listeners with high frequency hearing loss. 
     FIG. 5 is a graph  500  showing a comparison of a first sound output curve  502  reflecting a sound output of a moving-armature transducer assembly of the prior art, a second sound output curve  504 , reflecting a sound output of a moving-armature transducer assembly according to Design I of the present invention, described in connection with the discussion of FIG.  3 . FIG. 5 also includes a third sound output curve  506 , reflecting a sound output of a moving-armature transducer assembly according to Design II of the present invention, also described in connection with the discussion of FIG.  3 . It can be readily seen that each of the second and third sound output curves  504  and  506  reflects a greater frequency range than the first sound output curve  502  and also reflects a substantially higher sound level than does the first sound output curve  502 . Modifications of the design of a moving-armature assembly such as Design I or Design II can be made depending on a particular output curve desired. As indicated earlier, the diaphragm resonance frequency is higher than the frequency associated with the phase inverting cavity and ports. Namely, in output curves  504  and  506 , the diaphragm resonance frequency is seen to be 2700 and 3100 Hz, respectively. Similar output curves will be produced by the moving-armature assemblies of Design III and Design IV, with the selection of appropriate dimensions for those designs. 
     FIG. 6 is a flowchart  600  illustrating a method of sound enhancement for a moving armature transducer according to the present invention. At step  602 , a first sound is emitted from a first side of the transducer and a second sound is emitted from a second side of the transducer. At step  604 , the first sound is directed into a first acoustical cavity and out of the first acoustical cavity. At step  606 , the second sound is directed into a second acoustical cavity and phase-shifted to be in phase with the first sound, combining with the first sound so as to reinforce the first sound.