Miniature dual cavity ringer
BACKGROUND OF THE INVENTION 
     The invention relates to a ringer which is compact and provides a ring signal from a relatively low battery voltage source. 
     One-piece telephones and cordless telephones require small, inexpensive ringers. One-piece telephones are known which use piezo-ceramic disk type ringers. These ringers require a high voltage to operate effectively and are relatively large in diameter. The piezo-ceramic approach requires a large area, generally 40 millimeters or greater to implement. The acoustic output power is lower than desirable, due to insufficient battery voltage available from most cordless handsets, which commonly employ 3.9 volts direct current (DC) batteries. These ringers also tend to emit a high frequency sound which is very shrill and annoying, and are very susceptible to electromagnetic interference. Cordless telephones are known which use an earpiece transducer or speaker to emit the ring signal. However, such telephones have raised concern for possible accidental hearing loss to users. The speaker approach, whether or not used in the earpiece, is costly and requires a considerable volume in both area and depth to implement. Interface drivers and expensive tooling of case parts are required. 
     The U.S. Food and Drug Administration has recommended reducing the sound level from the earpiece of cordless phones to such a low level that the effectiveness of the ringer is substantially diminished. A separate ringer is now needed in cordless telephones to emit both a safe and loud sound. Size and power considerations are major limitations in known designs. 
     SUMMARY OF THE INVENTION 
     The invention provides for a telephone ringer which is substantially more compact, louder in sound level, more pleasant sounding, and which operates from a substantially lower DC voltage source than known devices. 
     The invention employs a low-cost miniature transducer as presently supplied by several manufacturers. The small diameter makes it substantially more suitable than the larger speakers and piezo-ceramic discs. Through use of a second reverberant cavity in front of the miniature transducer, a very large acoustic output power is achieved using the available 3.9 volts battery. A ringer according to the invention, due to its small relative size, may fit in the limited space inside a cordless telephone portable unit without costly tooling modifications. However, it is also suited for other uses, including for employment in compact one-piece telephones. 
     The small diameter of the ringer according to the invention allows sound to exit from the parting line of the handset and therefore does not require any special tooling changes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be better understood from a reading of the following detailed description in conjunction with the drawings in which: 
     FIG. 1 shows the ringer of the invention in cross section; and 
     FIG. 2 shows a circuit schematic employing the ringer of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A miniature dual cavity sound ringer according to the invention is illustrated in FIG. 1 and a ringer circuit schematic is shown in FIG. 2. 
     As shown in FIG. 1, the ringer of the invention uses a low-cost miniature transducer 1 as presently supplied by several manufacturers (for example, the Star Micronics QMB transducer). This transducer is an electromagnetic type consisting of a 0.47 inch diameter by 0.32 inch long cylindrical housing. The housing has a 0.08 inch diameter circular sound exit port 3 in front of the diaphragm 2, between which is located transducer reverberant volume VA. Port 3 has an aperture area AA, and the cavity length of volume VA equals LA. The diaphragm 2, which has thin magnetic material secured thereto, is driven by an electromagnetic coil 4 which in turn is driven by a trnsistor from a ring signal source (not shown in FIG. 1) as seen in FIG. 2. 
     This small transducer does not have sufficient acoustic output at 1 kilohertz (KHZ) to 2 KHZ to serve as a ringer output. Even at 4 KHZ, which is an optimum frequency for this size transducer, the output is not sufficient at the available 3.9 volts drive (Vcc) that is typical in cordless telephone portable units. 
     The ringer of the invention employs a small cylindrical slip-on cavity housing 5 comprised, for example, of plastic, which adds a resonant volume VB and a rectangular sound exit port 6 which in conjunction with transducer 1 give a 9 decibel (dB) increase in acoustic output. Port 6 has an aperture area AB, and the cavity length of volume VB equals LB. Volume VB can be adjusted by sliding cavity housing 5 along the exterior of transducer 1, for example, for consumer or producer variation of sound level. Alternatively, cavity housing 5 and transducer 1 can be mated by threading the interior of cavity housing 5 and the exterior of transducer 1 for engagement thereby. Volume VB adjustment could then be accomplishing by rotation of the one relative to the other for translation. 
     Test measurements at 10 centimeters away show that the original transducer 1 will emit at an 87 dB sound pressure level (SPL) at 1 KHZ from a 3.9 volts peak-to-peak (VP-P) source. When the cavity housing 5 is employed with transducer 1, then the measured sound level goes to 96 dB SPL, a significant increase in sound output. 
     As shown in FIG. 2, a sine or square wave signal at 1 KHZ, for example, is applied from the ring signal source via current limiting resistor R1 to the base of the transistor Q1, which amplifies the signal to the point of being in the switching mode and the collector then swings from the voltage level Vcc of a D.C. voltage source to ground. This voltage on the collector is then applied across the transducer coil 4 of ringer R. The coil which is an electromagnet moves the diaphragm 2 back and forth. A protective diode D may be employed in parallel with ringer R. 
     A cavity length ratio LB/LA of 3 along with an aperture area ratio AB/AA of 3 yields of 9 dB increase in acoustic level. The above ratio of each parameter generates a proper reverberation in volumes VA and VB. Sound is generated by said diaphragm and is emitted in a linear manner through volume VA, circular port 3, volume VB, and finally rectangular port 6. 
     While the present invention has been described in connection with a preferred embodiment thereof, it is to be understood that additional embodiments, modifications, and applications which will become obvious to those skilled in the art are included within the spirit and scope of the invention as set forth by the claims appended hereto.