TRANSMITTER ASSEMBLY FOR A TELEPHONE HANDSET

Abstract: A transmitter assembly for a telephone set uses a commer­ ciaily available cartridge electret transmitter, by modifying the response curve. This is obtained by an adapter housing the transmitter with a first volume defined between the end of the transmitter and the end of the bore in which the transmitter fits. At the end of the bore there is an end wall with a hole therethrough, the hole covered with a layer of acoustic resistance material. A second volume is formed on the other side of the end wall, by a hollow formation on the wall of the transmitter housing of the handset. A slot is formed in the transmitter housing wall within the formation. The first volume provides damping, the hole and acoustic resistance material provides dB limitation at a particular frequency (1000Hz) and the second volume and slot provide resonance at a second frequency (3000 Hz).

Description: This invention relates to a transmitter assembly for a telephone handset, and is particularly concerned with such an assembly which uses a cartridge-type microphone, i.e. an electret microphone of non-specialized form and adapts it to telephone use. Commercially available cartridge-type electret microphones as used in tape recorders and similar devices are very inexpensive but have a flat response curve. Electret microphones produced for telephone sets have not normally been of a cartridge-type and while having the desired response curve, are more expensive. A mounting using a standard telephone microphone, which is designed to reduce conversation interference by external noise is disclosed in JP-A-57131195 and has a tubular housing enclosing the microphone. A wall extends across the bore of the tube to create two volumes one of which is open and the other of which is closed by the microphone. The microphone communicates with the open-ended volume through a hole in the wall across the bore. The present invention uses a commercially available cartridge microphone together with a specifically designed adaptor which physically adapts the cartridge microphone for mounting in the handset but also shapes or modifies the response curve so that it is acceptable for telephone use. The present invention provides a transmitter assembly for a telephone handset comprising a cartridge electret microphone, means enclosing the microphone comprising a tubular housing defining a cylindrical bore, an end wall extending across a front end of the bore and defining with the microphone a first volume, a hole extending through the wall, a layer of acoustic resistance material extending over the hole and means defining a second volume on the side of the end wall remote from the first volume, characterized in that the adapter further comprises a ledge extending around the bore a pre-determined distance from the front end of the bore, deformable means extending into the bore at a rear end, the cartridge microphone being a snap insertion part the deformable means and being held between the deformable means and the ledge, and in that the second volume is a contained volume having a predetermined resonant frequency. The invention will be understood by the following description of an embodiment, by way of example, in conjunction with the accompanying drawings, in which:- Figure 1 is a cross-section through an assembled cartridge microphone and adaptor; Figure 2 is an exploded perspective view of a handset with the cartridge microphone transmitter and a receiver; Figure 3 is a cross-section on the line III-III of Figure 2 illustrating the assembly of transmitter to the housing of the handset; Figure 4 is a front face view of the transmitter housing; Figure 5 is a view, on the interior surface of the transmitter housing with the transmitter assembly omitted; and Figure 6 illustrates the performance curve desired for a telephone transmitter, and also the curve normally obtained with the cartridge microphone. As illustrated in Figure 1, a cartridge microphone, indicated at 10, is mounted in a hollow adaptor 11. The adaptor comprises a tubular housing part 12 with a front flange 13 which extends across one end of the tubular housing. A small hole 14 is provided in the flange 13. An annular rib 15 on the front face of the flange 13 defines a shallow recess 16 and a disc of acoustic resistance material 17 is fastened, as by welding or bonding, to the front face of the flange within the rib 15. The tubular housing has three slots 18 extending from the open back face of the adaptor. A deformable means 19 in the form of a shallow rib extends inwards at the back edge of the housing . The cartridge microphone is pushed into the housing deforming the wall outwards slightly as the cartridge is pushed past the rib 19, which then snaps in behind the cartridge. The front peripheral cover 20 of the cartridge is held firmly against a narrow ledge 21 extending around the inside of the housing. The ledge positions the front face of the cartridge a predetermined distance from the flange 13. Terminals 22 extend from the back surface of the cartridge for connection to the telephone circuitry. As illustrated in Figure 2, the cartridge microphone and adaptor, as in Figure 1, is mounted on a small circuit board 25, which also carries a modular jack 26. This assembly is mounted in the transmitter housing of the handset, the housing indicated at 27. The adaptor rests on a tubular formation 28 extending up from the inner surface of the front face of the housing 27. The circuit board 25 rests on the pillar 29 and is fastened thereto by a screw 30. An annular seal 31 is positioned between the adaptor 11 and the tubular formation 28. The modular jack 26 is aligned with an aperture in the wall of the housing 27 for reception of a modular plug. Figure 2 also illustrates the rest of the handset, which comprises the front and back members 35 and 36, the front member also having a receiver housing at 37, the receiver indicated at 38 and a sealing washer at 19. The electrical connections from the receiver to the circuit board 25 is by two wires 40. The assembly of the microphone 10, adaptor 11, circuit board 25 and jack 26 is seen in cross-section in Figure 3. In the front face 41 of the transmitter housing 27, there is formed an arcuate slot 42. The slot is within the formation 28 and is wider at its rearward part, as seen in Figure 3. For aesthetic reasons a groove 43 is formed in the front surface of the front face, continuing the narrower part of slot 42. However this groove does not extend through the front face. The groove 43 and slot 42 can be seen also in Figure 4 and the slot 42 is seen in Figure 5. The cartridge microphone, as stated, does not have, of itself, a satisfactory response curve, and does not meet telephone standards. The response is modified by the mounting of the microphone. Thus firstly the cartridge is mounted in the adaptor with a particular volume between the face of the cartridge and the end of the adaptor. In the end of the adaptor is a small hole 14, covered with the acoustic material 17. The volume between the cartridge and the acoustic resistance is such as to provide the required damping. The acoustic resistance, from the acoustic material 27, is chosen to limit the peak to 14 dB rise from 1000 Hz. The size of the hole 14 is related to the acoustic resistance of the material 17. The volume between the base of the adaptor and the front face of the housing 27 and the slot 42 cooperate to produce a resonance at 3000 Hz. This latter volume, referred to as the front volume, is sealed at the interface between adaptor 11 and the tubular formation 28 by the seal 31. Figure 6 illustrates the modified response curve obtained by the invention relative to an unmodified transducer. The chain dotted line 50 illustrates an unmodified transducer response curve and full line 51 illustrates the ideal or mean curve obtained with the invention. The dotted lines 52 illustrate a tolerance envelope acceptable for the curve 51.

Claims: A transmitter assembly for a telephone handset comprising a cartridge electret microphone (10), means (11) enclosing the microphone comprising a tubular housing (12) defining a cylindrical bore, an end wall (13) extending across a front end of the bore and defining with the microphone a first volume, a hole (14) extending through the wall (13), a layer of acoustic resistance material (17) extending over the hole and means (27, 28, 41) defining a second volume on the side of the end wall (13) remote from the first volume, characterized in that the adapter (11) further comprises a ledge (21) extending around the bore a pre-determined distance from the front end of the bore, deformable means (19) extending into the bore at a rear end, the cartridge microphone (10)being a snap insertion part the deformable means (19) and being held between the deformable means (19) and the ledge (21), and in that the second volume is a contained volume having a predetermined resonant frequency. An assembly as claimed in claim 1, the means defining a second volume characterized by a transmitter housing (27) of the telephone handset, the transmitter housing (27) having a front face (41), and a tubular formation (28) extending from an inner side of the front face (41), the adaptor (11) seated on an inner end surface of the tubular formation (28). An assembly as claimed in claims 1 or 2, the tubular housing defining the cylindrical bore characterized by a plurality of slots (18) extending from the back end of the tubular housing (12), and a shallow rib extending inward at the back end, the shallow rib forming the deformable means (19) extending into the bore. An assembly as claimed in claim 2, characterized by a slot 42 extending through the front face 41 within the tubular formation 28. An assembly as claimed in claim 4, characterized by the first volume being of a predetermined value to provide predetermined damping, the acoustic resistance of the acoustic resistance material 17 being of a value to limit the peak to 14 dB rise at 1000 Hz, and the second volume being of a predetermined value to produce, in combination with the slots 42, a resonance at 3000 Hz. An assembly as claimed in any one of the preceding claims, characterized by an annular rib 15 extending from a front surface of the end wall 13 coaxial with the bore, the layer of acoustic resistance material 17 being attached to the front surface within the annular rib 15.

Inventors: BUSCHE, ALAN CHARLES, FREEMAN, RAYMOND CARL, GUMB, BEVERLEY WILLIAM
Applicants: NORTHERN TELECOM LIMITED
Application year: 1985
Main classification: H04M1
IPC codes: H04R1, H04M1