1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 0 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%0(/.%G0G042!.3-)33)/.G0G015!,)49-%!352%-%.43G0G02%,!4%$G0G04/G0G030%#(,/5$.%3334!.$!2$G0G0#/.$)4)/.).G0G0-%4(/$G0G0 amended at Malaga-Torremolinos, 1984, Melbourne, 1988)1 Since the characteristics of car
2、bon microphones are strongly dependent on conditioning techniques, it isnecessary to follow a consistent procedure prior to measuring sensitivity/frequency characteristics in order to obtainreproducible results. The CCITT recommends that for best reproducibility, automatic mechanical conditioning be
3、 used.The following steps are specified for the standard conditioning method:a) Place the handset in a holding fixture with the handset clamped in a position corresponding to that in whichthe microphone is going to be measured e.g. loudness rating guard-ring position (LRGP) according toAnnex A of Re
4、commendation P.76.b) Connect the microphone or telephone set terminals as required to the d.c. feed circuit and appropriateterminating loading.c) Turn the feed current on. After 5 seconds, condition the microphone by rotating it smoothly. Rotation ismade such that the plane of the granule bed moves
5、through an arc of at least 180. The procedure isrepeated twice with the handset coming to rest finally in the test position. The time of each rotation cycleshould lie within the range of 2 to 12 seconds.2 When carrying out subjective tests with a carbon microphone telephone set, the conditioning of the handsetshould be done by the talker. This conditioning should conform to the conditioning for objective measuring asdescribed under 1 above insofar as it is practicable.
