Physikalisch-Technische Bundesanstalt, Lab. 1.51, Bundesallee 100, D-38116 Braunschweig, Germany
As early as 1991 the Physikalisch-Technische Bundesanstalt (PTB) in Germany decided that tests of electromagnetic susceptibility have to be performed, in general, with all measuring instruments submitted to PTB for pattern evaluation, among them also pure-tone and speech audiometers. A large shielded room is available where electromagnetic fields (10 V/m, 10--935 MHz, 1-kHz tone AM modulation) can be generated. No suitable test method and no limits were found in any known standard for audiometers including IEC 645-1, and no available acoustic measuring equipment did pass the EMC test by itself. So a new test method had to be developed based on loudness balance tests between the 1-kHz-interference tone caused by the electromagnetic field and the ordinary 1-kHz test tone of the audiometer. The tests are performed in the shielded room at fixed carrier frequencies. Taking into account the uncertainty of this subjective test method, an upper limit of 60 dB HL for the interference level was established. Of the first 25 types tested, 92% audiometers did fail the EM susceptibility test in the first run. All of them passed the test after reconstruction.