this effect was being explored it was found that the apparent reduction in trimming and the port trimming instability can be substantially mitigated if the temperature of the electret microphone is elevated above normal room temperature to between about 40 u00b0 c and 70 u00b0 c therefore if an electret microphone is to be trimmed by x ray radiation it may be preferred that the trimming process be carried out at a temperature elevated above normal room temperature for example it was found to be beneficial when the x ray trimming process was applied to an electret microphone with identification number em 100ptnl available from primo either separately or in an assembly including the microphone that the process be performed at about 40 u00b0 c this being a temperature within a range in which a typical electronic apparatus is expected to perform normally many variations of the process will be apparent in view of the preceding description and may be influenced by what is convenient for the particular forms in which the controller and the source of radiation are conveniently available or may be provided furthermore variations of the process may be influenced by what is convenient for determining the amplitude of signals on the telephone line as these signals may be generated in the transmit circuits of either of analog or digital type telephone sets for example in the process the x ray radiation and the amplitude measuring may be continuous until the desired transmission characteristic is achieved in one alternative the radiation may be delivered sequentially in.