Feasibility Study of Self Calibration of Audiometric Earphones

For clinical accuracy, it is important that verifiable information about the sensitivity of earphones used in conjunction with audiometric equipment be made obtainable to the user in the field. A form of reciprocity calibration has been examined and evaluated for 100 pairs of TDH-39's. The two earphones of one headset are alternately used as transmitters and receivers acoustically coupled in a small cavity. The electrical, acoustical, and mechanical constants of the transducers are lumped together by taking the absolute averages of the sensitivities of 100 pairs of earphones, where the absolute sensitivity is traceable to the NBS, and by integrating these sensitivity averages with the transmitter-receiver voltage transfer characteristic. A calibrating K factor is thus obtained and made available by the TDH-39 manufacturers laboratory. With this factor, the average sensitivity of a pair of earphones may be determined in the field from the relation Sav=K−(V1−V2), where V1 is the driver voltage and V2 the reeciver voltage. By using a third earphone, it is possible by an extension of the above analysis to determine the absolute sensitivity of any one of the three units.