The performance and calibration of TDH39 earphones fitted with Model 51 and MX41/AR cushions

Coupler measurements were made to compare the performances of new-style Model 51 cushions and old-style MX41/AR cushions fitted to TDH39 earphones. In addition, the calibration, using an artificial ear, of TDH39 earphones fitted with Model 51 cushions was compared with the calibration, using a reference coupler, of the same earphones fitted with MX41/AR cushions. It is concluded that the two cushion types can be taken as identical when fitted to TDH39 earphones, except at 6 kHz where differences in earphone sensitivity of up to 2.8 dB were observed. Likewise, output hearing levels of TDH39 earphones measured using a reference coupler and an artificial ear were found to be similar, except at 6 kHz where differences of up to 6.4 dB were observed. The data support the decision by ISO to declare the Model 51 equivalent to the MX41/AR cushion, thereby requiring TDH39 earphones to be calibrated using a reference coupler whichever cushion is fitted. However, they also highlight once again the ill-defined performance of TDH39 earphones at 6 kHz.