Coupler output level differences resulting from repeated transducer placement.
Coupler output levels were obtained for six repeated placements of a TDH-49 earphone in a P/N 51 cushion on a NBS 9A coupler and a B-71 and B-72 bone vibrator on a mechanical coupler from 250 to 4000 Hz using experienced (N = 9) and untrained subjects (N = 9) who were later trained. The largest minus the smallest coupler output levels across the repeated placements were determined for each subject and transducer at each frequency. The coupler output level differences were similar across subject group for each transducer at each frequency and were not reduced by training. The median and mean differences for the TDH-49 were less than 1 dB, consistent across frequency, and the highest range score never exceeded ANSI tolerance levels. The median and mean differences for the B-71 and B-72 were higher than for the TDH-49 but did not exceed ANSI tolerance levels and were frequency dependent corresponding to the high frequency resonance slope of the vibrator. However, for some subjects, the differences exceeded ANSI tolerance levels for both vibrators, especially for the B-72 at 250, 2000, and 4000 Hz.