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Influence of Temperature on the Output of a Mechanical Coupler

This research determined the influence of temperature on the output of two Bruel and Kjaer 4930 mechanical couplers using a Radioear 8–71, 8–72, and a Pracitronic KH 70 bone vibrator. When the temperature of the mechanical couplers was varied from 17°C (62.6°F) to 29°C (84.2°F) in steps of 3°C (5.4°F) and the bone vibrators had a constant temperature, the output of each mechanical coupler systematically decreased as its temperature increased for each bone vibrator type. The largest output differences for both couplers and each vibrator occurred at 4000 Hz. The average of these differences across the couplers and vibrators was 6.6 dB higher at 17°C than at 29°C which equates to a nearly linear output change of about 1 dB/2°C (1 dB/3.6°F). When the temperature of the bone vibrators was 17 and 29°C and the mechanical couplers were maintained at a constant temperature, there were little or no differences in the output of the mechanical couplers. The temperature of a mechanical coupler rather than that of the bone vibrator is a critical variable in bone conduction calibration. Mechanical coupler temperaturedependent output coefficients were determined for use in bone conduction calibration from 250 to 4000 Hz for all three bone vibrator types and at 6000 and 8000 Hz for the KH 70.