Temperature effects on mechanical couplers

Audiologists should know that the performance of the mechanical couplers used to calibrate bone-conduction devices is susceptible to temperature effects.  The effects of temperature are often overlooked but Audiologists need to keep in mind that errors calibrating audiometers are directly transferred onto the audiograms of patients.  The mechanical coupler system is often transported from place to place by the person performing the calibration and depending on the outside and inside temperature of various clinics, errors in calibration can occur.

Current IEC standards requires a temperature of 23^oC +/- 1 when using a mechanical coupler.  A popular mechanical coupler system is the Bruel and Kjaer model 4930 shown below.

  click to enlarge

When the temperature of the coupler is different from that stated in the standard, a correction value cannot be applied to measurements performed at these temperatures.  The reason for this is because a change in the temperature will affect both the force sensitivity of the device and its mechanical impedance.  When the mechanical impedance is altered, this affects the force output of the bone vibrator.      

Dowson and McNeill (1992) conducted a study on the effects of temperature on the performance of audiometric mechanical coupler systems.  They conducted a study to measure the performance of a mechanical coupler in temperatures varying from 18 – 33 degrees C.  In addition, they also measured the ability of the coupler to reach a room temperature of 23 degrees C from 5 degrees C. 

Dowson and McNeill (1992) found that measurements of force sensitivity level and mechanical impedance level generally gave standard deviations of less than 0.2 dB with isolated cases reaching as high as 0.6 dB.  Of the three couplers tested, all three showed a significant temperature effect at higher frequencies (4 & 5 kHz).  They stated that if measurements were made at 18 degrees C instead of 23 for the test couplers, then an error in calibration approaching 3 dB would occur at higher frequencies due solely to temperature effects. 

Dowson and McNeil (1992) also found a temperature dependence on voltage output when driven by a bone vibrator type B-71.  They gave an example whereby if a mechanical coupler was stored overnight in 13 degree C temperature and then coupled to a bone vibrator to calibrate an audiometer, an error of 5 dB would result at the higher frequencies based solely on temperature effects.  Audiologists need to know that if this happens, then this error gets built into the audiometer’s calibration.  These results may not be typical of all coupler and bone vibrator combinations.   

The thermal capacity of a mechanical coupler is used to determine how long it takes for it to reach the desired standard calibration temperature when previously stored at another temperature.  Dowson and McNeil’s (1992) test couplers required 3 hours to reach 20 degrees C from 5 degrees C, and a further 1.6 hours to reach 22 degrees C.  

It is important to keep in mind that with the exception of extreme cases, the effect of temperature on the mechanical coupler often may not be large enough to cause calibration errors resulting in clinically significant threshold differences.  What is important is that these effects get built into the audiometer's calibration and when combined with other possible errors in measurement variability, can result in an additive amount of error that can have significant effects on clinical results.  

The take home message here is that a simple element such as temperature can affect the calibration of your audiometer.  Since it is not something you can physically see it can be easily overlooked by both you and the person calibrating your audiometer.   

Note:  This site is best viewed using IE 4.X or higher under 800x600 resolution