Audiometer Calibration: 

Variability in Headband Force

Audiologists need to know that the headband force exerted by an earphone or bone conductor needs to be calibrated according to North American standards (ANSI 3.26-1981) or International Standards (ISO 389, 1991).  This however is often one of the most neglected areas of audiometer calibration and can be a cause for threshold measurement variability.   

International Standards requires that the static force exerted by the earphone or bone vibrator headband be within a certain limit when calibrating pure tone audiometers.  According to ISO 389 (1991), the headband should provide a nominal static force of 4.5 N +/- 0.5 N when the earphone is placed over the ears, but this figure has been changed in ISO 389-3 (1994) to 5.4 N +/- 0.5 N.  The 1994 ISO standard is similar to the North American standard.  ANSI S3.26-1981 standards states that the force should be 5.4 +/- 0.8 N.  

An average head width of 145 mm is said to comply with this standard for adult populations.  For forehead placement of the bone vibrator, 190 mm is said to comply within this standard.  Audiologists should know that there is no recommended procedure for making this measurement.

A study by Smith et al. (1997) surveyed two of the largest suppliers of audiometers in the UK and found that no routine measurement of force was made after the instrument was manufactured, and no quality control was in place.  In addition, Smith et al. (1997) looked at how the headband force varied over time and use, and how it varied with head size.  The earphones used in this study were the Telephonics TDH39P, TDH50P.  The bone vibrators were the Radioear B71.   

Results from Smith et al. (1997) found that the range of force across several clinical audiometers was larger for earphone headbands than for bone vibrator bands (2.8-7.6 N for earphones headbands vs. 4.0-5.8 for bone vibrator bands). 

The variability in headband force was examined over a 16 month period of use and time.  The maximum difference between measurements over this period of time were 0.4 and 0.3 N for earphone headbands and 0.4 N for bone vibrator headbands. 

The greatest causes of headband force variation was due to different separations and positions on the headband.  This variation was greatest for the bone vibrator headband because unlike the earphones, the Radioear B-71 headband is not adjustable.  Bone vibrator head band force varied by up to 59% for larger diameter separations.  Many of the results for force varied across diameter separation do not comply with ISO ANSI and British standards. 

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