Evaluation of the Credibility of Tone Audiometers in Determiniation of Air Conduction and Bone Conduction Hearing Thresholds

Background:
The result of audiometric measurement is the basis for medical decisions regarding treatment, surgery and medicines therefore its credibility is critical.

Objectives:
To examine the level of measuring deviations in a group of tone audiometers that have been calibrated and inspected by calibration centre and asses its impact on medical decisions.

Methods:
222 audiometers with headsets submitted for annual calibration and inspection were examined for the purpose of the study. Calibrations were performed on Brüel & Kjær and G.R.A.S. equipment according to ISO 60465 standard. The calibration corrections provided for each audiometer for 11 air conduction frequencies (125Hz-8000Hz) for both right and left earphone and 10 bone conduction frequencies (250Hz-8000Hz) were analysed.

Results:
In case of air conduction 11.3% audiometers required a correction of more than 5dB in case of one frequency, and 5.4% audiometers required a correction of more than 5dB in case of two frequencies. However in case of bone conduction the results were 85.6% and 49.1% respectively.

Conclusions:
We found significant differences between air conduction and bone conduction deviations which proves the importance of regular calibrations and inspections of audiometers. The deviations may lead to wrong determination of the type of hearing loss and false assessment of conductive hearing loss. We consider that the major cause is the fragility of bone conduction earphone. We therefore recommend that in case of assessed conductive hearing loss, the assessment should be double-checked on another audiometer and/or with Weber and Rinne tests using tuning fork.