This short article looks at ‘adjusting’ self-recording audiometry in order that it might be compared to manual audiometry such as pure tone audiometry.

This short article looks at ‘adjusting’ self-recording audiometry in order that it might be compared to manual audiometry such as pure tone audiometry.

There is much debate as to the utility of occupational hearing audiometry in NIHL cases.  The value and reliability of the previous audiometry might depend on which side is making the argument. It might also depend on whether the audiometry supports later audiometry or not.

Self-Recording Audiometry compared to Pure Tone Audiometry

Self-recording audiometry is where the frequency, the changes in hearing levels, and the recording are all done automatically.  This can be done as a continuous sweep, such as Békésy's original equipment, or at fixed frequencies.  For pure tone audiometry an audiologist manually changes the frequency and hearing levels, and manually records the results.

However, putting aside arguments as to the utility of such audiometry for another day, self-recording audiometry (as seen in the audiogram seen on this page) does not result in directly comparable readings to manual audiometry.  Firstly, the recorded thresholds are found to be ‘better’ than pure tone audiometry.  Secondly, self-recorded measurements are not in 5 dB steps as for pure tone audiometry.

Accordingly, BS 6655, as noted in ‘the Black Book’ (Guidelines for Medicolegal Practice, Assessment of Hearing Disability, King, Coles, Lutman & Robinson) gives a formula to adjust threshold readings from self-recording audiometry to manual/ pure tone audiometry.  

The formula

The formula is straightforward and easy to adjust:

“Add 3 dB to the self-recorded measurement, then round to the nearest 5 dB”

Example

The example in the Black Book gives results for two tests:

	1 kHz	2 kHz	3 kHz
Self-recording measurement	24	26	43
Manual recording equivalent	25	30	45

	1 kHz	2 kHz	3 kHz
Self-recording measurement	22	24	51
Manual recording equivalent	25	25	55

For Practitioners:

Use of occupational health audiometry, especially self-recording audiometry, is often controversial in NIHL cases.  Experts often disagree as to what degree such audiometry can be relied upon, if at all, and how accurate it is.  However, if it is to be considered, then it is suggested that it should be ‘adjusted’ as set out in BS 6655 and 'the Black Book' before being compared to later manual/ pure tone audiometry.