Air-Bone Gaps at 4 kHz Explained

10 mins
21 December 2021


I’ve noticed a consistent air bone gap at 4000 Hz during audiometry. Can you explain why this happens?

For a complete answer to this issue the reader is strongly urged to read Margolis et al. (2013) – citation below.

The short answer is that there appears to be an average air-bone gap of around 10 dB in normally hearing people and around 14 dB in people with sensorineural hearing loss at 4 kHz. It seems intuitive that with no middle ear disorders this air bone gap should not be there. Traditionally it has been attributed to air conducted radiation propagating down the ear canal, and plugging the ear when measuring bone conduction thresholds has been quite commonplace, although often ineffective (Tate Maltby and Gaszczyk 2015) . The paper by Margolis et al suggests the real cause of the air bone gap is a dependence of bone conduction thresholds at 4 kHz on the extent of sensorineural loss (hence why the effect increases from around 10 dB to around 14 dB with hearing loss). The paper describes how the Reference Equivalent Threshold Force Level (RETFL) used in calibration of bone conduction instruments could be adjusted by the above figures to compensate. 

References and caveats
Margolis, R.H.; Eikelboom, R.H. et al.  (2013) False air-bone gaps at 4 kHz in listeners with normal hearing and sensorineural hearing loss. International Journal of Audiology, 52 (8), pages 526-532

Tate Malty, M.; Gaszxzyk, D. (2015) Is it necessary to occlude the ear in bone-conduction testing at 4 kHz, in order to prevent air-borne radiation affecting the results? International journal of audiology, 54(12), pp.918-23 


Michael Maslin
After working for several years as an audiologist in the UK, Michael completed his Ph.D. in 2010 at The University of Manchester. The topic was plasticity of the human binaural auditory system. He then completed a 3-year post-doctoral research program that built directly on the underpinning work carried out during his Ph.D. In 2015, Michael joined the Interacoustics Academy, offering training and education in audiological and vestibular diagnostics worldwide. Michael now works for the University of Canterbury in Christchurch, New Zealand, exploring his research interests which include electrophysiological measurement of the central auditory system, and the development of clinical protocols and clinical techniques applied in areas such as paediatric audiology and vestibular assessment and management.

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