Air-Bone Gaps at 4 kHz Explained

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