The ‘air-bone gap’ is the difference in sensitivity threshold when measured by air and bone conduction transducers. It is used to differentiate between sensorineural and conductive hearing losses (and combinations of the two). The specific pattern of thresholds can also aid in diagnosing certain causes of conductive or sensorineural losses, for example otosclerosis or noise induced loss. However, while not unheard of, relatively few types of conductive loss would be expected to selectively produce air-bone gaps at 4 kHz1. Moreover, in many cases such air-bone gaps are seen in the absence of any other evidence for conductive hearing loss such as positive symptoms, otoscopic examination and tympanometry, hence the phrase ‘false’.
There have been a number of investigations into the causes of this apparently false reading centred on the Radioear B71 bone vibrator, but it was not until relatively recently that a full explanation was put forward (Margolis et al, 2013). The findings from this study attributed the false air-bone gap at 4 kHz to an error in the calibration reference levels (RETFLs). Other theories such as airborne radiation from the bone transducer that enters the ear canal where it can be heard via the air-conduction route are less able to fully explain the phenomena.
References and caveats
1Age related changes in middle ear function have been described in the literature which may affect the high frequencies in particular (e.g. Feeney and Sanford, 2004), as have partial ear canal collapse/occlusion, partial ossicular disarticulation and non-organic hearing loss (e.g. Mustain and Hasseltine, 1981).
Feeney, M.P. and Sanford, C.A. (2004) Age effects in the human middle ear: Wideband acoustical measures. Journal of the Acoustical Society of America 116 (6) pages 3546-3558
Margolis, R.H. et al (2014) False air-bone gaps at 4 kHz in listeners with normal hearing and sensorineural hearing loss. International Journal of Audiology 52 (8) pages 526-32
Mustain, W.D. and Hasseltine, H.E. (1981) High frequency conductive hearing loss: a case presentation. The Laryngoscope 91, pages 599-603