Blog 2018-2: How to quantify the gain (amplification) of a bone-conduction device; comment to the systematic review by Bezdjian et al. (2017)

During the last decades, several new types of bone-conduction devices (BCDs) have been released for patients with conductive or mixed hearing loss. One rather recent innovation is the semi-implantable (transcutaneous) Sophono device (Medtronic; Jacksonville, Fl, USA), which is based on the Otomag device (Siegert et al., 2013; chapter 2, this website). This device makes use of a transcutaneous magnetic coupling between the externally worn conventional BCD and the skull. Recently, Bedzjian et al. (2017) reviewed published clinical experience with this device. Their overview aimed at ‘functional improvement’ and peri-operative medical issues. The Bedzjian et al. paper has been discussed before (Snik, 2018); below, that discussion is elaborated.

The ’auditory’ or ‘functional’ gain

The ‘functional improvement’ or ‘auditory gain’, as introduced by the authors, was defined as the difference between aided and unaided sound-field thresholds. In sensorineural hearing loss, the ‘auditory gain’, better known as the ‘functional gain’, is a measure of the amplification provided by the (air-conduction) device. That is not the case for conductive or mixed hearing loss when using a BCD; BCDs directly stimulate the cochlea, bypassing the impaired middle ear and thus the air-bone gap. Owing to the definition of ‘auditory gain’, however, the width of that air-bone gap directly affects the ‘auditory gain’.

To illustrate this: in case of aural atresia, assuming a total hearing loss of 70 dB HL and a mean ‘auditory gain’ of 30 dB, the aided thresholds are poor, viz. 40 dB HL. In case of mild conductive hearing loss of 40 dB HL, e.g. owing to chronic otitis, an ‘auditory gain’ of 30 dB implies near-normal hearing with the device. Obviously, the latter patient has more adequate amplification, although the ‘auditory gain’ is the same for either patient. As has been suggested before, it is more useful to analyse the aided thresholds in relation to the cochlear thresholds (bone-conduction thresholds, see Appendix 2.3, this website).

Bedzjian et al. (2017) reported that the ‘auditory gain’, averaged over studies, was 31.6 dB and they concluded that this was a satisfactory result. However, it only indicates that the BCDs did work but obviously not how adequately the BCDs were fitted.

Averaging the ‘auditory gain’ over studies is also debatable because some studies comprised patients with conductive hearing loss and other studies patients with single-sided deafness. In single-sided deaf patients, the BCD is implanted at the deaf side and works as a (transcranial) CROS device, a totally different application than a BCD for conductive hearing loss. Reading the original papers, its is clear that in single-sided deaf patients the ‘auditory gain’ was defined as the thresholds of the normal ear that was blocked with an ear plug minus the aided thresholds obtained with the CROS-BCD. Thus, to determine the ‘auditory gain’ the thresholds of the blocked normal ear were considered as ‘unaided thresholds’.
Clearly, the ‘auditory gain’ of a CROS-BCD, defined as the thresholds of the blocked normal ear minus the aided thresholds obtained with the CROS-BCD, doesn’t assess the benefit of the CROS-BCD as, amongst others, it depends directly on how effectively the normal ear was blocked. Nevertheless, oftenb done (references)

In summary. To illustrate the real capacity of any BCD, aided thresholds* should be related to bone-conduction thresholds for patients with conductive or mixed hearing loss. For patients with single-sided deafness, aided thresholds should be compared to the air-conduction thresholds of the normal hearing ear to see whether or not cross stimulation with the BCD compensates effectively the acoustic head shadow (see also previous blog 2018-1).
The ‘auditory gain’ or ‘functional gain’ is not an appropriate measure to assess the effectiveness of any BCD fitting.

* Note that using aided thresholds to assess the gain of a device is not straightforward if non-linear amplification (compression) is applied, see Appendix 2, this website. Using Bezdjiran et al.’s data, it seems to be justified to use the aided threshold to assess gain, as amplification seemed to be linear. That is concluded from the fact that the reported mean unaided minus aided free-field tone thresholds (assessing gain for low-level sounds) was comparable to the mean unaided minus aided speech reception thresholds (assessing gain for mid-level sounds), namely 31.6 dB versus 33.6 dB.


1. Bezdjian A, Bruijnzeel H, Daniel SJ, Grolman W, Thomeer HGXM, Preliminary audiologic and peri-operative outcomes of the Sophono™ transcutaneous bone conduction device: A systematic review, Int. J. Pediatr. Otorhinolaryngol. 101 (2017) 196-203.

2. Siegert R, Kanderske J, A new semi-implantable transcutaneous bone conduction device: clinical, surgical, and audiologic outcomes in patients with congenital ear canal atresia, Otol. Neurotol. 34 (2013) 927-934.

3. Snik A. How to quantify the ‘auditory gain’ of a bone conduction device; comment to the systematic review by Bezdjian et al. Int. J. Pediatr. Otorhinolaryngol. (2018). E-pub ahead of publication