Appendix 1

About the author: Ad Snik studied physics at the Eindhoven University of Technology and got his master degree in 1976. In 1982, he got the doctor’s degree from the same university. The title of his dissertation was ‘Visco-elastic properties of monomolecular layers”. He started to teach Physics and Electronics at a college for Bachelor students. In 1984, he entered a training to become a medical physicist and he was registered a few years later as medical physicist/ audiologist. Since 1988 he is employed at the ENT department of the Radboud University Medical Centre, where he was appointed as professor in 2006. Apart from Audiology and surface science, he wrote articles on conflicting streams in traffic and muscle fatigue. He published over 250 papers.

Affiliation: Donders Hearing & Implants. Donders Institute for Brain Cognition and Behavior, Department of ENT, Radboud University Medical Centre,

Department of Biophysics. Radboud University, Nijmegen, the Netherlands

Relevant memberships: AUROnet is an international consortium of individuals dedicated to a unified mission of improving patient health through development and application of comprehensive evaluations of the effectiveness of therapies for rehabilitation in patients with mixed or conductive hearing loss.

Scientific rankings:


Research gate Score 40,79 (March 2016).


Expertscape objectively ranks people and institutions by their expertise in more than 26,000 biomedial topics. The author is listed in the top 10 of experts in the fields of Hearing , Hearing Impairment, Hearing Aids, Bone Conduction and Sensory Aids (December 2015).


The author has and had no financial interests in any of the devices described in this blog, nor in any alternative device. This blog is partly based on research carried out at RadboudUMC, which was supported by Nobelpharma, Entific, Cochlear BAS, Symphonix, Med-El, Otologics, KNO fonds Nijmegen, Heinsius Houbolt fonds and the William Demant Foundation

Appendix 2: Validation of the auditory implant fittings; objective outcome measures

A2.1 Introduction

Outcome measures used to validate the fitting of a hearing device can be divided into subjective measures and objective measures. Subjective refers to the assessment of patient’s opinions with regard to the remaining disability, remaining handicap, satisfaction, and, more generally, the quality of life. Mostly, when referring to objective measures, it concerns hard data, like electrophysiological data, stapedius reflex data, etc. However, when evaluating hearing devices, objective refers to all data that are measurable, specifically (aided) hearing thresholds and (aided) speech perception (e.g. Humes, 1999). The most popular objective measure is speech recognition in quiet and/or in noise, obtained in the sound field (Gatehouse, 1998; Gurgel et al., 2012). In case of bilateral devices, other objective tests are added like directional hearing and speech recognition in noise with spatially separated speech and noise sources (Snik et al., 2015).

To quantify the outcome of an intervention in conductive or mixed hearing loss, surgeons often use hearing thresholds to evaluate reconstructive surgery (Toner & Smyth, 1993; Gurgel et al., 2012). The difference can be referred to as the functional improvement. In the nineties, certain rules were developed that could be used to see whether the results of surgery in terms of hearing thresholds were adequate; e.g. the Belfast rule of thumb and the Glasgow benefit plot (Toner & Smyth, 1993). Now that there is an alternative for reconstructive surgery, namely auditory implants with favourable aided hearing thresholds, these rules might become obsolete (e.g. regarding atresia repair, see chapter 5.1).

Talking about hearing devices, the difference between unaided and aided threshold values is indicative for the gain (amplification) of a device; however, although widely used, this measure has limitations.

A2.2 Validating the gain of an auditory implant in patients with sensorineural hearing loss

Probably the most widely used measure to assess the gain of a hearing device is the ‘functional gain’, which is, by definition, the difference between unaided and aided hearing thresholds. Essentially, functional gain assumes linear amplification.

Nowadays, most auditory implants don’t use linear amplification but instead non-linear amplification or compression. The most frequently used type of compression amplification (wide dynamic range compression; WDRC) implies that for soft sounds the gain provided by the device is relatively high, while for louder sounds, the gain is gradually reduced to zero. This kind of compression has been introduced to deal with pathological ‘accelerated’ loudness growth in sensorineural hearing loss (Dillon, 2012). Compared to linear amplification, compression ‘boosts’ low-level sounds; consequently ‘functional gain’ is boosted as well. When using compression, the ‘functional gain’ assesses just the gain of the device for soft sounds and overestimates the gain at higher input levels like normal conversational levels.

Snik and Cremers (2001) discussed two options to estimate the gain of the auditory implant at more realistic listening levels. Their first option was the determination of ‘gain at most-comfortable-listening level (MCL)’. In short, the patient’s MCL is determined at octave frequencies twice, once unaided and once with the hearing implant active, and the results are subtracted. In this way, the ‘gain at MCL’ is determined, frequency specific. Second option was the ‘gain in speech reception threshold (SRT)’; the SRT is determined twice, once unaided and once aided. The two SRT values are subtracted; the difference is the (non-frequency specific) ‘gain in SRT’. Essentially, a SRT level is typically 20-25 dB above pure-tone thresholds and, MCL levels, mostly even more than that, therefore, these two measures are less affected by compression. Snik and Cremers compared the gain measures in 14 patients using the VSB with 304 processor using compression to deal with the limited maximum output of the device. Averaged at 0.5, 1, 2 and 4 kHz, the ‘gain at MCL’ was indeed approx. 10 dB higher than the  ‘gain-at-threshold-level’. The ‘gain in SRT’ and ‘gain at MCL’ were comparable. It is obvious that gain at normal listening levels is more important for communication than ‘gain-at-threshold-level’. Rameh et al. (2010) published ‘gain-at-threshold’ data and SRTs obtained pre and post intervention, in 112 auditory implant users with sensorineural hearing loss. Using their data, similar differences (10 dB) or even higher were found between ‘gain-at-threshold-level’ and ‘gain in SRT’. So, ‘the gain-at-threshold level’ or the ‘functional gain’ is not a representative gain measure whenever compression is used.

Further, it should be noted that aided hearing threshold measurements might be affected by advanced signal processing: viz. by expansion to prevent the patient from hearing the microphone noise of the device) or by noise reduction algorithms (that might interpret test signals as noise and reduces their level). Concerning any interfering algorithms, these should be switched off before threshold measurements are carried out.

To evaluate hearing aid fittings in sensorineural hearing loss, the ‘functional gain’ is often used. However, when evaluating devices with non-linear amplification ‘functional gain’ doesn’t have the usual significance. Better option is to report the ‘gain in SRT’ or ‘gain at MCL level’. Whether or not devices use expansion or other algorithms that might affect threshold measurements should be clearly stated by the manufacturer and such options should be deactivated before the measurements, if possible. 

A2.3 Validating gain in patients with conductive or mixed hearing loss

To assess benefit of hearing devices in patients with mixed or conductive hearing loss, the ‘functional gain’ is not an appropriate measure. It should be realized that middle ear implants (with their actuator coupled to one of the cochlear windows) as well as bone-conduction implants directly address the cochlea; consequently, the mal-functioning middle ear doesn’t play a role. However, the status of the middle ear (width of the air-bone gap) does affect the outcome of a ‘functional gain’ measurement.

To illustrate this, let us assume a patient with a pure conductive hearing loss of 60 dB HL (bony aural atresia) using a device that addresses the cochlea rather perfectly (aided thresholds at 15 dB HL). Then, following the definition, the ‘functional gain’ is 60-15=45 dB. However, if the air-bone gap is 40 dB instead of 60 dB (partial atresia) while using the same device, then the ‘functional gain’ is (only) 40-15=25 dB. This example shows that owing to its definition, the ‘functional gain’ depends on the width of the air-bone gap, and, therefore, it is not a measure of the gain (amplification) provided by the implant.

If a patient with pure conductive hearing loss is fitted with a powerful bone conductor and if some every-day sound of 40 dB is indeed perceived as 40 dB by the patient, evidently, the gain is 0 (as the gain is the difference between output and input). In other words, the bone conductor has effectively compensated the conductive hearing loss. In the audiogram, this leads to aided thresholds coinciding with the cochlear (thus bone-conduction) thresholds. This gain, the difference between aided thresholds and the bone conduction thresholds, has been referred to as the ‘bone-conduction gain’ (Carlsson and Hakansson, 1997) or the ‘effective gain’ (Bush et al., 2017). The ‘effective gain’ might be negative, indicating that the aided thresholds are worse than the bone-conduction thresholds. That means that every-day sounds are (still) perceived attenuated and in the audiogram, a persistent although reduced air-bone gap is seen. When a patient has mixed hearing loss with an obvious sensorineural hearing loss component (SNHLc) of e.g. 50 dB HL, an ‘effective gain’ of 0 is not enough. Part of the SNHLc should be ‘compensated’ as well, to make speech sufficiently audible. The ‘effective gain’ should be positive, e.g. 22 dB (calculated target gain using the adapted NAL-rule; see Chapter 4); then, a 28 dB HL sound will be just audible for his patient, using that amplification.

Note that the ‘functional gain’ equals the ‘effective gain’ plus the air-bone gap of the patient. The ‘functional gain’ is dominated in most cases by the width of the air-bone gap. Therefore, in contrast to the ‘effective gain’, the ‘functional gain’ is not a direct measure of the quality of the device fitting.

It should be noted that the bone-conduction thresholds should be properly masked in case a middle ear implant is chosen. However, unmasked bone-conduction thresholds should be considered when a bone-conduction device is fitted, as any bone-conduction device doesn’t only stimulate the ipsilateral cochlea but also the contralateral cochlea. Where the sound is perceived depends on the sensitivity of either cochlea.

Two limitations of measuring aided thresholds have to be discussed. In case of a predominant conductive hearing loss, ambient noise level in the test booth should be low; if not the noise might affect the measurements falsely. Furthermore, expansion and/or compression might have been applied. Compression, to deal with a relatively low maximum output (Chapter 3), results in low (thus favourable) aided thresholds and expansion (to deal with device noise) results in higher (worse) aided thresholds (Dillon, 2012; chapter 10), respectively over- and under-estimating the gain provided by the device.

Unfortunately, gain at supra-threshold presentation levels (e.g. ‘gain in SRT’ or ‘gain at MCL level’) cannot be determined easily in conductive or mixed hearing loss. This is caused by limitations of the bone-conduction transducer (part of the audiometer) used to determine the bone-conduction thresholds. The standard B71 bone-conduction transducer has a relatively low maximum output (might be close to the patients MCL levels instead of LDL levels) affecting the MCL measurements. Its frequency response is poor in high and low frequencies (Alomari, 2014) affecting proper amplification of broadband sounds like speech. Additionally, the distortion of low-frequency sounds by the B71 is significant. The relatively new B81 bone-conduction transducer is the better option although the maximum output is still limited (Alomari, 2014).

If auditory implants are used in patients with conductive or mixed hearing loss, the ‘functional gain’ doesn’t give any information on the amplification provided by the implant. The ‘effective gain’ should be used, as it is a fitting-quality measure. The ‘effective gain’ is by definition the difference between bone-conduction thresholds and aided thresholds and, therefore, might be negative.

A2.4 References, Appendix 2

Alomari HM. Binaural hearing with bone conduction stimulation.  Thesis, 2014, University of Southampton

Busch S, Lenarz T, Maier H. Comparison of Alternative Coupling Methods of the Vibrant Soundbridge Floating Mass Transducer. Audiol Neurootol. 2016; 21(6):347-355

Carlsson PU, Håkansson BE. The bone-anchored hearing aid: reference quantities and functional gain. Ear Hear. 1997;18(1):34-41

Dillon H. Hearing aids. 2012. Second edition, Thieme Verlag, New York

Gatehouse S. Speech tests as measure of outcome. Scand Audiol Suppl. 1998;49:54-60

Gurgel RK, Jackler RK, Dobie RA, Popelka GR. A new standardized format for reporting hearing outcome in clinical trials. Otolaryngol Head Neck Surg. 2012;147(5):803-807

Humes  LE. Dimensions of hearing aid outcomes. J Am Acad Audiol. 1999; 10:26-39

Rameh C, Meller R, Lavieille JP, Deveze A, Magnan J. Long-term patient satisfaction with different middle ear hearing implants in sensorineural hearing loss. Otol Neurotol2010;31:883-892

Snik AFM, Agterberg M, Bosman A. How to quantify binaural hearing in patients with unilateral hearing using hearing implants. Audiol Neurootol. 2015;20 Suppl 1:44-47

Snik AFM, Cremers CWRJ. Vibrant semi implantable hearing device with digital sound processing. Effective gain and speech perception. Arch Otolaryngol Head Neck Surg 2001;127:1433-1437

Toner JG, Smyth GD. Comparison of methods of evaluating hearing benefit of middle ear surgery. J Laryngol Otol. 1993;107(1):4-5

Appendix 3: History of the blog and acknowledgements

In September 2014, Franco Trabalzini organised the EAONO meeting in Siena and asked me to organize a panel on Consensus on Auditory Implants. Preparing for that panel, I wrote a document what can be considered as the first version of the text of the present blog. Later that year, an updated version was send to my colleagues from Auronet (Auronet is a private initiative to develop a core set of patient-centred outcome measures which enable the search for the best treatment options for a patient with conductive or mixed hearing loss; Tysome et al., 2015).  Comments by my Auronet colleagues, especially Bill Hodgetts and Penny Hill, encouraged me to go on. Bill added valuable comments.

Chapters 1 to 3 were published on the internet on April 22th, 2015. Chapters 4 and 5.1-5.2 were added on May 6th and June 16th.

During the Osseo meeting in Lake Louise, Alberta (May, 21-23, 2015), the website was introduced to the public for the first time.

On July 28th, chapter 5.3 and 6 were uploaded. Martijn Agterberg critically read this part of the blog before it was published. Chapter 7 was added on August, 18th.

Owing to comments by Arjan Bosman, Martijn Toll, Tove Rosenbom and Marc Flynn, the text of chapters 1 to 4 was revised and re-published on September 7th and October 8th, 2015. During the latter update it was decided to skip the option for the readers to respond on line. From the start until early October, more then 90 responses were collected and all of them were spam. On March 29th 2016, chapters 5 and 6 were revised and Appendix 2 was added.

On August 12th 2016, chapters 5 to 7 and Appendix 2 were updated and revised. The remarks made by Hannes Maier are acknowledged.

Chapter 8, on the use of auditory implants in patients with pure sensorineural hearing loss, was added on August 20th 2016.

January 20th 2017, data regarding relatively new devices was added (Chapter 3). This concerned the Sophono Alpha 2 processor, the Cochlear Baha Attract and the Med-El BoneBridge.

On May 2nd 2017, chapter 1, 2, 3 and 5 were updated and revised. Amongst others, new data was added concerning new super-power percutaneous bone conductors. Appendix 2 was rewritten and Appendix 3 updated. 

In the month of November 2017, this website was updated and fully re-designed.

August 2018, Chapters 3, 4 and Appendix 2 have been re-written and updated.

December 2018, Chapters 6 and 8 are being re-written and updated. Blog 4 added.

A3.1 Reference Appendix 3:

Tysome J. et al., The Auditory Rehabilitation Outcomes Network: an international initiative to develop core sets of patient-centred outcome measures to assess interventions for hearing loss. Clin Otolaryngol. 2015;40:512-515