MICA: Novel applications of microphone technologies to hearing aids.

The purpose of the network is to develop and pursue ideas for improvement to hearing aid technology. The network will focus on technologies associated with microphones. Four initial ideas are proposed, but the network will work to develop more. Two of the four are subject to parallel applications for funding from EPSRC. Concrete proposal for pilot work on the fourth idea are included. The four initial ideas are:-

(1) Small microphones produce their own noise that, once powerfully amplified, becomes audible to the user. An associated application to the EPSRC will develop novel low-noise microphones to address this problem using MEMS technology. MEMS microphones could also facilitate multiple microphone noise reduction techniques (see (4)).

(2) Strong amplification means that any sound leakage from the ear canal may be picked up by the nearby microphone and reamplified, causing a whistling feedback loop. The leakage can only be prevented by a tight seal, but in order to combat the occlusion effect, many hearing aids are deliberately "vented"; a hole is drilled in the moulding to make the user's voice sound more natural. Moreover, many "instant fit" hearing aids make no attempt to block the canal in the first place. Instead, modern digital hearing aids attempt to remove the feedback using digital signal processing. This works by attempting to model the ever changing feedback path and subtracting the predicted feedback signal from the microphone input or by detecting the presence of a whistling sound and briefly cutting amplification at that frequency in order to break the loop. These methods often fail to discriminate between sustained tones in the environment, notably those in music, and the whistle of feedback, so it has a bad effect on enjoyment of music. We will address methods of more accurately idendifying genuine feedback.

(3) In day-to-day usage, the required amplification is often not achieved. It is difficult to verify that a hearing aid, once fitted, is producing the right level of amplification. It can be measured by an audiologist in a skilled procedure known as real-ear measurement, but this takes specialised time and equipment and is only reliable at low frequencies. Consequently, higher frequencies are not amplified for fear that excessive sound levels may occur. we will explore ways of better monitoriing the sound level in the ear canal and of delivering the right amplification across the freqeuncy spectrum.

(4) Finally, even one sound is amplified to the right degree, this amplification helps users little with their principal difficulty of understanding speech in environmental noise, such as a room full of backgruond conversation. This is because a damaged auditory system has a wider range of deficits than a mere loss of sensitivity. Given the degraded state of the user's auditory system, removal of this background noise is the only established way to improve intelligibility. We will explore novel methods of reducing background noise, particularly through the use of multiple microphones. This idea is the subject of 3 linked proposals to the EPSRC.

The network will conduct a series of meetings and workshops to bring forward these ideas. It will sponsor network participants to attend conference outside their immediate area of research and develop a special session on future hearing aids at the 2016 BSA annuanl conference.

Four preliminary ideas will be explored:-

A small, sensitive, low-noise microphone could be created using MEMS technology. MEMS microphones with a low-noise characteristic could be made at some cost in terms of distortion, but existing techniques can be used to remove this with digital signal processing, and that processing could be built into an integrated low-noise digital microphone chip.

Insertion gain could optimised by placing additional microphones within the ear canal, perhaps using MEMS technology once again, to provide in-situ real-time measurement of ear-canal sound levels. This approach could also save time in the fitting process. The technique could be verified using laser vibrometry. Alternatively laser vibrometry of the tympanic membrane could itself act as the internal microphone.

Intra-canal microphones could also support more sophisticated approaches to feedback suppression. The time delay between signals detected from internal and external mikes could be used to discriminate between genuine external sounds and feedback (whose delay would match the acoustic transmission time from the loudspeaker to the external mikes).

Existing hearing aids generally use just two microphones and adaptive beam-forming to suppress a single interfering noise in different directions at different frequencies (they produce one "null"). Such systems are programmed to null out the most prominent sound in the rear hemifield and thus improve the SNR of speech sources in front of the listener, but they ultimately have quite low directional sensitivity. Only a small number of commercial systems have attempted alternative approaches (Siemens triano and Phonak's stereoZoom). The key to stronger beam-forming is the use of larger numbers of mikes, perhaps distributed over a pair of glasses. For instance, in principle, an N-microphone systems are capable of directing N nulls in each frequency channel. They can also be used in conjunction with sparse coding.

External impact will be supported by the communication plan. With an aging population, and an increasing retirement age, the number of individuals living and working with hearing impairment is rising. Current hearing aids are of limited benefit, resulting in unsatisfactory levels of acceptance. One NHS commissioning authority in England has even announced plans to decommission hearing aid provision for mild and moderate hearing loss, judging that their funds would be better directed to other areas of health provision. The project is intended to generate innovations in commercial hearing aids that will make them more effective and thereby more likely to used and valued by the hearing-impaired population. Such improvements will increase working efficiency and sustain the social engagement of hearing impaired people. It may also reduce costs to the health service, because prospective monitoring of individual health has shown a link between hearing impairment at study onset and later diagnosis of dementia (Lin et al. 2013). If one infers that this linkage is likely mediated by social withdrawal, then there is a good prospect of delaying dementia and its associated care costs through more effective audiological intervention.

These benefits can only be delivered through encouraging hearing-aid manufacturers to develop and invest in new technologies. Hearing aid companies all have active research and development divisions, but have limited resources for blue-skies innovations. The network will therefore bring together academics, hearing-aid technologists and end users in order to communicate the potential benefits and how they might be delivered in an acceptable way. Internal impact will occur directly through this interaction. A space will be created for the development of a long term vision of the future development of hearing aids, which we term the "roadmap". The roadmap will provide different potential routes and identify key technological obstancles to be overcome in following those routes. It will thus provide direction to those member projects that have won EPSRC funding and inspiration for the development of new collaborative research projects. The presence of two hearing aid companies in the mix will guarantee that plans are realistic and bring the new ideas generated directly to those in good position to implement them. The presence of hearing aid users, both within the network itself and through reference to focus groups will also act as a guiding influence.