I must profess at the outset that I am not an expert on (central) auditory processing disorders (CAPD or APD). Far from it. I am a hearing aid person; and the solutions I propose would only be appropriate if the description of or the requirement for the solution to the problem is valid. In that regard, because a possible problem of APD is difficulty with speech understanding in noise, amplification that provides a signal-to-noise ratio (SNR) improvement should be a good solution. The recently reported successful use of Frequency Modulation (FM) devices on this population (e.g., Johnston et al, 2009) is encouraging. On the other hand, I would like to posit that hearing aids, especially today’s digitally wireless hearing aids, could offer a more acceptable solution than FM for the SNR problem experienced by children with APD.
An FM device achieves SNR improvement through the use of analog wireless transmission technology. The speaker’s voice is picked up by a remote microphone, which is then wirelessly transmitted to an FM receiver. The FM receiver may be integrated into a hearing aid, worn independently as earphones, or as a stand-alone behind-the-ear (BTE) hearing aid. In the later case, the BTE is worn in an open-ear manner to avoid occluding the ear canal and to allow direct entrance of environmental sounds. The main reason for its effectiveness, which is reported to be as great as 15-20 dB SNR improvement (Hawkins, 1984), is that the remote microphone bypasses the effects of distance, reverberation and interfering noise which degrade a speaker’s speech to the listeners.
But there is nothing magical about an FM device in achieving the SNR improvement. It is an acoustic device; and as such, it must obey the same acoustic constraints that such devices follow. For example, the input stage of an FM microphone typically goes into compression limiting at around 70-75 dB SPL. This means a speaker who speaks slightly louder or has the FM mic closer to the mouth will likely create a “muffled” sound for the listener. Furthermore, FM systems typically have an output bandwidth below 7000 Hz. This could restrict the audibility of very high frequency sounds like the /s/. In addition, the 15-20 dB SNR improvement that one sees reported on FM would largely disappear when it is worn in an open-ear manner. This is because the direct sound (noise in this case) will mix with the transmitted sound and yield a lower effective SNR. Indeed, the higher the environmental noise level, the greater diminishment of the reported SNR. Unless someone is willing to wear an FM system in quiet only, in the FM alone mode, and/or with an occluded earmold (or headphone), the reported SNR improvement will NOT be realized. Another practical consideration is how many teenagers are willing to walk around with a big BTE over their ears and stick a remote FM mic to every person to whom they are going to talk? What if there are multiple speakers? A more practical solution is needed.
An acceptable solution should be cosmetically appealing, ensures a favorable SNR, allows portability, and should not be dependent on speaker participation. Thus the use of personal hearing aids may be an option. In addition to the advantage of portability, the recent miniature thin-wire receiver-in-canal (RIC) hearing aid (e.g., Widex Passion) provides a cosmetic motivation for children to accept and wear the device. From the acoustic standpoint, hearing aids have a higher input saturation level (as much as 105 dB SPL in the CLEAR hearing aids), broader bandwidth (Passion bandwidth in excess of 8400 Hz in a 2 cc coupler) and a better sound quality than an FM system. In addition, almost all current day hearing aids have a directional microphone and a noise reduction algorithm that could help improve the SNR of the listening environment. Kuk et al (2008) reported on the use of a mild-gain open-ear hearing aid in 14 children (between 7 and 11 years) diagnosed with APD. They were evaluated on the NU-6 words and the Auditory Continuous Performance Test (ACPT) in noise. The hearing aids were 
evaluated in the omnidirectional microphone mode only, omnidirectional microphone with noise reduction mode, and directional microphone with noise reduction mode. The results showed that the use of the hearing aids in the omnidirectional microphone mode alone did not improve speech identification in noise over the unaided condition. However, the inclusion of the noise reduction algorithm and directional microphones improved speech understanding in noise. Improvements in real-life and school activities were also reported.
Despite the availability of a directional microphone and noise reduction algorithm on almost all current hearing aids, the maximum theoretical SNR effectiveness of hearing aids is admittedly not the same as the FM. But the difference between hearing aids and FM will likely narrow when both are used in an open-ear manner. Furthermore, it is important to recognize that hearing aids can be worn everywhere without the constraint of the wearer holding a separate remote microphone to the talker. And if the wearer desires the SNR improvement of the FM microphone, one should choose hearing aids that are also digitally wireless. This offers the advantages of a higher input saturation level, a broader bandwidth (up to 11 kHz on WidexLink) and better sound quality. A remote microphone can be connected to the TV (or M) DEX devices to achieve the SNR improvement. This way, children with APD may use the hearing aids in its standard mode for most daily situations, and use the aids with the DEX/remote microphone whenever it is necessary.
A main reason people hesitated using hearing aids for children (or adults) with APD is the amount of gain specification since these individuals have normal hearing. In our preliminary study, we found that 10-15 dB gain for very soft sounds, 5-10 dB for conversational sounds, and close to 0 dB gain for very loud sounds would be acceptable. As a reference, the natural resonance that is provided by one putting his/her hand behind the pinna is approximately 10-12 dB in the 1000-2000 Hz region. The maximum power output (MPO) control should be adjusted to a level typical of wearers with normal hearing (<100 dB SPL). The hearing aids should also have a directional microphone (preferably multichannel fully adaptive) and noise reduction algorithm. The hearing aids should be worn binaurally in an open-ear manner. Although an open-ear fitting ould compromise the SNR offered by the directional microphone and noise reduction algorithm to about 1-2 dB, it is 1-2 dB more than what the APD child would otherwise receive.
Dr. Francis Kuk, PhD, is Vice President of Clinical Research, Office of Research in Clinical Amplification (ORCA) at Widex Hearing Aid Company, USA. He received his doctorate from the University of Iowa. Since then, Dr. Kuk has assumed clinical, research and faculty positions at the University of Iowa Hospitals and Clinics and the University of Illinois-Chicago. He has published over a hundred articles in various peer-reviewed and trade journals, and book chapters. Dr. Kuk is currently a board member of the American Academy of Audiology Foundation.
References
- Hawkins D (1984) Comparisons of speech recognition in noise by mildly-to-moderately hearing-impaired children using hearing aids and FM systems. J Speech Hear Disord 49(4): 409-418.
- Kuk F, Jackson A, Keenan D, Lau C. (2008) Personal amplification for school-age children with auditory processing disorders. J Am Acad Audiol. 19(6):465-480.
- Johnston K, John A, Kreisman N, Hall J, Crandell C. (2009) Multiple benefits of personal FM system use by children with auditory processing disorder [APD] Int J Audiol 48(6): 371-383.
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