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Hearing Aid Noise Reduction Technologies

2014-12-04

One of the most pressing difficulties of persons with hearing loss is an inability to effectively understand speech in the presence of noise. Studies have shown that around 40% of adult wearers continue to be unsatisfied with their ability to hear in noise after being fitted with hearing aids (Kochkin, 2010). Numerous research studies have shown that children with hearing loss experience even more difficulty understanding speech in noise when compared to their peers with typical hearing or adults with hearing loss (McCreery et al., 2010; Stelmachowicz et al., 2001). Hearing aid manufacturers have developed numerous technologies to improve performance in noise, including directional microphones, digital noise reduction (DNR), wind noise reduction, and dereverberation algorithms.

Directional microphone technology typically employs two or more microphones to amplify sounds coming in from the front as prescribed, while limiting or reducing amplification for sounds coming from the sides and back. This approach assumes that the wearer will face the signal of interest in a noisy environment, and consequently, the speech will be enhanced and the surrounding noise will be reduced. Numerous research studies have shown that directional microphones improve speech understanding in noise more than any other technology currently built into hearing aids (although directional microphones do not provide as much improvement in speech understanding in noise as remote microphone radio frequency systems—also commonly known as FM systems—which are an assistive technology that may be used with hearing aids to provide the most improvement in speech recognition in noise (Schafer & Thibodeau, 2004)). As a result, directional technology is routinely recommended for adult hearing aid wearers. 

In contrast, there are conflicting recommendations regarding the use of directional microphones in children (Bagatto et al., 2010; King, 2010; McCreery et al., 2012; Ricketts et al., 2010). For instance, the Ontario guideline for fitting hearing aids for children discourages the use of directional microphones for children, while the Australian guideline recommends the use of directional hearing aids for children (Bagatto et al., 2010; King, 2010). The hesitation associated with directional microphone use in infants centers around the concern that directional amplification may limit a child’s access to important sounds that arrive from behind him/her. We know that incidental listening, the term used to describe a child’s tendency to listen to speech that is not directed specifically to him or her, is responsible for a great deal of a child’s vocabulary and social development. In fact, Dr. Carol Flexer has estimated that as much as 90% of what a child learns during the first few years of life comes from incidental listening (Cole & Flexer, 2010). Could directional microphones, which inherently limit access to sounds arriving from behind a child, interfere with incidental listening? There has not been enough research examining directional hearing aid use in young children to fully answer that question yet.   

Ask your audiologistIt should be noted that many contemporary hearing aids feature what is often referred to as adaptive directionality, in which the hearing aid amplifies sound from all directions equally (e.g., omni-directional) in quiet environments and automatically switches to directional mode in noisy environments. In fact, some hearing aids even go a step further and remain in omni-directional mode if the primary signal arriving from behind the user is speech. With this approach, the risk of missing out on important speech signals arriving from behind or from the side of a hearing aid wearer is limited. It is likely that there are differences in adaptive directional algorithms from one manufacturer to another, and consequently, there is not enough evidence to determine whether they are suitable for children.

Although directional microphones may improve speech recognition in noise, it is probably prudent to refrain from their use until a child is old enough to consistently orient toward the signal of interest and provide verbal feedback about the potential perceived advantages and limitations of directional amplification. Hopefully, ongoing research and development will clarify the question of whether young children should use adaptive directional amplification and possibly even result in a hearing aid that limits the potential disadvantages of directional amplification for children.

Digital noise reduction (DNR) is another hearing aid noise technology designed to improve performance in noisy environments. DNR analyzes the sound arriving to the hearing aid, determines whether it is speech or noise, and reduces the aided gain when background noise is the dominant input. Research conducted with adult hearing aid users has shown that DNR significantly improves listening comfort in noise, and wearers consider DNR to be one of the most important features in their hearing aids (Powers et al., 2006; Kochkin, 2010). Other studies have indicated that DNR provides at most a modest improvement, and in many cases, no change in speech understanding in noise (Bentler, 2005; Peeters 2009). Most studies have shown no degradation in speech recognition in noise for adults using DNR. Like directional technology, DNR is routinely recommended for adult hearing aid wearers. 

As with directional hearing aids, we are still examining the appropriateness of DNR for young children. Research with pediatric hearing aid users has essentially shown that DNR does not degrade or improve speech recognition in noise (Bentler et al., 2010; McCreery et al., 2010). Bentler and colleagues (2010) did show that DNR may improve listening comfort for children in noisy situations. They also showed that children’s novel word learning abilities were improved with the use of DNR, a fact they attributed to a decrease in the cognitive processing load afforded by improved listening comfort associated with DNR. Taken collectively, these findings suggest that DNR may be quite beneficial for children and should be considered for use with pediatric hearing aid wearers (McCreery et al., 2010; Stelmachowicz et al., 2010). 

However, it is important to remember that not all hearing aids work alike, and it is possible that some DNR algorithms may reduce gain for speech. It is imperative that the audiologist verify that gain is not reduced when speech is present. Contemporary hearing aid testing equipment typically allows for this type of verification. The interested audiologist is referred to an excellent review by McCreery and colleagues on electroacoustic assessment of hearing aids with DNR (McCreery, Gustafson, & Stelmachowicz, 2010). Once the audiologist does determine that DNR does not reduce gain when speech is present, DNR should be considered for pediatric hearing aid wearers. 

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