Effect of Directional Strategy on Audibility of Sounds in the Environment

Abstract

Directional microphones in hearing aids have been well-documented to improve speech recognition in noise in laboratory conditions. The real-world perceived benefits of directionality have been less dramatic. The development of directional technology during the past decade has focused on improving laboratory benefit by means of adaptive behavior, and more recently, binaural beamforming made possible by earto-ear audio streaming. In contrast, Beltone has pursued a strategy for applying directional technology that takes advantage of auditory processing by the brain, with the goal of optimizing real-world benefit. In this study, Beltone CrossLink Directionality 2 is compared to two commercially available binaural beamformers to explore the possible advantages and disadvantages of these very different approaches to applying hearing aid directionality. Introduction Directional microphones amplify sound coming from a particular direction relatively more than sounds coming from other directions. They are the only hearing aid technology proven to improve speech understanding in noisy situations. However, some conditions must be met in order to benefit from a directional microphone. For one thing, the signal of interest must be spatially separated from the noise sources. In addition, the signal of interest must be located within the directional beam and should be within two meters of the listener. Directional microphones in hearing aids are designed to have a forward-facing beam as worn on the head. This means that the hearing aid wearer must face what they want to listen to. By constructing a test environment that fulfills these conditions, the benefit of directional microphones in hearing aids is easily demonstrated. However, real-world environments bear little resemblance to contrived laboratory test environments. Real listening environments are unpredictable in terms of the acoustics, the type and location of the sounds of interest, and the type and location of interfering noises. To complicate matters further, any of these sounds may move, and the listener may want to shift attention from one sound to another. A sound that is the signal of interest one moment may be the interfering noise the next. It has been observed that the benefit of directional microphones is not perceived to the degree that laboratory tests of directional benefit would imply1. There are numerous acoustic and personal intrinsic factors that play a role in this discrepancy. An additional factor is simply that directional microphones can interfere with audibility of the sound of interest when it does not originate from the direction that the hearing aid wearer is facing. It is assumed that individuals wearing directional hearing aids will orient their heads toward what they want to hear. However, in daily life it is not at all unusual to listen to sounds that one is not facing. In fact, it has been shown that more than 30% of adults’ active listening time is spent attending to sounds that are not in front, where there are multiple target sounds, where the sounds are moving, or any combination of these2. Beltone has taken an unconventional approach to applying directionality that considers both the advantages and disadvantages of this type of technology. CrossLink Directionality 2 leverages the brain’s ability to compare and contrast the separate inputs from each ear to form an auditory image of the listening environment. By providing access to an improved signal-to-noise ratio (SNR) for sounds in front while maintaining audibility for sounds not in front, CrossLink Directionality 2 allows hearing aid wearers to focus on specific sounds, to stay connected to their auditory environment, and to shift their attention at will3,4. The CrossLink Directionality 2 strategy controls the microphone mode of each hearing aid depending on the presence of speech and noise in the environment, as well as the direction-ofarrival of the speech. The possible configurations that can result include bilateral Personal Sound ID, bilateral directionality, asymmetric directionality with directionality on the right side, and asymmetric directionality with directionality on the left side5. The rationale behind CrossLink Directionality 2 contrasts sharply with the advanced directional technologies in other premium hearing aids. The focus of development of those technologies has been to maximize SNR improvement in laboratory environments. The most recent advancement in this area is to use an array of all 4 microphones on two bilaterally worn dual microphone hearing aids to attain a greater degree of directionality, commonly referred to as binaural beamforming. A binaural beamformer creates one monaural signal which is delivered to both ears. While there may be additional features that attempt to preserve some cues for localization, the overall effect of this approach is to eliminate the contrasts in the per ear acoustic signals that enable binaural hearing. Speech-in-noise testing under laboratory conditions has shown either modest or insignificant improvements in directional benefit for speech in front compared to traditional directionality 6,7. The lack of greater differences is perhaps related to the unavailability of binaural hearing cues that is a result of binaural beamforming. How big of a trade-off is being made for the additional benefit provided by binaural beamformers? In other words, how does the advantage of a modest improvement in speech recognition in noise for speech in front weigh against possible disadvantages of reduced audibility for speech coming from other directions? To begin to answer this question, it is of interest to explore how performance on speech recognition in noise under laboratory conditions is affected when speech arises from varying directions. The investigation described in this paper compared performance for participants fitted with Beltone hearing aids with CrossLink Directionality 2 and two commercially available premium hearing aids with binaural beamforming. The research questions were: • Is there a difference in speech recognition in noise for speech in front for CrossLink Directionality 2 versus binaural beamformers and, if so, how great? • Is there a difference in speech recognition in noise for speech from the side or behind for CrossLink Directionality 2 versus binaural beamformers and, if so, how great?

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Cite this paper

@inproceedings{Jespersen2017EffectOD, title={Effect of Directional Strategy on Audibility of Sounds in the Environment}, author={Charlotte Jespersen}, year={2017} }