Scientific investigations of binaural hearing processes have provided clinicians with a wide body of evidence to support the use of two hearing aids in the presence of bilateral hearing impairment. Over the years, this evidence has been applied to clinical practice, and the industry has seen a 20% growth in binaural adoption rates since the early 1990s.1
Given the widespread acceptance for the fitting of bilateral hearing aids, manufacturers have been looking for ways to enhance the benefits of bilateral hearing aid fittings via exchange of wireless data between the devices. Schemes for applying device-to-device communication are often described as “binaural processing.” However, manufacturers make quite different choices regarding what the hearing instruments actually do when this “binaural processing” is active.
This article discusses some of the ways wireless device-to-device communication is applied, as well as the implications for the hearing instrument user’s binaural perception of their auditory environment.
Binaural Hearing and Bilateral Fitting
In most clinics, it is a relatively safe generalization to say that two hearing aids are the de facto recommendation in the absence of an ear with significantly poor word recognition scores or thresholds not within the limits of amplification. This recommendation has gained acceptance from both consumers and practitioners because hearing-impaired individuals can, in many cases, enjoy the same benefits of binaural hearing as those with normal hearing.
For example, a sound presented to both ears is perceived as louder than the same sound presented only to one ear. This phenomenon, known as binaural summation, can be applied to hearing aids in that two devices will require less gain from each individual instrument than from one instrument used alone. In turn, this reduction in gain can result in lower levels of distortion, a lower occurrence of feedback, and a smoother sound quality from the instruments. Other examples of how binaural hearing can be of benefit are related to hearing in noise, such as the binaural squelch effect and the influence of head shadow effects on speech understanding in background noise.
For many patients, bilateral losses can be helped with two hearing instruments. A bilateral fitting of amplification generally can contribute greatly to binaural hearing benefits. However, some characteristics of hearing instrument processing may alter the acoustic information delivered to the ear in such a way that natural binaural processes are disrupted. For example, Keidser and colleagues2 point out that some asymmetries in how bilaterally fit hearing aids process the sound affect interaural time differences. This, in turn, may cause degradation of the wearer’s ability to localize sounds—an important binaural hearing phenomenon.
Bilateral Signal Processing Versus a Binaural Strategy
The use of two instruments operating independently of each other for a patient with bilateral hearing loss can fairly be called a bilateral hearing instrument fitting. But what if the hearing instruments are communicating with each other such that they become a system? Does this fitting enhance binaural hearing abilities?
Binaural hearing may be defined as “the perception of sound by stimulation of two ears.”3 Perception is not done by the ears; it is done at the level of the brain. Hearing instrument systems can support binaural processing, but cannot actually perform it. So, even if they are operating as a system, hearing instruments perform “bilateral” processing. Depending on how that processing is applied, a binaural strategy that facilitates binaural processing by the brain may be achieved.
Just as manufacturers have taken different philosophical and technical approaches to other aspects of sound processing, such as compression and noise reduction, there are also different approaches to applying wireless device-to-device communication. Some wireless hearing instrument systems that share information aim to maximize audibility of a signal identified by the system. These systems use the hearing instruments to choose which sound inputs are presented to the user, without providing the user with all sound inputs to allow for user intent. For example, the system may assume that the loudest speech signal in the environment is always the signal of interest. However, this may not always be the case. The user is then at the mercy of the signal processing, and the listening intent of the individual may be all but ignored.
Bilateral signal processing schemes share the common trait where the focus is on decision-making by the hearing instruments. With a binaural strategy, on the other hand, the hearing aid processing and user intent converge. The focus is on the user and the natural sound delivery process to the brain.
A Real-World Example
Perhaps the best way to understand how the application of wireless device-to-device communication can differ among manufacturers is to use an example. Let us consider a family dinner as shown in Figure 1.
The woman in black holding the fork is the hearing instrument user. Everyone is talking and laughing. The voice of the woman holding the baby behind the user is the loudest signal to the hearing instrument on the right side, but the user would actually like to continue listening to a story being told by the man at the end of the table. In other words, the user intends to listen to the man’s story, even though he is farther away than the closest speech signal in her environment.
One current wireless hearing instrument system will attempt to increase audibility of the voice of the woman holding the baby by reducing gain and increasing noise reduction for the user’s left hearing instrument, and increasing gain and reducing noise reduction for the right. This not only results in an unbalanced and unnatural sound picture, but it also reduces audibility for what the listener actually wants to hear.
Another system will also identify the loudest speaker, and automatically adapt settings in both hearing aids to increase audibility for this signal. It will most likely switch to backward-facing directionality, or it may drastically reduce gain for the microphone input on the left, and stream the signal picked up by the right ear over to the left. As in the previous case, the user’s signal of interest will be reduced, and other non-salient sound inputs will be enhanced.
A binaural strategy for applying wireless device-to-device communication would handle this situation very differently: it would allow for user intent. Although the voice of the woman with the baby is detected as the loudest signal, a system supporting binaural hearing would not presume this signal is what the user wants to hear. Therefore, it would optimize the listening environment so the user can choose the signal she would prefer to hear most.
One way to accomplish this would be to have the right hearing instrument go to an omnidirectional microphone response to ensure audibility of the woman behind the user. Since there is speech concurrently arising from the front of the user, a directional response would be appropriate for the left hearing instrument. The user can take advantage of the favorable signal-to-noise ratio (SNR) for the speech from in front to listen more comfortably to the man telling the story. She can improve the SNR even further by looking up at the speaker, as is also the most natural response people use when they attend to a certain signal of interest. If the woman behind the hearing instrument user says something of interest, such as “Julie, would you like one of my home-baked rolls?” Julie will still be able to detect the sound. Her most natural response when this change of events occurs will be to turn her head toward the woman to reply, “Yes, please!”
Binaural Signal Processing
In hearing instruments, a binaural strategy does not attempt to “take control” of the processing. This is more elegantly and efficiently done by the human auditory system, not by machines like hearing instruments. Instead, a binaural hearing instrument strategy strives to provide auditory input that facilitates natural binaural processing abilities. It provides for sounds presented independently to each ear to be perceived as a fused auditory image and gives the auditory system the opportunity to take advantage of the ear with the best SNR. One of the ways a binaural signal processing strategy provides this user benefit is through application of a range of different microphone responses.
Asymmetric directionality, which combines two directional patterns, can give subjects the benefits of omnidirectional awareness without sacrificing the signal-to-noise ratio benefits associated with directional hearing. The literature supports that, compared to a bilateral directional fitting, equivalent SNR benefit is provided, and greater ease of listening can be achieved.4-6 This results in an excellent balance between comfort and speech intelligibility.
Occasionally, situations arise where a bilateral directional or an omnidirectional microphone response is best. A bilateral directional response provides the greatest benefit when the speech signal is predominantly in front of the listener.7 In quiet environments, a bilateral omnidirectional response is strongly preferred by users.5,8
In these cases, new technologies can be applied to resolve such issues while still supporting true binaural processing. Device-to-device communication is one technology that holds much promise for accomplishing this. By applying knowledge of microphone mode preferences, device-to-device communication will allow for a dynamic microphone steering strategy that can provide the optimum combination of directionality and omnidirectionality for any listening situation. In addition, data exchange between the devices will enable a more accurate analysis of the acoustic environment. Consequently, any processing relying on this environmental analysis can be synchronized to provide the most balanced sound, for a transparent and natural-sounding listening experience.
Wireless device-to-device communication is a huge technological leap for hearing aid manufacturers. Bilaterally fit hearing aids that can communicate with one another have the potential to make coordinated and more intelligent choices about how and when to adapt (eg, noise reduction, gain, and directional settings). Adaptive algorithms can be used more effectively, helping to ensure that the settings of each hearing aid are in sync with one another.
Brent C. Kirkwood, PhD, is a senior audiologist at GN ReSound A/S in Ballerup, Denmark, and Stephen A. Hallenbeck, AuD, and Tammara Stender, AuD, are senior audiologists at GN ReSound in Glenview, Ill. Correspondence can be addressed to Dr Kirkwood at .
- Kochkin S. Marketrak VIII: 25-Year Trends in the hearing health market. Hear Jour. 2009;10:12-31.
- Keidser G, Rohrseitz K, Dillon H, Hamacher V, Carter L, Rass U, Convery E. The effect of multi-channel wide dynamic range compression, noise reduction, and the directional microphone on horizontal localization performance in hearing aid wearers. Int J Audiol. 2006;45:563-579.
- Walden TC, Walden BE. Predicting success with hearing aids in everyday living. J Am Acad Audiol. 2004;352:342-352.
- Walden BE, Surr RK, Cord MT, Dyrlund O. Predicting hearing aid microphone preference in everyday listening. J Am Acad Audiol. 2004;15:365-396.
- Cord M, Walden BE, Surr RK, Dittberner A. Field evaluation of an asymmetric directional microphone fitting. J Am Acad Audiol. 2007;18:245-256.
- Hornsby BWY, Ricketts TA. Effects of noise source configuration on directional benefit using symmetric and asymmetric directional hearing aid fittings. Ear Hear. 2007; 28:177-86.