Tech Topic: Binural Fittings | Hearing Review July 2014
Previous research and current movement in the endeavor to provide the most natural, binaural auditory experience for hearing instrument users will be discussed, in addition to the technological developments that have made these advances possible.
By Tammara Stender, AuD
The benefits of bilateral hearing aid fitting—that is, the real-world advantages provided when a hearing instrument is fitted for each ear in the common case of aidable hearing loss diagnosed for both ears—are generally well accepted. When patients with bilateral hearing loss inquire, “Do I need hearing instruments for both ears?” the typical response of the hearing professional may be simply stated as: “Yes, because you have hearing loss in each ear.”
But, as we know, the complete answer to this question extends far beyond this 5-second response. The long answer is that a partial list of benefits arising from bilateral amplification includes:
1) Better localization abilities, or the ability to decipher the location of sound sources1-3;
2) Improved speech intelligibility in noise,4-6 in part due to better spatial release from masking7;
3) Prevention of auditory deprivation of the unaided ear, when that ear also has aidable hearing loss,8-12 which may result in decreased speech recognition for the unaided ear11; and
4) Possible relief from tinnitus.13
Although much of these benefits are findings from older studies, new research in bilateral versus unilateral fittings continues to emerge. A study published in 2014 concluded that patients fitted with both hearing instruments at the same time were more compliant with their usage than those who were fitted with one hearing instrument initially and the other a month later.14 In fact, sequentially fitting one hearing aid after the other was already fitted resulted in reduction of use for both hearing instruments.
Additionally, the results of a relatively recent study involving veterans as participants provided further support for the argument that two hearing instruments are better than one in terms of speech perception in noise.15
Of course, as with any broad-stroke recommendation involving humans, there are a few patient considerations that may preclude successful use of binaural amplification as compared to unilateral. One such consideration is binaural interference due to the coexistence of hearing loss and certain auditory processing disorders (for a review, see Holmes 200316). But, as Holmes also asserts, the benefits of fitting hearing instruments for both ears far outweigh the costs, in nearly all cases—and especially as patients age and as hearing loss progresses.
It is important to note that just fitting two wide dynamic range compression (WDRC) hearing instruments for a patient is not a surefire way to provide each benefit listed above. A prime example implicates the compression itself as a barrier to accurate localization. By virtue of the audibility benefits provided by compression, where soft sounds above a kneepoint are amplified to a greater degree than loud sounds, a high-frequency sound approaching from one side of a listener will be assigned lower gain, due to its higher sound level, than the same sound at the far, opposite ear, due in part to head-shadow effects. This can effectively reduce the interaural level difference that the brain uses in its natural binaural processing to deduce localization information. Bilateral fittings in and of themselves do not provide support for the brain to produce its important, inherent binaural processing advantages.
Bilateral Fittings Are Not Inherently “Binaural”
The digital technology that we enjoy today in hearing instruments performs impressive calculations and manipulations of the input signal, to such noble ends as radically suppressing feedback and reducing environmental noise for the listening. But in many ways, this technology is performed in a vacuum, at the level of each device individually.
This means there is no reference to what is happening for the other hearing instrument in a fitted pair. For some features, this individual hearing instrument control can be advantageous; for example, problems such as feedback and wind noise are often best controlled when each hearing instrument is allowed to process and control the problem for each ear.
But for other features, such as directionality and acoustic noise reduction, this automatic processing can have undesirable results for the user in the very environments where they are designed to provide user benefits. One hearing instrument might switch audibly to a directional mode, while the other might be slower to make the change. Or one hearing instrument could in fact have a strong level of noise reduction activated, while the other one might remain in a neutral, noise-reduction-off setting, based on the environmental classification at each ear. In general, these advanced features could possibly detract from the binaural benefits and natural sound quality the patient should receive.
To address this problem, new features using wireless device-to-device communication were introduced. The design of these new features focused on many goals, including improving audibility of the loudest speech signal in the environment, and/or improving the signal-to-noise ratio (SNR) of this primary speech signal.
For example, one such feature chooses a bilateral omnidirectional, bilateral reverse directional, or bilateral adaptive directional response, based on the environmental inputs provided by detectors on each hearing instrument. But, regardless of the goal of the feature, both hearing instruments provide input into the final response at each hearing instrument. And because both hearing instruments are involved, the term “binaural” is used in the marketing and advertising material.
In fact, perhaps due in part to the recent emergence of academic research focusing on cognitive effects of amplification (eg, Arehart et al, 201317), the very label “binaural” has become a catchphrase in itself, as it is becoming the new vogue term to market newly developed, wirelessly communicating hearing instruments.
But the term “binaural” can extend so much farther than the basic act of fitting two hearing instruments for a patient, or even the two hearing instruments working together toward a common goal. Often, the end result of such processing is a response that makes sense only in certain situations. In the case of some of the current coordinated directionality features, the hearing instruments work together to find a signal to focus on (typically the loudest speech signal), and force the listener to attend to that by making other signals less audible. While the loudest speech signal may be what the user would sometimes choose, this is not true in every case. The user may not detect another softer speech signal in the environment if it is outside of the directional beam, and in some cases, this user may have preferred to listen to the softer speaker rather than the louder speaker.
Further, this directed beamwidth is not a natural listening pattern; unaided listeners are used to being able to choose and focus on important signals at different azimuths in their environment (ie, a binaural hearing process), because surrounding sounds are largely audible. Thus, it is important that these wireless features that are purported to be “binaural” actually have the true binaural benefits they are represented to possess.
A Better Definition of Binaural Hearing Support
Developing technology that supports the natural processes of human hearing is a key goal of ReSound. Beginning with the launch of ReSound Verso and continuing in the newest fully featured product offering, ReSound LiNX, many advanced features using device-to-device wireless communication have been designed for one unified purpose: to support the processes that the brain does so naturally.
These processes include auditory scene analysis,18 which involves the ability to detect new and novel sounds in the environment (such as a television commercial), and the option to choose which sounds to focus on (for example, a grandchild asking a question). It is only when the brain receives a true representation of the sound environment, provided by the inputs at the ears, that it can do its work well.
Binaural Directionality is the current premier directional option when fitting two ReSound hearing instruments for a patient. It steers the microphone response of both devices to support patients’ awareness of the auditory environment, and to allow them to choose what is most salient in that environment.
Based on academic published research on user preferences for directionality in different listening environments,19,20 as well as lab testing to determine the microphone responses yielding the best speech intelligibility in noise21-26 and ease of listening22 in these complex listening situations, Binaural Directionality offers patients four bilateral microphone responses. A bilateral omnidirectional response is chosen in quiet and speech-only environments; a bilateral directional response is provided when speech is directly in front of the listener in a noisy environment; and an asymmetric response (omnidirectional on one side and directional on the other) is chosen when speech is not directly in front of the listener in a noisy environment, or in a noisy environment without speech (Figure 1).
Each of these four possible bilateral microphone responses is selected based on the classification of the environment by the Environmental Classifier. Users can employ their brains’ binaural processing powers because they can hear the sounds in the environment via the omnidirectional response, and choose for themselves the signal of interest since there is no “tunnel hearing” to the directional beamwidth only. This feature truly incorporates the principle that the ears should be providing inputs to the brain so that the person can choose to focus on what is important.
Added Benefits of Directional Mix
Binaural Directionality, like all ReSound directional options, includes bandsplit processing, termed Directional Mix. Low frequency sounds are processed in an omnidirectional fashion, while high frequency sounds are provided with a directional response when it is selected (such as in a bilateral directional or an asymmetric directional response via Binaural Directionality). This means that, even if Binaural Directionality automatically selects a bilateral directional response, there will be some environmental awareness via the omnidirectional processing in the low frequencies.
This is especially true for closed fittings, which receive amplified sound in the low frequencies. For certain open fittings, where much of the low frequency sound escapes through the ear canal, the effects of Directional Mix Processing may not be apparent. However, in an open fitting, the user also receives direct, unamplified sound inputs, including low frequency sound, which aids in environmental awareness.
Additionally, providing omnidirectional processing for low frequencies restores audibility due to the inherent low-cut response of directional microphone functionality, without the artificial noise induced by low frequency equalization features (ie, “bass boost”). This also minimizes the audible switching that may be heard when the hearing aid switches between omnidirectional and directional patterns.
Bandsplit processing also affords wind noise reduction, as compared to a hearing instrument with a full directional response and bass boost, because wind noise has been found to be more problematic when processed through a full-spectrum equalized dual-microphone directional response.27,28 Further, low-frequency longer-wavelength sounds naturally produce an omnidirectional-like response as they move around the head. The similar treatment of low and high frequency sounds provided by Directional Mix Processing (Figure 2) translates to improved front/back localization performance, as compared to a full-spectrum directional response.29
Directional Mix is preset for the hearing instruments in one of four levels, ranging from “very low” to “high,” depending on the patient’s low frequency hearing thresholds and the hearing instrument model fitted. This setting is inversely related to the crossover frequency between omnidirectional and directional processing (eg, a “very low” Directional Mix setting will have a higher crossover frequency than a “high” setting). Regardless of the level selected, however, testing revealed significant directional benefit as compared to an omnidirectional setting, for both open and closed fittings.30
Bilateral fittings, where two hearing instruments are programmed for a patient, may fall short in providing the brain the support it needs to conduct binaural processes. These binaural processes, including the patient’s ability to detect sounds in the environment and then choose which sound is of the greatest significance at that time, can be disrupted when advanced hearing instrument features automatically reduce audibility via focused directional beamwidths and/or the lack of bandsplit directional processing. Provision of directional benefit in conjunction with auditory environmental awareness allows hearing instrument users to detect and choose the signal of interest. Bandsplit directionality affords users with additional support for better binaural hearing instrument.
Correspondence can be addressed to Dr Stender at: [email protected]
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Original citation for this article: Stender T. On the benefits of binaural. Hearing Review. 2014;21(7):28-31.