Programming a new hearing instrument to provide satisfactory speech perception in quiet is a straightforward process. It usually requires no more than a single mouse click to set it to within a few decibels of desired amplification. The real challenge is providing optimal speech perception and a more natural auditory experience across a range of listening environments. Kochkin notes that consumers expect high performance in multiple listening situations.1 Only 1 in 4 hearing aid wearers are satisfied in as many as 75% of listening situations.
In an earlier survey, Kochkin indicates that satisfaction and performance are improved across a wider range of listening situations by using hearing aids that combine multiple features, such as multiple memories, channels and microphones.2 Combining features provides the professional with greater flexibility and the ability to tailor amplification for specific listening situations.
High performance hearing aids take technology a step further. Instead of providing multiple memories, multiple channels, or multiple microphones that are programmed with a static set of parameters, hearing aids can also provide several adaptive features. Sets of adaptive features can engage and disengage automatically in different listening environments without the wearer ever touching the device. For example, the Liaison™ hearing aid by Unitron Hearing provides Dynamic SoundScene™ processing. Dynamic SoundScene provides a fully automatic solution for consumers by integrating the functions of five different adaptive features: 1) Adaptive Beamformer; 2) Auditory Contrast Control; 3) Intelligent Noise Reduction 2.0; 4) Realtime Feedback Canceller; and 5) Wind Noise Manager.
The integration of these adaptive features is designed to provide the consumer with a flexible responsive device requiring almost no adjustment to maintain optimal performance. However, even a hearing aid this adaptive requires fine-tuning. No hearing aid ever “knows” what the wearer is really trying to hear. Therefore, optimizing hearing aid parameters to the needs of the individual wearer is still an essential component for a successful fitting.
There are two ways to optimize the performance of adaptive features. Some features, such as the real-time feedback canceller and wind noise manager are fully automatic. When they are enabled during the fitting, they engage and disengage as needed in response to listening conditions. Therefore, the clinician can only enable or disable these features. Similarly, Dynamic SoundScene always engages the directional microphones at high noise levels. Directional microphones can only be disabled in louder environments by leaving Dynamic SoundScene or by ordering an omnidirectional product (eg, a CIC). However, the fitter can optimize the auditory contrast control and the noise reduction features. These two controls allow the customization of the hearing aid to match the wearer’s listening needs and preferences.
Customizing Awareness of Scene Switching
Directional microphones can improve SNRs in the presence of high-level off-target noises.3-5 A device that switches between omni and directional microphones when moving from a quiet to a noisy environment is usually desirable; however, automatic switching takes some control away from the listener.
For some people, the awareness of the switching, called “hard switching,”, indicates to them that the hearing aid is working. However, other people find hard switching too distracting. They prefer a seamless transition between microphone states known as “soft switching.” Auditory Contrast Control provides the fitter with the ability to adjust the wearer’s perception of the switching across a continuum, from very soft and nearly transparent to very hard and obvious. This control adjusts the frequency/gain function in directional mode to provide complete directional microphone equalization (“soft”) down to no equalization at all (“hard”).
Figure 1 shows the effect of the auditory contrast control for signals in front of the aid. The dark grey line shows the frequency response in omnidirectional mode; the blue line shows the effect of minimum contrast in adaptive directional mode. The only significant gain reduction in directional mode occurs below 400 Hz. The orange line shows the effect of maximum contrast. The result is a gain reduction for signals from 2500 Hz and below. Therefore, minimum contrast is used for people who prefer a seamless transition between microphone modes. Contrast is increased to provide ever more awareness of the switching between microphones.
Optimizing Noise Reduction to Wearer Preferences
Noise reduction is by far the most customizable of the adaptive features in Dynamic SoundScene. Historically, noise reduction has been offered as a feature that can be adjusted by degree (moderate/maximum) or merely turned on or off. In Liaison, the degree of noise reduction may be set at one of three positions: mild, moderate, or maximum. Additionally, a new capability has been added to this noise reduction algorithm: the ability to set the activation threshold. These two controls provide the dispensing professional with the ability to respond directly to issues related to the aggressiveness of the noise reduction.
Just as some people prefer the microphone switching to be more obvious, there are also those who prefer more aggressive noise reduction. The differences in preference may be due to the type of hearing loss or the listening environments where the hearing aid is commonly worn. For example, a person who has a moderate sensorineural hearing loss (SNHL) might find that a moderate setting for the degree of noise reduction works well for conversation while riding in the car. However, if that same individual frequently uses a motorcycle—creating a situation where speech intelligibility is less important than comfort—a maximum setting may be more desirable.
If this example is extended to an individual with a severe SNHL, the question of audibility becomes a factor. Someone with a severe hearing loss and a relatively narrow dynamic range may prefer only enough gain to keep speech slightly above threshold. This will make the speech audible and keep it comfortable. In such a case, even moderate noise reduction inside a moving car may reduce amplification enough to drop the speech signal below threshold, limiting audibility and reducing the perception of loudness. Therefore, the complications due to the severity of the hearing loss preclude any setting more aggressive than mild noise reduction. In the moderate SNHL example, the listening environment dictated the choice of degree, whereas the limiting factor in the severe SNHL case was the hearing loss itself.
Traditional noise reduction attempts to suppress noise whenever there is no speech present in the signal regardless of how quiet the listening situation. Soft environmental sounds, such as the refrigerator running or a gentle rain, may be suppressed even if these sounds are not annoying or interfering with speech perception. Suppressing these sounds may make hearing aid wearers feel removed from their auditory environment. This problem can be avoided by providing activation level settings that are optimized for each wearer. Both hearing loss and listener preferences are taken into consideration and play a role in the choice.
For example, a person with near-normal hearing or a mild hearing loss in the low frequencies will likely have a well-vented hearing aid, and a very low activation threshold may be appropriate. Soft environmental sounds will be audible through the vent for someone with good low-frequency hearing. Applying noise reduction in such a case will not lead to the sense of being cut off from the auditory environment. However, with even a moderate hearing loss and limited venting, sounds like computer fans and automobile turn signals can become inaudible at the lowest activation level setting. Raising the activation level by even 6-12 dB will allow softer sounds to pass through the aid with no noise reduction applied.
Furthermore, low activation levels are problematic for individuals with severe hearing losses for the same reason that maximum noise reduction is of limited value. For these individuals, it takes very little noise reduction to make most environmental sounds inaudible, such as the sound of footsteps of someone walking up behind them. For this group, the highest activation level is often appropriate. By fitting them with mild noise reduction at the highest activation level, an input signal will need to be quite loud to engage the algorithm, and its effect when engaged will not be overwhelming. Audibility will be maintained for all but high level inputs.
Figure 2 shows examples of how noise is processed differently at three separate activation level settings. Three waveforms are shown. The amplitude of the waveforms increases from left to right. In each case the waveform is passed through the hearing aid without any noise reduction applied as long as the amplitude of the signal is below (to the left of) the noise reduction activation level. The activation level setting goes from low to high for each waveform from bottom to top respectively. Note how much louder the noise is allowed to become for a high activation level than for a low level before noise reduction is engaged. However, the same degree of noise reduction is applied in each case so the amplitude of the noise above the activation levels is equal for three waveforms.
Summary
The success of a hearing aid is directly related to the number of situations where it can deliver desired performance. The addition of multiple adaptive features can unobtrusively provide this performance without requiring constant adjustment by the hearing aid wearer. However, for these features to provide optimal benefit, the dispensing professional should be able provide adequate adjustments based upon the needs of the hearing aid wearer.
| This article was submitted to HR by Donald Hayes, PhD, manager of audiology research and training, Unitron Hearing, Kitchener, Ontario, Canada. Correspondence can be addressed to Donald Hayes, PhD, Unitron Hearing, 20 Beasley Dr, PO Box 9017, Kitchener, ON N2G 4X1, Canada; email: [email protected]. |
References
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2. Kochkin S. Customer satisfaction and subjective benefit with high-performance hearing instruments. Hearing Review. 1996;3(12):16-26.
3. Hawkins DB, Yacullo WS. Signal-to-noise ratio advantage of binaural hearing aids and directional microphones under different levels of reverberation. J Speech Hear Disorders. 1984;49(3):278-286.
4. Leeuw A, D.W., Speech understanding and directional hearing for hearing-impaired subjects with in-the-ear and behind-the-ear hearing aids. Scand Audiol. 1987;16(1):31-36.
5. Valente M, Fabry D, Potts L. Recognition of speech in noise with hearing aids using dual microphones. J Am Acad Audiol. 1995;6(6):440-449.
