Multi-channel processing in hearing aids: is more better?

For a consumer, getting more while paying the same (or even less) is a strong incentive. who would not want to get two suits for the price of one, or better yet, four suits for the price of one?

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Written by Francis kuk , ph.d, vp clinical research, Widex ORCA-USA

Technology articles

That is precisely the consumer psychology that some marketers go after when they advertise that their hearing aids have the most number of channels on the market. Inadvertently, it gives the impression that these hearing aids have the most value. But are hearing aids with more channels really better than those with fewer channels? 

What is a channel?

A channel is a filter that covers a range of frequencies that is analyzed, processed, and reproduced by the hearing aid in the same manner. For example, a broadband or single channel hearing aid with a bandwidth between 100 Hz and 8000 Hz treats all incoming sounds within that frequency range as one sound. The average level of that sound across all frequencies (100-8000 Hz) is computed (or analyzed). It is processed with the same gain across all frequencies, and produced (or outputted) as one sound with its original details, except louder.

A multi-channel hearing aid does not mean a hearing aid with a broader bandwidth (i.e., covering more frequencies). Rather, it means that the hearing aid divides the incoming sounds into different frequency regions for separate analysis, processing, and reproduction. For example, a 3-channel hearing aid may treat all sounds below 1000 Hz as the low frequency channel; all sounds between 1000 and 3000 Hz as the mid frequency channel, and all sounds above 3000 Hz as the high frequency channel.  The average level of that sound in each low, mid, and high frequency region will be computed (or analyzed). Depending on the hearing loss of the wearer in each frequency region, a different gain is assigned. The final output is the sum of the outputs in each channel. 

So, what are the benefits of multiple channels?

The advantages of a hearing aid with multiple channels can be seen in the following features:

  • Complex auditory scene analysis – the sound pressure levels in more discrete frequency regions can be identified more precisely, leading to a better identification of the sound environment for proper subsequent processing (such as compression and noise reduction).
  • Multi-path active feedback cancellation – the action of the gain limitation during the Dynamic Cancellation Optimizer (DCO) can be limited to a narrower range of frequencies, thus preserving the audibility of the other frequencies.
  • Noise reduction - the algorithm can identify more precisely which frequency region is noise (and which speech) and reduce gain only in the frequency region where noise is present. This preserves speech audibility while improving listening comfort.
  • HD Locator - the directional microphone can change to different directional polar patterns for different frequencies. This preserves and improves speech audibility in noise (see Figure 1 below).
  • Target gain match - the frequency response and compression settings can be matched to more configurations of hearing losses, especially the atypical ones. This ensures consistent speech audibility.
  • Narrow band Automatic Output Control (AOC) - output limitation is restricted to a narrow frequency range, preserving audibility of other sounds.

Figure 1: Different polar patterns in each of the 15-channels of the HD Locator.

Is there any downside to a multichannel hearing aid? 

The previous examples indicate that multiple channels are desirable for analysis and processing of sounds. However, having multiple channels increases the risk of spectral and temporal smearing, which is a reduction of the contrasts between the peaks and valleys within the speech signal.

Listeners, and especially those with more severe hearing losses, need such contrast for proper speech identification. The following figure illustrates how spectral smearing occurs with multiple channels.

Figure 2A shows the unaided spectrum of a vowel with the three formant frequencies identified as F1, F2, and F3 respectively. The difference in level between the peak of F1 and the valley of F2 is the spectral contrast. Figure 1A can also be true for the aided spectrum of a single channel hearing aid where the same gain is applied to all formants.
 
Figure 2B shows the aided formants using a two-channel system. The amount of gain in the low and high frequency channels is shown by the red bars. Notice that more gain is applied to the high frequency channel than the low because more hearing loss is encountered in the high frequency than the low frequency. Note the shallowing of the spectral contrast, especially between F2 and F3.
 
Figure 2C shows the gain and output of a 15 channel system with individual gain at each channel. One can clearly notice even more smearing of the spectral contrast (Figure 2D) as channel number increases. This could negatively affect speech understanding.

So, is multichannel good or bad?

The use of multiple channels in hearing aids can be likened to the use of prescription drugs. For example, a baby aspirin (80 mg) a day is said to minimize the risk of a heart attack. A regular aspirin (320 mg) can relieve pain and headache as well. But taking excessive dosages could result in stomach ulcers, tinnitus, and even kidney and liver damages. A hearing aid designer has to decide on the optimal number of channels that deliver the largest number of advantages with the fewest drawbacks. Because the auditory system can be modeled as a series of 1/3 octave filters (or channels), we at Widex decided on a 15-channel hearing aid as our premium product. In this product, each channel has approximately a 1/3 octave bandwidth. In addition, an anti-smearing algorithm is also incorporated within the design so that despite the advantages of the 15-channel system, minimum or no smearing occurs. 

Look beyond the number of channels

While the number of channels is definitely a deciding factor in selecting a hearing aid, how the hearing aid handles potential smearing, along with the other signal processing algorithms (such as those listed earlier) within the hearing aid are also important considerations. These algorithms also determine if the hearing aid has good performance. Their merits will be described in future blogs. Remember, multiple channels are good; but more channels does not necessarily mean a better hearing aid for every user. Other features within the hearing aid (such as True Input technology, for example) are also important determinants of a hearing aid’s performance.

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