How Filter Bank Design Impacts the User Experience with Hearing Aids

Dana Helmink, Au.D.

Modern hearing aids are a marvel of electromechanical engineering. Guaranteeing that their performance meets the expectations of wearers involves a complex choreography of signal processing choices within a microelectronic space. This feat of engineering, in the hands of knowledgeable hearing care professionals, has improved quality of life for millions of hearing-impaired people.

Among other risks, research indicates that hearing loss can lead to social isolation, and studies from around the world show that social isolation is associated with decline in cognitive function. But it is important to understand that for many, the challenge is not just about hearing more (volume); it is about hearing better (clarity). It’ is about picking up the voices of friends and family in a noisy restaurant, for example, or understanding clearly and naturally the words people say. Achieving this with hearing aids depends on many different parameters, but signal processing is at the core.

On the most general level, digital hearing aids process sound through a similar pathway. An analysis filter bank splits the original sound signal into multiple frequency bands, or channels. These bands are then processed using various methods such as compression or noise reduction. After processing, the signal is put back together through a synthesis filter bank and delivered to the auditory system of the wearer. The goal of this signal processing is providing the gain necessary to compensate for hearing loss while delivering an experience as close to that of natural hearing as possible.

Many wearers grapple with the tinny and artificial sound that results from a delay in sound processing. That is because signal processing in hearing aids takes time—in most hearing aids about 5 to 8 milliseconds. This delay in processed audio, when coupled with the direct, immediate sound that comes through a hearing aid’s venting, degrades sound quality for the wearer. Known as the comb-filter effect, this mixing of processed and direct sound in the ear canal can cause distortion, tinniness, and general discomfort when listening over time.

This is where advances in signal processing come into play— and where the choice of signal processing technique can positively impact the natural sound experience for hearing aid wearers. The goal is to minimize delay and quash the comb-filter effect, resulting in more natural sound. Through a combination of high sampling rate and time-domain filter banks, this natural sound in hearing aids is achievable.

How People Hear

Recreating human speech is a very complicated endeavor. The ear is a logarithmic system, with higher sensitivity at lower frequencies and lower sensitivity at higher frequencies. This is the result of a physiological difference with larger areas of the cochlea dedicated to lower frequencies and smaller areas to higher frequencies (similar to the layout of the pure-tone audiogram). In speech, spoken consonants are short in duration, but high and wide in frequency. Spoken vowels are longer in duration but lower and narrower in frequency. When we look at a speech spectrogram (Figure 1), we can visualize the difference.

What does this mean in practice? It means that speech signals may most effectively be processed by a system that mimics the ear - where the high frequencies have higher time resolution while the low frequencies have better frequency resolution. For this reason, the fundamental design of a signal processing system is critical to the user experience.

The trade-off between time and frequency

Two main types of filter banks in hearing aids are time-domain and frequency-domain filter banks. Frequency-domain filter banks are the popular choice for hearing aid manufacturers, and there are some convincing reasons why. But to deliver the most natural sound, time-domain filter banks offer significant advantages.

The obvious question is of course what the difference is between the two. The answer lies in the relationship between time and frequency resolution. In any filter bank, the wider the frequency band, the greater the time resolution. Therefore, a bandwidth that is five times wider has a time resolution that is five times higher (faster). Conversely, a narrower bandwidth means poorer time resolution (slower).

Frequency-domain filter banks limit the system to frequency bands of the same width across the entire frequency range. In practice, this means that all bands are relatively narrow because bandwidth is set based on the bandwidth needed for the lowest frequencies, where the ear’s frequency sensitivity is the highest, and the bands therefore must be narrow. Because of the trade-off between time and frequency, this also means that all bands—both high- and low-frequency— operate with the same, relatively poor, time resolution and long processing delay.

On the other hand, time-domain filter banks offer the flexibility to use filters that vary in bandwidth. This means that hearing aid designers can set bandwidths any way they like. With a frequency-domain filter bank, all bands have the same width and time resolution, but with a time-domain filter bank, the bands can vary in width and, therefore, also in time resolution. This is important when it comes to a hearing aid use case.

What do variable widths mean in practice? Time-domain filter banks allow hearing aid designers to narrow the bands at lower frequencies and broaden them at higher frequencies, resulting in the same trade-off between time and frequency as humans have in their ears.

Avoiding Down-Sampling for Better Sound Quality

One more thing to consider about filter bank design in hearing aids is the role of down-sampling. For wearers, hearing aids are vital for their daily life. Ideally, they should last a long time before needing to be recharged, which means power-consuming functions like signal processing need to be optimized. Down-sampling can help accomplish this, but it can also introduce annoying artifacts.

Frequency-domain filter banks with narrow filters make down-sampling straightforward, and fewer samples mean less power consumption, but also the risk of artifacts that are inherent to non-linear processes like down- and up-sampling. With a time-domain filter bank, the benefits of downsampling are minor and do not outweigh the risk of artifacts. Therefore, choosing a time-domain filter bank means that other creative solutions may be necessary for keeping power consumption sufficiently low, but by avoiding downsampling and preserving the original input signal, much is achieved: higher fidelity, reduced risk of artifacts, and a more natural sound quality.

Although choosing a time-domain filter bank can make it more challenging to optimize power consumption and certain other aspects of signal processing, both are achievable. And ultimately, a time-domain filter bank better complements the human auditory system and helps optimize sound quality. In other words, when it comes to signal processing in hearing aids, time-domain filter banks will deliver the best result for the wearer. ■

With more than 15 years in global product management, Dana Helmink, Au.D. applies her experience in user-centered design to develop innovative educational experiences. She now works as the Sr. Director, Audiology/Clinical Education at Widex US. She can be contact at dana.helmink@ widexsound.com