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Creating natural sound experiences

Natural sound: It’s all around us. The voices of our loved ones; waves crashing and birds singing, symphonies playing, traffic passing by. But many people have a hard time enjoying natural sound — or any sound for that matter.

According to the National Institutes of Health (NIH), one in eight people aged 12 years and older has hearing loss in both ears. Twenty-five per cent of adults aged 65 to 75 and half of adults aged 75 and older experience disabling hearing loss. All told, it’s estimated that about 26 million adults in the US have suffered permanent hearing damage, with estimates that a further and 28.8 million could benefit from using hearing aids.

But for many, there’s a problem with wearing hearing aids in that they don’t sound natural. In fact, sound quality and the “naturalness” of sound remain among the most-cited reasons people don’t pursue hearing assistance, despite the great strides in hearing aid innovation made over the last decades.

Ironically, the unnaturalness of hearing aid sound is, to a great extent, the by-product of that innovation. When hearing aids went digital, drastic improvements in noise reduction and advanced signal processing came at a price.

In the beginning, hearing aids were built to compensate for hearing loss and make speech audible again. Of course, the reality was — and remains — that ambient noise and the consequences of hearing loss are variable and listener specific.

Today, high end hearing aids are intelligent. They can process and separate out unwanted noise, like background conversation in a coffee shop, while simultaneously enhancing nearby speech so the wearer can better hear their companion. Really good digital hearing aids can even automatically adapt to different listening environments so the wearer doesn’t have to, and they eliminate feedback that might have been associated with analogue technology.

They do all this through advanced digital signal processing, but therein lies the rub, and the reason for further innovation in creating a more natural sound experience for the millions of people seeking it.

Unnatural sound

Taking an analogue sound wave, converting it to a digital signal, processing it, then re-converting it back into a sound wave that the ear can perceive, takes time — in most cases 4 to 8 milliseconds. A signal is passed through the input stage and converted to digital numbers, passed through a filter bank, which splits it into various channels for processing, for example to compensate for a person’s unique hearing deficiencies in one frequency range but not another.

Finally, the digital signal again needs to be converted from numbers back to an analogue sound wave at the output stage of the hearing solution. Studies have shown that a processing delay within that 4-to-8-millisecond span can adversely affect sound quality.

The challenge with a system delay of approximately 4 to 8 milliseconds is simple: if processed sound and unprocessed sound mix in the ear canal of a hearing aid user, the delay will have a degrading effect on the perceived sound quality, also called the comb filter effect.

It’s become increasingly important to get this effect under control, because more and more hearing aids are fit with discrete and open- or vented-fit ear sets, particularly for people with mild to moderate hearing loss.

As a result, more direct, unprocessed sound passes through the opening around the ear set and mixes with the processed sound from the hearing aid. When that happens, the perceived quality of the user’s own voice becomes tinny and artificial; environmental sounds come out distorted and unusual; and overall, the resulting sound becomes strenuous to listen to over time.

For anyone embarking on a new hearing life with hearing aids, the expectation is a world of sound as it was — natural. And many end up disappointed in their first experience of amplified sound, because, despite great strides in hearing aid development, until now, a more natural sound has been elusive. Smarter, more efficient processing holds the solution.

Signal processing

The challenge is to build a digital signal path with virtually no system delay, so that the mixing of signals in the ear canal would not be detrimental.

Engineers at Widex looked at the situation and realised they could dramatically speed up the digital signal processing in hearing aids. Building off a determined focus on high-fidelity signal processing design, Widex is already an industry leader with the lowest system delay. The company’s time domain filter bank, along with its 32kHz sampling rate and 16kHz digital bandwidth, results in very high-fidelity sound and the perfect backdrop for engineers to tackle the comb-filter effect.

The solution is an alternate, innovative, high-speed signal pathway through the hearing aid. Widex calls it the ZeroDelay pathway, and is intended for people with mild to moderate hearing loss.

The new accelerated pathway results in a processing delay of just .5 milliseconds, effectively eliminating the comb-filter effect and the tinny sound many hearing aid users notice when direct and processed sound come together and cause distortion.

In this pathway, all signal processing is adapted to the ZeroDelay design to deliver the best and purest sound quality. Several core functions, such as acoustics stability, adaptive gain control, and enhanced signal-to-noise ratio, still take priority, while others are modified to fit it to the new paradigm of natural sound design in the faster ZeroDelay pathway.

The two pathways - ZeroDelay and “Classic” – exist, side-by-side on the Widex platform. Depending on the needs of the wearer, a hearing care professional can program one or the other as the default mode. The fact is, in real life, hearing naturally and with focus depends on context and intent. The wearer of hearing aids with the two distinct pathways can switch between them based on where they are and what they want to hear, thereby enjoying far great sound quality and a more natural experience in most situations.

Better hearing

Why is this important? Why innovate to create a more natural-sounding hearing aid, beyond the fact that natural sound is what we all crave? One of the biggest reasons is so more of the people who would benefit from hearing aids will embrace the technology that’s available today.

Most first-time hearing aid users are in their 60s and 70s, and although their hearing problems started years earlier, data shows it can take them, on average, seven years to give hearing technology a try. Having waited seven long years to try hearing aids, they shouldn’t put on their first pair and hear tinny distortion.

What the development of a ZeroDelay pathway shows us is that there continues to be room for innovation in recreating natural sound. Could there be additional processing pathways, tailored further for prevalent hearing challenges? In what other ways can engineers shape sound frequencies and process digital signals based on a person’s preference, lifestyle, or listening intent?

Having accomplished the delivery of natural sound, engineers are committed to further pushing the envelope on personalised, natural sound.

Author details: Lise Henningsen is Global Head of Audiology for Widex

Lise Henningsen

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