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The holy grail for digital hearing aid developers is to allow wearers to hear speech more clearly in the presence of background noise.
‘Noise’ simply put is sounds that we do not wish to hear and make it difficult for us to hear what we do want to hear. This can be anything from the hum of a refrigerator, traffic (cars and planes), wind, doors slamming, road works, cutlery and the biggest gripe of all … the background babble of many people speaking (know as cocktail party noise).
Background noise can typically be split onto three general forms:
Hearing aid developers have come up with an array of technology to try and reduce background noise and allow their users to hear speech better (static noise reduction, wind noise reduction, impulse noise reduction, directional microphones) – but all this technology is up against it because of the damage within the cochlear (the inner ear) – for this lesson we are dealing with age related hearing loss (presbyacusis) which is damage in the cochlea / inner ear.
So in order to understand why we can’t hearing in noise we need to understand what presbyacusis is.
The above image is not to scale. The cochlea is only a few millimeters across and yet can contain 20,000 hair-cells, set within a fluid.
A microscopic view of the hair-cilia on the ends of the nerves.
As we discussed in lesson 2 – sound is an air pressure wave, and this pressure wave generates friction which gradually wears down the hair-cells in the cochlea and causes irreparable damage.
A microscopic view of damaged hair-cells on the right.
This damage occurs mostly at the entrance to the cochlea where the high frequency hair-cells are located. Presbyacusis is a high frequency hearing loss.
As previously shown, this results in a loss of clarity of speech due to the important parts of speech being in the high frequencies (on the right of the graph).
So we can see:
Volume on the left (low-mid range) picked up by better hair-cells
Clarity to the right (mid-high range) picked up by damaged hair-cells
Cocktail party noise mainly comprises of the low frequency babble of many people speaking – the low frequencies being the louder elements (volume on the chart above) – this babble drowns out speech clarity (as on the chart above) even for people with good hearing, and we rely partly on catching ‘parts’ of the clarity along with lip reading and filling in the gaps via knowledge of sentence structure – the more noisier it gets the more we reply on these other speech cues.
People who have presbyacusis hearing loss typically say they are fine one-to-one, but cannot follow conversation in noise.
If you have a hearing loss then it becomes much harder to hear any of the speech clarity among all the babble and so it becomes impossible to complete sentence structure and you then reply more so on lip reading alone, which can become very difficult and draining, and can lead to the point where people just give up and don’t join in.
People who wear hearing aids want to hear better in social gatherings – this is where they struggle the most.
Digital hearing aids amplify sounds based on prescriptive fitting to match the level of hearing loss for each patient. That is the easy part.
But the amplified sound is being sent to a damaged cochlea, which is never going to hear perfectly again. And the better part of the cochlea is hearing all the babble noise in the background, which is also masking the more important speech clarity.
The hearing aids have to try and break the varying sounds apart and determine not only what is speech and what isn’t, but what is the speech you are listening to.
There is an array of hearing aid technology within hearing aids which tries to assist overcoming these handicaps and we shall deal with that our Guide to Buying Hearing Aids.
It may be though that for some people hearing in background noise will always be a struggle due to the damage in their cochlea and the inability to process sounds like they once used to.
