I can’t hear in noise! Wireless microphones for hearing aids
Posted 31st March 2017by Adam Chell
“I can’t hear in noise!” is without doubt the biggest complaint I get as an audiologist. The reason for this is hearing loss does not come alone. You may have heard of the ‘cocktail party effect’. It is defined as the ability to focus one’s auditory attention on a particular stimulus while filtering out a range of other stimuli. To do this effectively the auditory pathway to the brain needs to be intact.
When one or both ears are damaged, the brain is starved of the input it requires to process the environment and create this filtering effect. As such, hearing aids try to mimic this effect to support the wearer in background noise. My experience is that people think it is easy to achieve but, as you will see, they come with their limitations. I suspect this is because modern headphones are able to block out noise so effectively so I want to explain why the two are very different.
It is much simpler to block out noise in headphones because the interfering noise and the music/stimulus you want to listen to have different sources. The stimulus is directed in to our ears through the direct connection with a media player. There is no outside source of interference with the stimulus until it is transferred in to acoustical energy by the speaker. The speaker is very close to the ear drum and so even if there is a lot of background noise you can still hear your music well.
However, there are times when the noise outside is very high such as when you are on the London underground. In times like this, it can be difficult to hear your music and so you have to increase the loudness of your headphones to improve the signal to noise ratio (SNR). This level of noise is very unsafe for our ears and so the clever manufacturers introduce noise cancellation technology to help overcome this.
Passive Noise cancellation
Noise cancellation can be achieved through a passive and/or active process. Passive noise cancellation is the use of closed cup headphones with good insulation to absorb acoustic energy. These are often very bulky and not very stylish. Many individuals like to use the ear buds and so manufacturers had to come up with another solution which is called active noise cancellation.
Active Noise Cancellation
This uses a microphone which samples the external noise and uses a process of destructive interference to cancel out the noise. The reason it is so effective is because it does not determine whether the sound is useful speech information or unwanted background noise; it just cancels out everything. This is because when using headphones what you want to listen to is not the external world. Instead, you have plugged yourself in to a manufactured source of sound which is easily separated from real world noise. But what happens when these two worlds collide and you want to pick out relevant information such as speech from the real-world background noise?
Needle in a haystack
Trying to pick out a speech signal in background noise can be like trying to find a needle in a haystack. This is what hearing aids have to try and achieve. The problem is background noise is not always predictable. You can have repetitive noise such as a blowing fan or a fridge hum. You can have fluctuating broadband noise such as road traffic. The hardest one though is other conversations interfering with your own such as in a busy bar or restaurant. Put simply it requires extremely sophisticated processes to try and decipher a speech signal.
Modern hearing aids have incredible technology that allows them to sample and recognise the type of environment you are in. The hearing aids use a process called directionality which can help to adapt to a given environment and pick out speech signals to help you hear. They also have many different algorithms for other sources of noise such as wind, echo, traffic, and impulse sounds. Hearing aid manufacturers are world leaders in noise cancellation technology but unfortunately they aren’t very effective beyond what is called the critical distance.
Let’s pretend you are standing next to somebody and having a conversation in background noise. The closer you are to the person you are speaking to, the easier it is for you to hear. If you were to gradually start stepping away from the speaker it would get harder to hear them. You keep stepping back until the speaker and the background noise appear to be the same loudness. This point is called the critical distance. Beyond this point, noise reduction technology within the hearing aid has little to no benefit. This critical distance varies depending on the loudness of the background noise. If you are in a very loud restaurant it is closer than if you were in a quiet room. It also depends on your age.
Carol Flexor, a renowned paediatric audiologist, always talks about conversation at an arms distance away. Not YOUR arm though; the arm of the individual you are communicating with. For example, if you are talking to a new-born baby, their arms are very short and so communication should be almost face to face to ensure proper stimulation of the ‘hearing brain’. As they grow, we can increase the distance until they become adults and effective communication happens at about half a metre away. So what do we do when we can’t engage within this critical distance? We have to bridge the gap.
Bridging the gap
Imagine you are in a meeting. The room has hard flat walls and high ceilings. You are sitting at a long executive table and the person talking is at the other end of the room. There is office noise from outside and people in the meeting are muttering amongst themselves. You are trying to listen to the speaker but because of the distance you struggle to keep up. Your brain then uses a process called top down processing which uses your experiences with language to in the gaps.
You may or may not have noticed that I missed out the word ‘fill’ in the previous sentence. Your brain likely noticed this and hesitated for a moment to try and process what word should have been there. It may have ruined the flow of your reading, but fortunately you could pause before carrying on. In the real world though, there is no pause button. You miss word after word and your brain quickly tires. This is where the use of wireless microphones comes in.
Wireless microphones are produced by every major hearing aid manufacturer. They vary in quality and price but they all aim to improve the clarity of speech beyond the critical distance of hearing aids.
There is a chronic lack of awareness around the benefit of wireless microphones and too much emphasis is placed on the hearing aids to pick up the slack which results in unnecessary appointments and poorer patient outcomes.
Every audiologist should be talking about wireless microphones with every hearing aid user . I used to work for the NHS and I used to be shy about discussing products that needed to be purchased by the patient. However, as I have developed as an audiologist I realise that it is my duty to make every patient aware of any technology that I feel may benefit them, even if it does come at a cost.
Hearing aids are far from perfect at mimicking the cocktail party effect, especially beyond the critical distance. Wireless microphones help to overcome some of these issues but awareness needs to be raised. It is a simple concept that has incredible benefits but is unfortunately underutilised.
If you are reading this and feel you would benefit from a wireless microphone contact us today.
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