Written by Courtney Morrison, M.Cl.Sc.
Today’s hearing aids have come a long way from the big
beige bananas that may first come to mind.
Not only do they amplify sound, they work to enhance meaningful signals
(i.e. speech) in order to compensate for a damaged auditory system. When you
realize how much your little hearing aid is actually doing second by second,
you may find you have a new appreciation for the tiny battery that keeps it going
for 100 hours or more!
Acoustic Environment
Classification
Almost any new hearing aid you will try is constantly
analyzing the sounds around you in order to classify the listening environment
into one several categories. More basic hearing aids are generally determining
between a ‘quiet’ or a ‘speech in noise’ environment, whereas more advanced
hearing aids are often sorting the incoming signal between quiet, speech in
noise, noise alone and music categories.
The hearing
aid makes this classification based on various characteristics of the incoming
sound including the overall level and the frequency and variations in amplitude
of the sound waves that are picked up at the microphones.
By classifying the acoustic environment, the hearing
aid can then automatically enable or disable other features which reduce noise and are designed to
make speech audible and understandable.
Noise
Reduction
One of the main goals of signal processing in hearing
aids is to make listening comfortable and noise less bothersome. Imagine
sitting in a café, in conversation with a friend, when suddenly the loud
humming sound of a blender cuts in. Your hearing aid detects this hum,
determines it is an unwanted sound and then uses an algorithm to reduce the
noise. Many of today’s hearing aids can reduce noise levels by up to 10 dB!
Noise attenuation is particularly effective for what we call ‘steady-state’
sounds – think of the fairly constant sounds from fans, refrigerators or vacuum
cleaners. Studies have shown that noise
reduction techniques employed in hearing aids improve listening comfort, reduce
listening effort and offer preferred signal quality for hearing aid users.
Directional
(Beam-forming) Microphones
One of the biggest issues for new hearing aid users is
that unwanted and distracting background sounds are amplified, along with the
things that you are trying to hear.
Directional microphones have been proven to improve speech understanding
in noise, reducing some of the background noise, thereby boosting the signal-to-noise-
ratio (SNR). They work by changing the direction of microphone
sensitivity, generally making the microphones more responsive to sounds coming
from the front and less responsive to sounds from the sides and rear.
Essentially, they act like a spotlight, directing the focus to the front and minimizing
other distractions from your surroundings.
Imagine you are in the café when several groups of
people enter and sit at tables nearby. In this case, the background noise that
is interfering with your ability to communicate with your friend is the babble
and chatter coming from the people around you. Noise reduction as discussed
above is likely to be less effective in this case, as the babble of speech is
much more dynamic then steady-state sounds; however, the directional
microphones of the hearing aid have kicked in and are now focusing their beam
of sensitivity towards your friend sitting across from you, and dampening
sensitivity to the noisy table behind you.
Feedback
Cancellation
At one time
or another, you have likely heard a whistling hearing aid, perhaps from the
person sitting next to you in church or when you’ve gone in to hug your
great-aunt Myrna. That whistling sound, termed feedback, occurs when
amplified sounds leak out around a hearing aid and are then picked up by the
microphone and re-amplified. Having a properly fitted earmold or custom aid is
the first step in ensuring feedback is minimized; however, advanced hearing
aids also use various algorithms to stop the whistling quickly or before it
occurs. As with most digital processing strategies, exactly how and when this
is done varies between hearing aid manufacturers. Certain adaptive feedback
managers will generate a sound wave opposite to that of the whistle,
essentially cancelling it out, while others will change the output frequency
slightly so that it is no longer troublesome. These advances in digital
feedback management have allowed receiver-in-canal (RIC) style aids
to be possible!
This is just some of the advanced digital processing
that goes on inside of hearing aids. Other notable technologies include
frequency compression, frequency lowering, wind noise and impulse noise
reduction. Certainly there is a lot more going on in today’s tiny hearing aids
than meets the eye!