2. Some hearing aid basics…
• All hearing aids have a microphone(s) to pick
up sounds, an amplifier to make sounds
louder, and a speaker to put the amplified
sound into the ear canal
• All hearing aids are powered by a battery
• Note: hearing aids are much more complicated
than this, but you will learn more about that in
future courses
3. Some hearing aid basics…
• Hearing aids come in a variety of styles and
sizes:
▫ In-the-ear hearing aids
▫ Behind-the-ear hearing aids
6. The Goal of Hearing Aids
• To amplify the sounds in the hearing aid user’s
environment so those sounds are audible and
comfortable
• All of the amplified sounds from the hearing aid
must fit within the patient’s dynamic range
(DR, the dB range from threshold of hearing to
loudness discomfort level)
7. SNHL and Hearing Aids
• Even when hearing aids amplify sounds so that
they are audible to the hearing-impaired listener,
the perception of those sounds is usually abnormal
due to distortion in the cochlea
• Patients will report:
▫ That the sounds are unclear and distorted
▫ That it is hard to hear comfortably over a wide range
of sound levels
▫ Difficulty in understanding speech in noisy
situations or reverberant rooms
8. SNHL and Hearing Aids
• Much of these complaints are due to the more
linear response of the cochlea/basilar membrane
due to outer hair cell loss
• So how do hearing aids overcome the loss of the
non-linearity of the cochlea?
▫ By using non-linear sound processing!!!
▫ Modern hearing aids attempt to mimic the non-
linearity of a healthy cochlea
9. Non-Linear Sound Processing in
Hearing Aids
• Modern hearing aids employ a combination of
compression, expansion, and linear processing to fit
sounds within the patient’s DR
• Expansion is a reduction of very soft/low intensity
sounds
▫ Purpose is to reduce circuit/microphone noise
generated by the hearing aids operation
• Compression can be thought of as an “automatic
volume control”. It makes soft sounds louder (turns
them up) and loud sounds softer (turns them down)
without the patient ever having to touch the volume
control. Compression maintains audibility and
comfort.
10. Overcoming Reduced Frequency
Selectivity
• Many patients have the greatest degree of hearing
loss in the high frequencies, since damage due to
aging and noise exposure affects the high frequency
hearing first.
• In patients with this sloping, high-frequency hearing
loss, the amount of gain or amplification in hearing
aids is greater at high frequencies than at low
frequencies.
• This rising frequency-gain characteristic will help to
alleviate the effects of the upward spread of
masking.
11. Cochlear Dead Regions
• Cochlear damage may result in complete loss of
function of inner hair cells over a certain area of the
cochlea, called a dead region.
▫ The nerve cells that are “dead” cannot transmit any
sounds of the specific frequencies that those nerve
cells are responsible for up to the brain
▫ People with extensive dead regions often do not
benefit much from a hearing aid
May benefit from amplifying frequencies up to an octave
above the edge frequency of a high-frequency dead region
May also benefit from frequency transposition
12. Frequency Transposition in HAs
• For patient’s with very severe-to-profound
hearing loss in the high frequencies OR for
patient’s with cochlear dead regions,
amplification at those frequency regions will
often be ineffective at making those sounds
audible to the patient.
• Frequency transposition or frequency shifting
literally moves the frequencies that are inaudible
down to a lower octave, so they can be perceived
by the patient.
13. Loss of the Pinna Effect/Ear Canal
Resonance with Hearing Aid Use
• Recall that the
resonance of our
ear canal and the
pinna effect result
in our most
sensitive hearing
from 2-5 kHz
▫ Our ears have a
12–15 dB
advantage near
3000 Hz
Image from: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/
14. Loss of the Pinna Effect/Ear Canal
Resonance with Hearing Aid Use
When a hearing aid is blocking the ear canal and/or
the microphone placement of the hearing aid is
outside of the ear canal, it affects the natural
resonance of our ear
o This natural resonance is lost for a behind-the-ear aid
with a closed mold,
o Reduced for an in-the-ear aid,
o Shifted to higher frequencies for a completely-in-the-
canal aid.
This should be considered when fitting hearing aids,
though many hearing aid manufacturers have
“corrections” for this change in the software.
15. Hearing Aids and Conductive Hearing
Loss
• Because the cochlea is healthy and there is no
distortion in the processing of sound in the
cochlea, CHLs are very easy to fit with hearing
aids
• These patient’s just need an increase in volume
at the affected frequencies
• Of course, hearing aids should only be
considered AFTER medical options for
treatment have been exhausted and the patient
is medically cleared for the fitting of hearing aids
