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Selecting hearing devices: features
and parameters
Contents
Key to resources for activities
2
Compression
3
Other types of signal processing
6
Glossary
11
Feedback to activities
13
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Key to resources for activities
Note: Print this Key to resources for activities and refer to it as you work
through the task guide.
Resource
number
Resource title
1
Dillon, H (2001) Hearing Aids, Boomerang Press, Sydney
Chapter 6: Compression systems in hearing aids
2

Synopsis

Section 6.1: Compression’s major role: Reducing the signal’s
dynamic range
Dillon, H (2001) Hearing Aids, Boomerang Press, Sydney
Chapter 7: Advanced signal processing schemes for hearing aids
Now go to Your case studies on Miss Bennett beginning on slide 4.
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Compression
The most important and valuable means of sound modification, apart from
amplification and filtering, is compression. As you may remember in your
study of how we hear, a sensorineural hearing loss doesn’t just make it more
difficult for a person to hear soft sounds, but also makes it more difficult for
the person to tolerate loud sounds. The term for this is recruitment, and it
describes the reduced dynamic range people with sensorineural losses
usually experience.
A hearing aid (device) without compression will amplify all incoming
sounds by the same amount. This means that a soft sound will be made
audible, which is lovely, but a loud sound may become uncomfortably loud,
or even unbearable, which is not so lovely.
Amplification like this (otherwise known as linear amplification) depends
on the quick action of the person wearing the aid (device) to reach up for
their volume control and turn the volume down.
There are two problems with this. Some people with a particularly small
dynamic range will need to be adjusting their volume control very
frequently. Also, some loud sounds are very sudden, and the hand to the
volume control just isn’t fast enough.
A better option is for the hearing aid (device) to adjust the amount of
amplification it gives to the sound before it sends it into the unsuspecting
person’s ear. The way it does this is to squash, or compress, the amount of
amplification it gives to louder signals. This is the basis of the concept of
compression. Compression can be performed in a number of ways.
Go to Resource 1
Activity 1
Write down the three types of compression described.
1.
2.
3.
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Go to Resource 2
Activity 2
Write definitions for the following words and phrases as you read through
Resource 2.
1.
Attack time:
2.
Release time:
3.
Adaptive release time:
4.
Compression threshold:
5.
Compression ratio:
6.
Input-controlled compression:
7.
Output-controlled compression:
8.
Write down the two main reasons why we might want to compress
different frequency regions by different amounts.


Go to Resource 1
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Activity 3
As you read Resource 1, write down the key advantages of compression
over linear amplification for each of the following sound levels, and note the
most useful form of compression and compression threshold.
For example:

For very loud sounds:
High thresholds, output controlled (also known as output limiting
compression) reduces distortion; preferable for all hearing losses
except those who will benefit from the additional SPL generated by a
peak clipping aid.
Now answer the following:

For loud sounds:

For medium sounds:

For soft sounds:
In Section 6.5.2, Benefits of multichannel relative to single-channel
compression, Dillon cautions against using high compression ratios (greater
than 3:1) in multichannel devices because there can be a loss of
intelligibility. However, most modern hearing aids (devices) do have more
than one channel, and many have more than 10.
Activity 4
Write down three reasons for having multiple channels (other than
compression).
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An important note here is that the terms ‘multiple channels’ and ‘multiple
bands’ can have two definitions.
In most cases, the terms ‘channel’ and ‘band’ are synonymous – that is, a
channel or band is a frequency range in which all processing is performed
independently of other channels or bands.
Less frequently, ‘channel’ and ‘band’ are used to mean different things: a
‘channel’ refers to a frequency range in which all processing is performed
independently of other channels, and a ‘band’ is a frequency range within,
or independent of, a channel.
This second distinction can be rather confusing; fortunately most
manufacturers use the first definition.
So compression’s main role for people with sensorineural hearing loss is to
‘squash’ all incoming signals into their reduced dynamic range. The
opposite of compression is expansion, where the amplification of soft
sounds is less than the amplification of mid-level sounds, and the
amplification of loud sounds is greater than that of mid-range sounds. There
is no application of expansion for louder sounds for people with hearing
loss, however expansion is useful for the very soft sounds, such as the
hearing aid (device) internal noise. Expansion, which is also known as ‘soft
sound squelch’ and ‘microphone noise reduction’ can also be used to reduce
the amplification of quiet environmental humming noises such as the fridge
or computer.
Other types of signal processing
Now we move on to Chapter 7, which looks at other ways the electroacoustics of the hearing aid (device) can be used to improve the sound
signal for particular clients.
We will focus on three features:

directional microphones

noise reduction technology

feedback suppression.
Directional microphones
Go to Resource 2
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You may have heard clients say, “The main problem with my hearing is that
I can’t seem to understand what people are saying in background noise”.
This is probably the most common difficulty for people with sensorineural
hearing losses. Unfortunately, it is also one of the things that hearing aids
(devices) are notoriously bad at alleviating.
The directional microphone is the most effective of the various features that
hearing aid (device) manufacturers can use to improve the signal-to-noise
ratio for the people who are wearing their products.
Activity 5
Which of the following statements is false? Circle the incorrect one.
(a) A directional microphone system is a single microphone with an
opening to the front and the rear, in which the signal from the rear is
delayed slightly and subtracted from the signal from the front.
(b) Directional microphones consist of an array of two or more
microphones, in which the signal from the rear microphone is
delayed slightly and subtracted from the signal from the front
microphone.
(c) Directional microphones need to be separated by a large enough
distance.
(d) Adaptive directional microphones change from omnidirectional to
directional when the noise level reaches a predetermined level.
(e) An adaptive directional microphone is a directional microphone
array in which the directivity pattern varies depending on where an
unwanted noise is coming from.
Noise reduction
As mentioned, the use of directional microphones is the single most
effective way of dealing with the problem of background noise. There are
other ways, however.
Go to Resource 2
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Activity 6
1.
Which style of hearing aid (device) would be most helped by single
microphone noise reduction?
2.
Write down the names of the two key means of noise reduction.
3.
What effect do both key means of noise reduction have on a noisy
signal?
Noise reduction can be fixed, user-selected or adaptive. With adaptive noise
reduction, the amount of noise reduction increases as the level of ambient
sound increases. When used with adaptive directional microphones,
particularly automatic adaptive directional microphones, the hearing aid
(device) can maintain optimum comfort and intelligibility in most situations,
without any user adjustments.
When noise reduction and/or directional microphones are not automatic, the
use of two or more separate programs, or memories, may be necessary. In
this case the hearing aid (device) wearer manually chooses the level of noise
reduction and/or directionality by selecting one of two or more preset
programs or memories. Multiple-program hearing aids (devices) usually
have a switch or button which allows the user to change program in much
the same way that you might change from a sport channel to a movie
channel on your TV. The programs can also be accessed via a remote
control where fitted.
The hearing aid (device) programs are usually set by the audiometrist or
audiologist (with recommendations by the hearing aid (device)
manufacturer), and allow the hearing aid (device) to provide optimum sound
quality for different listening situations.
For example, one program may be set without noise reduction and without
directional microphones to provide optimum sound quality for a quiet
situation; another program might be set with noise reduction and directional
microphones to provide optimum signal-to-noise advantage, for use in noisy
situations. Another program might be allocated for the telecoil.
Multiple-memory/program hearing aids (devices) became increasingly
common in the mid to late 1990s, as digitally programmable analogue
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hearing aids (devices) increased in prevalence. As digital hearing aids
(devices) have made possible more adaptive and automatic adjustments, the
need for multiple manually controlled programs is reducing. Multiple
programs/memories are still necessary for selection of the telecoil, although
many hearing aids (devices) can be set to select telecoil when a phone is
brought up to the aid.
Feedback reduction
Have you ever sat with someone whose hearing aid (device) is whistling?
This annoying and very embarrassing problem can be alleviated by clever
electronics.
Go to Resource 2
Activity 7
Describe the three methods of feedback management currently used with
hearing aids (devices).



Electro-acoustic methods of feedback such as these, and particularly the last
method, have made possible the development of the Open Fitting BTE, also
known as the Thin-Tube or Slim-Tube BTE. With this hearing aid (device),
the friction hook and standard 2mm tubing have been replaced by very thin
(0.9mm or 1.3mm) replaceable tubing, and the standard ear mould has been
replaced by a tiny, floppy dome which holds the thin tubing in place without
blocking the ear canal. An alternative to the dome is a custom-made shell
which acts as an anchor for the tubing, but is otherwise a hollow cylinder
shaped like the ear canal.
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Open Fitting BTEs greatly reduce the incidence of occlusion effect, and so
can be used for people with normal and near-normal low-frequency hearing.
Go to Resource 2
Although modern hearing aids (devices) really are a far cry from the hearing
aids (devices) of only a decade ago, it’s useful to remember that they are
still hearing aids (devices). People with sensorineural losses have lost some
of their hearing and it cannot be replaced with electronics.
At this point, go back to the task guide: Identify different types of hearing
devices in this unit and reread the descriptions of the two creatively named
hearing aids (devices). You should now have a pretty good idea just how
similar they are.
In the real world it is unlikely that two hearing aids (devices) with such
similar features will have greatly different prices. However it is quite
common for two hearing aids (devices) with similar prices to have different
combinations of features.
Your job, as you help your client decide which hearing aid (device) options
best meet his or her hearing loss and hearing needs, is to match frequency
response and features with audiogram and needs. This is a balancing act, to
be sure, and one which is not helped much by the highly creative and
evocative terms different manufacturers use to describe the same things. But
once you’re able to translate those terms so that the true hearing options can
be compared, it’s a task which is quite possible – and even fun.
A very good place to start the comparison is the measurement of the sound
output of a hearing aid (device). The measurement of a hearing aid (device)
allows you to estimate how suitable its gain is, relative to your client’s
requirements. It also allows you to evaluate how well it is doing its job once
your client is wearing it.
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Glossary
Acoustic hearing
aid (device)
effects
Anything non-electrical that affects the sound reaching the
eardrum (pinna, microphone tubing, ear hook, ear mould tubing,
ear mould/ear shell, sound bore, venting, ear canal)
ALD
Assistive listening device
BTE
Behind-The-Ear hearing aid (device)
CIC
Completely-In-the-Canal hearing aid (device)
Digitally
programmable
The settings of a digitally programmable hearing aid (device) are
adjusted by a computer or programming device. This allows for
much greater flexibility in adjustment than is possible using
trimpots. Note that a digitally programmable hearing aid (device)
is an analogue aid (device)
Electro-acoustic
hearing aid
(device) effects
Anything that pertains to the hearing aid (device) transducers
(microphone, receiver and everything between them)
ITC
In-The-Canal hearing aid (device)
ITE
In-The-Ear hearing aid (device)
MC
Mini-Canal hearing aid (device)
MPO
Maximum power output : this is usually used to mean OSPL90
NRS
National Relay Service, an Australian Government service which
allows a person using a TTY to communicate with someone who
does not have a TTY. See <www.relayservice.com.au> for more
information
OSPL90
The (output) SPL (sound pressure level) produced by a hearing
aid (device) when it receives a 90dB input
Peak clipping
A method of maximum output limiting in which the peaks of the
signal that would otherwise be greater than the maximum level
are ‘clipped’. As a result there is no reduction in gain, but there is
significant distortion
REIR
Real Ear Insertion Response. REIG stands for Real Ear Insertion
Gain, while IG stands for Insertion Gain. Note that REIR, REIG
and IG are generally used interchangeably
REUG
Real Ear Unaided Gain; note that REUR and REUG are generally
used interchangeably
REUR
Real Ear Unaided Response
Signal-to-noise
ratio (S/N or SNR)
A measure of the level of the desired signal, relative to the level
of undesired signal (background noise). The larger the SNR, the
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less the undesired signal masks the desired signal
Trimpot
A tiny potentiometer on a hearing aid (device) which allows the
settings of the hearing aid (device) to be adjusted (using a
screwdriver)
TTY
Telephone typewriter: a device which allows a hearing-impaired
person to send text via an ordinary phone. The text can only be
read by someone else with a TTY
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Feedback to activities
Activity 1 feedback
Write down the three types of compression described.
1. high-level compression, of which one type is compression limiting
2. wide dynamic range compression (WDRC)
3. low-level compression
Activity 2 feedback
Write definitions for the following words and phrases as you read through
Resource 2.
1. Attack time: The time taken for the compressor to react to an
increase in signal level
2. Release time: The time taken for the compressor to react to a
decrease in input level
3. Adaptive release time: The release time is short for brief intense
sounds, but becomes longer as the duration of an intense sound
increases
4. Compression threshold: The SPL above which the hearing aid
(device) begins compressing
5. Compression ratio: The change in input level needed to produce a
1dB change in output level; note that the compression ratio describes
how much the gain decreases with input increase
6. Input-controlled compression: Compression controlled from a point
on the input side of the volume control (the compression threshold is
independent of the volume control)
7. Output-controlled compression: Compression controlled from a
point on the output side of the volume control (the compression
threshold is independent of the volume control setting)
8. Write down the two main reasons why we might want to compress
different frequency regions by different amounts.

Hearing loss usually varies with frequency.

Signals and noises in the environment have more energy in some
frequency regions than in others.
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Activity 3 feedback
As you read Resource, write down the key advantages of compression over
linear amplification for each of the following sound levels, and note the
most useful form of compression and compression threshold.

For loud sounds: (medium or low compression threshold – inputcontrolled) increases listening comfort

For medium sounds: (low compression threshold – input-controlled)
reduces the need for the volume control

For soft sounds: (low compression threshold – input-controlled)
increases audibility
Activity 4 feedback
Write down three reasons for having multiple channels (other than
compression).

Multiple channels enable the gain-frequency response to be most
easily and flexibly controlled.

Noise suppression is more effective.

Feedback suppression can be more effective.
Activity 5 feedback
Which of the following statements is false? Circle the incorrect one.
(a) A directional microphone system is a single microphone with an
opening to the front and the rear, in which the signal from the rear is
delayed slightly and subtracted from the signal from the front.
(b) Directional microphones consist of an array of two or more
microphones, in which the signal from the rear microphone is delayed
slightly and subtracted from the signal from the front microphone.
(c) Directional microphones need to be separated by a large enough
distance.
(d) Adaptive directional microphones change from omnidirectional to
directional when the noise level reaches a predetermined level.
(e) An adaptive directional microphone is a directional microphone array in
which the directivity pattern varies depending on where an unwanted
noise is coming from.
(d) is false: this is an automatic directional microphone, not an adaptive
directional microphone (note that you can have both, ie an automatic
adaptive directional microphone)
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Activity 6 feedback
1.
Which style of hearing aid (device) would be most helped by single
microphone noise reduction?
The CIC, which does not have enough space for a directional
microphone.
2.
3.
Write down the names of the two key means of noise reduction.

Wiener filtering

spectral subtraction
What effect do both key means of noise reduction have on a noisy
signal?
They decrease the gain most at those frequencies where the signal-tonoise ratio is worst.
Activity 7 feedback
Describe the three methods of feedback management currently used with
hearing aids (devices).

reduction of gain at a frequency or frequencies where feedback is a
possibility

variation of the phase response of the gain at any frequency where
feedback is a possibility

production of a second, intentional feedback path with just the right
gain and phase response to cancel the external leakage path.
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