Download noise-induced hearing loss

 5th Congress of Alps-Adria
Acoustics Association
12-14 September 2012, Petrčane, Croatia
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NOISE-INDUCED HEARING LOSS
Davor Šušković, mag. ing. el. techn. inf.
[email protected]
Abstract: One of the most common types of hearing loss is noise-induced hearing loss. Such hearing loss is recognized by
the characteristic audiogram on which is clearly visible a larger loss of hearing in the area around the frequency of 4 kHz.
Noise-induced hearing loss is the result of constant exposure to noise, of working in noisy environment, of frequent listening
to loud music, etc. Since hearing loss is permanent, hearing can be obtained only with appropriate hearing aids. Today's
digital technology enables sound processing strategies that can fulfil the requirements of this hearing loss.
Recommendations are set for this hearing aids; to use open fitting, work in at least 6 channels, have advanced algorithms
for noise supression and focusing on speech, and to use hearing aids on both ears. With FM technology hearing can be
provided in the most difficult hearing situations.
Key words: hearing, noise-induced hearing loss, hearing aids, open fitting, sound processing, channels, fm technology
1. NOISE
Nowadays, noise is one of the main culprits for the
majority of hearing losses. Exposure to noise during the
day permanently damages our hearing. Noise generated
by traffic, noise that we are exposed at work, listening to
loud music and most importantly, the neglect of hearing
protection are the main causes of noise-induced hearing
loss (NIHL).
In the context of the medical literature, the noise can be
defined as extremely intense sound capable of producing
damage to the inner ear. High levels of sound pressure
cause damage to the cilia on the basilar membrane, which
are responsible for the transmission of impulses to the
auditory nerve. As the resonant frequency of the external
ear canal is about 3200 Hz, the sounds of these
frequencies come to the eardrum increased up to 20 dB.
With the greater sensitivity of cilia to the higher
frequency sounds, it is expected that cilia responsible for
the adoption of sound frequencies around 4 kHz are the
most vulnerable.
In acoustic trauma, which is caused by short exposure to
intense sound, hearing loss similar to the one caused by
noise may occur. Level of the sound pressure capable to
produce this kind of damage is around 130-140 dB and
varies between individuals. The outer ear examination
often shows no damage, except in rare cases of the
damage of the eardrum. However, audiogram sometimes
shows a curve characteristic for the noise-induced hearing
loss, or more often down-sloping or flat curve.
2. NOISE-INDUCED HEARING LOSS
Chronic noise-induced hearing loss, in contrast to acoustic
trauma, is a disorder that worsens over time due to
exposure to noise. The amount of sound capable of
creating damage may be shown by the so-called “Damage
Risk Criteria”. It has been shown that short exposure to
loud noise has the same effect as longer exposure to noise
that is somewhat quieter. Since the increase of the sound
intensity of 3 dB corresponds to a twofold increase in the
sound pressure level, it has been concluded that the
exposure to a the intensity of sound higher by every 3 dB
will result in a twofold decrease in the exposure time
allowed. However, as the noise level varies over time,
there is a prescription that leads to an increase of 5 dB in
half the time. For example, OSHA (Occupational Safety
& Health Administration) prescribes the permissible noise
exposure to 90 dBA for 8 hours, or 115 dBA for 15
minutes.
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Even though the noise-induced hearing loss varies from
individual to individual, American College of Medicine
Occupational Noise and Hearing Conservation Committee
specified the main characteristics:
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Hearing loss is always perceptive
(sensorineural)
Almost always symmetrical and binaural
Rarely will cause deafness
Will not deteriorate after exposure to noise
The degree of hearing loss is reduced as the
threshold increases
Frequency of 4 kHz is most affected, and
also the higher frequencies (3 - 6 kHz) are
more vulnerable than lower (500 Hz - 2
kHz)
The maximum hearing loss occurs after 1015 years of exposure to noise
Continuous noise is more dangerous than
the one that is intermittent with silence
Emergence of noise-induced hearing loss can be divided
into two phases. The first phase is characterized by the
Temporary Threshold Shift (TTS). Slight loss of hearing
occurs after exposure to noise which is completely
withdrawn after a period of rest. It is recognized by light
distinctive "ringing in the ears" and occurs after, for
example, listening to loud music. Recovery time can take
up to 48 hours, depending on the duration of exposure to
noise. This damage can be considered as ear fatigue and
many studies show that there is no damage to the sensory
cells.
After frequent exposure to noise capable of producing
TTS, the Permanent Threshold Shift (PTS) will occur.
This is the second phase of the emergence of chronic
noise-induced hearing loss that cannot be repaired. At this
point, there is permanent damage to sensory cells - cilia.
Fig. 1. Audiogram of the person with NIHL
Comparing the position of the phonemes with the
audiogram, it can be noticed that a patient with NIHL will
have problems with hearing of occlusives, fricatives and
affricates characterized by noise at higher frequencies.
Also, due to the higher frequencies of basic formants of
female speakers, there will be more problems with
listening to female vocies than male ones. Such hearing
loss is causing problems in understanding speech and
creates difficulties in social contact. It is often the cause
of separation from society because of the inability of full
participation in the discussions. Understanding speech is
even more harder if more people talk at the same time or
if the conversation takes place in noisy environments
(Fig. 2. and Fig. 3.).
2.1. Characterstics of the Noise-Induced Hearing Loss
NIHL is determined by the specific audiogram. The curve
shows normal hearing at low frequency sounds and a
decrease after 1 kHz (about 20 dB per octave) to a
frequency of 4 kHz, followed by growth towards higher
frequencies (Fig. 1.).
Fig. 2. Position of phonemes in relation to the NIHL
audiogram
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Fig. 3. Position of some sounds in relation to the NIHL
audiogram
3. RECOMMENDED HEARING AIDS FOR
PERSONS WITH NIHL
With regard to the specific audiogram and requirements
that the hearing aid must meet, from a wide range of the
hearing aids offered by the industry nowadays, there are
some that stand out because of their mode of operation
and characteristics to meet the needs of these patients.
Hearing aids can be roughly divided into two groups:
ones that are worn behind the ear (BTE) and the ones that
are wholly in the ear (ITE). BTE hearing aids conducts
sound to the ear by the tube and the earmold which is
placed at the beginning of the ear canal. ITE hearing aids
can be more or less visible in the ear canal, depending on
the construction of the housing.
People with normal hearing of lower frequency sounds
often have problems with a sense of blockage or
occlusion when wearing hearing aids. It is the effect that
occurs due to three sound sources brought to the eardrum:
the amplified sound from the hearing aid, the direct sound
through the vent of hearing aids and the sound that occurs
due to bone conduction (Fig. 4.). The difference between
the amplified and two natural sounds can lead to
somewhat strange sound, and in these situations patients
complain that their own voice sounds strange, too deep,
boomy or hollow, with the frequent observation that
sound “as if speaking from the barrel” to themselves. This
problem is a result of the inability to conduct sounds of
lower frequencies than can be heard, which are only
listened by the bone conductivity. The solution to this
problem is to expand the vent, but today the technology
can overcome this problem with open fitting.
Fig. 4. Three sounds that come to the eardrum
On ITE hearing aids, on which insist patients who care
about discretion, larger vents are done, often of different
shapes and sizes in order to obtain sufficient acoustic
mass which would anull the effect of occlusion. On the
other hand, vent size is limited by the size of hearing aids
as well as by hearing aid amplification due to the greater
possibility of whistling. Calculation of optimum vent size
and shape is done by a software based on algorithms that
correspond to the theoretical setting, and on the results of
various studies (Fig. 5.).
Fig. 5. Various vents – D shape and “Shark bite”
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Recommendations of experts for noise-induced hearing
loss still refer to the "fit as open as possible" rule - that is,
the use of open fitting technology in every possible case.
Open fitting leaves enough space for the passage of
audible sounds of lower frequencies in a natural way due
to which the sound is more natural, it is easier to
determine the direction of the arrival of the sound and
there is no occlusion. Amplified sound from the BTE
hearing aid is led by a thin, barely noticeable tube to the
ear canal, and on the top of the tube there is a small dome
that serves as the protection against cerumen.
A newer version of open fitting hearing aids separates
receiver (speaker) of the hearing aid casing which reduces
the dimensions of hearing aids. Even more importantly,
gain losses are reduced by placing the receiver in the ear
canal. This technology (CRT - Canal Receiver
Technology, RITE - Receiver in the Ear or RIC Reciever in the Canal) appears to be the best solution for
people with NIHL (Fig. 6.).
In addition to the required algorithms - options for noise
suppression and focusing on the speech, another option
provided by today's hearing aids is recommended.
Algorithm that performs a nonlinear transformation of the
higher frequency sounds to the lower frequency range
could provide people with a higher degree of hearing loss
with the ability to hear high frequency sounds (around 4
kHz).
Because of the relatively symmetrical hearing loss on
both ears and in order to further increase the speech
clarity, it is advisable to wear hearing aids on both ears.
With binaural hearing aids fit today's technology allows
communication between hearing aids and the use of the
network of two pairs of microphones. In this case, the
sound from the front would be even more prominent over
the background noise. Binaural wearing of hearing aids
greatly helps in locating the direction of the arrival of
sound and increases the user satisfaction.
3.1. FM technology
Fig. 6. CRT hearing aid – Phonak Audéo S SMART
Today's technology and multi-channel digital audio
processing, which is done by the hearing aids hardware,
provides a variety of options which may greatly benefit
patients with noise-induced hearing loss. Very specific
audiogram curve that is broken in several spots and
difficulty in understanding speech in difficult hearing
situations create demands that today's hearing aids can
fulfill.
Firstly, consider the requirements for the number of
channels, with reference to the number of points in which
it is possible to set up gain of hearing aids. Although
today's nonlinear fitting formulas describe very well
different hearing losses, audiogram of NIHL makes the
request of at least five points of gain tuning. Since the
difference between the number of bands of signal
processing (the number of channels), and the number of
bands in which the gain is determined (the number of
tuning points) is set only at the hearing aids with a smaller
number of channels, a minimum of five to six bands in
which gain could be determined is recommended.
Because of the complex signal processing algorithms and
applications for suppression of noise while focusing on
the speech signals, at least six channels of signal
processing are recommended, therefore six bands in
which the noise is suppressed and speech sounds are
focused on.
Although advanced technology enables excellent speech
understanding in difficult hearing situations, there are still
situations where ambient noise makes it impossible to
listen well. Distance from the speaker makes it difficult to
listen with hearing aids and the technology that would
solve this problem, or the problem to listen to speakers
who are not facing the person with hearing loss, is
necessary. Rooms in which there is an echo or
reverberation, or even outdoor spaces provide a difficult
task for the hearing aids.
Technology of the transmission of the frequency
modulated audio signal has been shown as an optimal
solution in such a difficult hearing situations. A radio
transmitter located near the speaker and a radio receiver
as a part of the hearing aid allow transmission of the highquality speech signal directly to the hearing aid
eliminating the problem of distance and ambient noise
(Fig. 7.).
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Fig. 7. FM transmitter and FM receiver
FM technology proves to be particularly effective in
solving problems with understanding speech that occurs
in people with noise-induced hearing loss, but also in
children during classroom instruction. Schools for
children with special needs, as well as regular schools
attended by children with hearing loss are increasingly
equipped with FM systems that enable them to continue
normal monitoring of school programme.
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