The
Essential Guide to Hearing Loss
Part
1: Understanding Hearing and Hearing Loss
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Sound
and speech
The nature of sound
and how speech is produced.
How
we hear
How the outer, middle, and inner ear work together, enabling
us to hear an incredible array of sounds.
Types
and causes of hearing loss
The differences between conductive and sensorineural ("nerve")
hearing loss and common causes of hearing impairment.
Informal
tests and checklists
Observational checklists for very young children and self-tests
for older children and adults help you decide whether to take
the next step.
Hearing
tests for infants and young children
How the audiologist tests for hearing loss in patients too young
to respond the usual way.
Hearing
tests for older children and adults
How the audiologist determines the nature, cause, and degree of
hearing loss.
Decibels
and the perception of loudness
What it really means if we increase the sound by a certain
number of decibels.
Ranges
of hearing loss
Definitions of mild, moderate, severe, and profound hearing loss.
Sound and Speech
Our ear
is specially designed to receive, amplify, and transmit speech
sounds to the brain. How does it do this?
Well,
for starters, the outer ear, with all its curves and grooves,
is like a specialized antenna. In the world around us, we see
all types of antennas. These antennas are designed to pick up
certain types of electromagnetic waves, (radio, TV, microwave,
etc.). Like antennas, our ears are designed to pick up waves of
a different type - sound waves. Let's see how these sound waves
are made in the first place.
The
Nature of Sound
Sound
waves are nothing more than vibrations of air molecules. The vibrating
air molecules are set in motion by vibrating objects. We have
no trouble visualizing how a plucked guitar string can vibrate
rapidly back and forth for several seconds. Every time it moves
to and fro it compresses air molecules, thus pushing out waves
of sound like ripples in a pond. (Hence the term, "sound
waves.") Strings that vibrate at a higher rate send out more
waves per second and this results in higher-pitched sounds. But
what about speech?
Producing
Speech -- Our Unique Vocal Tract
The vocal
cords are two leathery sheets, with a gap in the middle, that
stretch across the top of the trachea (the windpipe). A number
of muscles control the tension of the vocal cords. When we talk,
the muscles contract, stretching the vocal cords and creating
more tension, while narrowing or closing the gap. This leaves
a very thin opening. As we talk we exhale air and it is this stream
of air, passing through the narrow passage, that causes the taut
vocal cords to vibrate and produce sound. It is like blowing air
through the reed of a clarinet.
Children
have shorter vocal cords than adults. As a result, they vibrate
more rapidly and produce higher-pitched sounds. The quantity of
air passing through determines the loudness.
The sounds
produced by the vocal cords are shaped into an incredible array
of utterances and words by our tongue, teeth, mouth and lips.
In addition, our sounds are made richer and more complex as they
pass around and through our vocal tract: the larynx, throat, sinus
cavities, and mouth. The size, shape, and flexibility of these
structures vary from individual to individual and give us our
unique "voice personality."
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How We Hear
The
Outer Ear -- Catching the Sounds
The outer
ear catches the waves of sound and funnels them down the ear canal
(about an inch long) and flush up against the ear drum. The ear
drum (tympanic membrane) is the boundary between the outer ear
and the middle ear.
Now it
gets interesting. The sound waves or vibrations impinging on the
eardrum can be pretty faint. It is the job of the middle ear to
pick up these unique patterns of vibrations, amplify them, and
pass them on to the inner ear. There the physical vibrations will
be converted into electrical signals and passed on to the brain.
If the sound patterns aren't amplified, the brain will have little
to work with. So, how do the organs of the middle ear do this?
The
Middle Ear -- Powering Up
The work
is done by a trio of very tiny bones: the malleus picks up the
vibrations from the eardrum, passes them to the incus which then
passes them to the stapes. The stapes terminates in a tiny footplate
that fits precisely into the contact point or window of the inner
ear. The sound is strengthened in two ways. First, since the sound
energy is collected from the relatively large surface area of
the eardrum, and then funneled down and focused onto a much smaller
surface area -- the window of the inner ear, 1/80th its size --
it has a magnifying effect. Secondly, these same bones also act
as levers, further amplifying the sound waves. So these vibratory
messages, traveling through the middle ear, arrive in a strengthened
state at the tiny window of the inner ear.
The
Inner Ear -- Converting Mechanical Movement to Electrical Impulse
The window
of the inner ear is the contact point of the cochlea, a fluid-filled
chamber. The vibrations set up rolling waves in the cochlear fluid
which stimulate different areas of a free-floating membrane. The
membrane, in turn, rubs against specialized cells called hair
cells. (They are called hair cells because they are attached to
the membrane by very fine hairs.) This friction creates electrical
impulses in the hair cells which are then passed to the cochlear
nerve and on to the brain.
So, in
essence, all sound is produced by vibrating material. Vibrations
create waves of air molecules that are captured by the outer ear
and funneled down the ear canal where they hit the eardrum. The
middle ear picks up and amplifies the mechanical movements of
the eardrum and passes them to the fluid-filled cochlear, where
waves are created that activate different areas of a free-floating
membrane. This membrane, in turn, physically stimulates the hair
cells which convert this mechanical energy into electrical energy
and pass it onto the brain, via the cochlear nerve.
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Types and Causes of Hearing Loss
People
can suffer from conductive hearing loss, sensorineural
hearing loss, or a combination of the two.
Conductive
hearing loss occurs
when sound waves are physically prevented from reaching the inner
ear. These problems can almost always be corrected through medical
or surgical treatment (and sometimes take care of themselves on
their own). Common causes of conductive hearing loss include:
- Perforated
Eardrum. The eardrum is like the head of a drum. If it
is punctured, it cannot vibrate in time with the vibrational
patterns it receives, and so the sound cannot be picked up
on the other side by the tiny bones of the middle ear. In
time, this condition usually repairs itself. (But see a doctor
anyway.)
- Ear wax buildup
in the ear canal. This can partially block or muffle the sound
waves impacting on the eardrum. Wax can easily be removed
by a doctor or nurse.
- Fluid in the
middle ear.
This is very common in children. Known as Otitis Media, it
is caused by an infection of the middle ear or by a cold when
fluid is backed up into the middle ear. (Fluid normally drains
through the eustachian tube to the throat .) In either case,
fluid fills the middle ear, preventing the tiny bones from
vibrating properly and sending the sound impulses onto the
inner ear. The problem can be remedied through antibiotics
or by surgically installing a tiny tube in the child's ear
so the fluid can drain.
- Abnormal growth
of the bones of the middle ear. As in the condition above,
the bones of the middle ear are unable to move properly and
cannot transmit the sound. Severe hearing loss can result.
Surgery is usually very effective in this situation.
Sensorineural
hearing loss occurs
when the sensory cells of the inner ear (the hair cells) or the
auditory nerve itself are damaged through aging, exposure to loud
noise, drug reaction, head injury, or genetic factors. The most
common cause is gradual exposure to excessive noise over a number
of years or one or more intense exposures. (That's why ear protectors
are so important.) Unfortunately, this damage usually cannot be
corrected.
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Informal tests and questionnaires to see
if there is a hearing problem
(Note: The material
in this section was obtained from the National Institutes of Health,
National Institute of Deafness and other Communication Disorders)
Important: If
you know you have hearing loss, you should always see a doctor,
for the impairment may be caused by a medical condition unrelated
to the ear. For example, it may be the result of circulatory problems
caused by diabetes, vascular (blood vessel) problems, or a heart
condition. If you experience a sudden loss of hearing,
see a doctor at once.
General
Observations:
People
with hearing loss may experience some or all of the following
problems:
- Difficulty
hearing conversations, especially where there is background
noise
- Hissing,
roaring, or ringing in the ears (tinnitus)
- Difficulty
hearing the television or radio at a normal volume
- Fatigue
and irritation caused by the effort to hear
- Dizziness
or problems with balance
Self-Test
Ask yourself the following
questions. If you answer "yes" to three or more of these
questions, you could have a hearing problem and should see a doctor.
Do I have a problem
hearing on the telephone?
Do I have trouble hearing
when there is noise in the background?
Is it hard for me to
follow a conversation when two or more people talk at once?
Do I have to strain
to understand a conversation?
Do many people I talk
to seem to mumble (or not speak clearly)?
Do I misunderstand
what others are saying and respond inappropriately?
Do I often ask people
to repeat themselves?
Do I have trouble
understanding the speech of women and children?
Do people complain
that I turn the TV volume up too high?
Do I hear a ringing,
roaring, or hissing sound a lot?
Do some sounds seem
too loud?
Behavioral
Checklist for Infants and Children
Important!
Infants and young children
with hearing problems can have difficulty developing speech and
language.
Some babies are born
with hearing problems. Other children are born with normal hearing
and begin to have hearing problems as they grow older.
You can help your child's
doctor to decide if your child's hearing needs to be tested. Hearing
problems can be temporary or permanent. Hearing problems can happen
because of ear infections, injuries, or diseases.
Read the hearing checklist.
Find your child's age. Indicate "yes" or "no"
for every item. After you complete the checklist, show it to your
child's doctor. Ask the doctor questions. Talk about the items
checked "no". If you think your child has trouble hearing,
tell the doctor right away.
Birth to 3 Months
Reacts to loud sounds?
Is soothed by your
voice?
Turns head to you when
you speak?
Is awakened by loud
voices and sounds?
Smiles when spoken
to?
Seems to know your
voice and quiets down if crying?
3 to 6 Months
Looks upward or turns
to a new sound?
Responds to "no"
and changes in tone of voice?
Imitates his/her own
voice?
Enjoys rattles and
other toys that make sounds?
Begins to repeat sounds
(such ooh, aah, and ba-ba)?
Becomes scared by a
loud voice?
6 to 10 Months
Responds to his/her
own name, telephone ringing, someone's voice, even when it isn't
loud?
Knows words for common
things (cup, shoe) and sayings ("bye-bye")?
Makes babbling sounds,
even when alone?
Starts to respond to
requests such as "come here."?
Looks at things or
pictures when someone talks about them?
10 to 15 Months
Plays with own voice,
enjoying the sound and feel of it?
Points to or looks
at familiar objects or people when asked to do so?
Imitates simple words
and sounds?
Uses a few single words
meaningfully?
Enjoys games like peek-a-boo
and pat-a-cake?
15 to 18 Months
Follows simple directions,
such as "give me the ball."?
Uses words he/she has
learned often.
Uses 2-3 word sentences
to talk about and ask for things?
Knows 10 to 20 words?
18 to 24 Months
Understands simple
"yes-no" questions (Are you hungry?)?
Understands simple
phrases (in the cup, on the table)?
Enjoys being read to?
Points to pictures
when asked?
24 to 36 Months
Understands "not
now" and "no more."?
Chooses things by size
(big, little)?
Follows simple directions
such as "get your shoes" and "drink your milk."?
Understands many action
words (run, jump)?
Talk
to your doctor if you think your child has a hearing problem.
Let
him know of the following information and observations:
- Whether
others in the family, including brothers or sisters,
have a hearing problem.
- Whether
the child's mother had medical problems in pregnancy
or delivery (serious illness or injury, drugs or medications).
- If
the baby was born early (premature). Weight at birth:
_______
- If
the baby had physical problems at birth.
- If
the child rubs or pulls an ear(s) often.
- If
the child had scarlet fever.
- If
the child had meningitis.
- The
number of ear infections in the past year: _______
- How
often the child had colds, allergies, and ear infections
(once a month or more often).
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Hearing Tests: infants and children
Infants:
Two hearing
tests are often used to screen babies. In both tests, no activity
is required from your child other than lying still.
- Otoacoustic
emissions (OAE) tests can show whether parts of the ear
respond properly to sound. During this test, a sponge earphone
is placed into the ear canal. The ear is stimulated with sound,
and the "echo" is measured. The echo is found in
everyone who hears normally. If there is no echo, it could
indicate a hearing loss.
- Auditory brain
stem response (ABR) tests
check how the brain stem (the part of the nerve that carries
sound from the ear to the brain) and the brain respond to
sound. During this test, your child wears earphones, and electrodes
are placed on the head and ears. A mild sedative may be given
to help keep your child calm and quiet during the test. The
nurse or doctor sends sounds through the earphones and measures
the electrical activity in your child's brain when he or she
should be hearing.
Older
Infants and Toddlers:
Two screening tests recommended by the American Speech Language
Hearing Association are:
- Visual
reinforcement audiometry (VRA) is highly recommended.
Whenever the child looks toward a sound source, she is rewarded.
(Six months to two years of age)
- Conditioned
play audiometry (CPA). The
child performs a task (puts a block on the table, touches
a toy, etc.) every time a sound is heard. (Two to three years
of age)
Pre-schoolers:
- Conditioned
play audiometry (CPA).
(See above.)
- Tympanometry
tests the responsiveness of the eardrum. (Next section.)
- Additional
testing of
acoustical reflex (a tiny ear muscle) and air volume test
of ear canal.
School
Age Children:
- Conditioned
play audiometry (CPA) and
tests for adults. (Next section.)
Note:
For
a great deal more info on screening tests, including important
risk factors, visit the American Speech-Language-Hearing Association
at www.ASHA.org
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Hearing Tests: Older Children and Adults
The audiologist
may conduct some or all of the following basic tests:
1.
Pure Tone tests.
The air
conduction test is the most general hearing test as it evaluates
the entire mechanism: the outer, middle, and inner ear as well
as the hearing nerve. Soft tones are played through a set of headphones
to determine the hearing threshold (the softest sound one can
hear at least 50% of the time) for different pitches.
If
there is hearing loss, the bone conduction test can
help determine whether the problem is in the outer, middle, or
inner ear. A small vibrator is placed on the skull bone directly
behind the ear. When sound is transmitted through this device,
it bypasses the outer and middle ear and delivers the sound vibrations
directly to the inner ear. If the hearing improves during this
procedure, then the audiologist knows there is a problem involving
the conduction of sound through the outer or middle ear. (It can
also be a combination of both conductive hearing loss and
sensorineural loss.) If the hearing loss is the same after this
test, then the problem most likely lies in the inner ear. (How
We Hear) Once the results of the tone tests are in,
the audiologist can prepare an audiogram, a picture of our hearing.
2.
Tympanometry tests the flexibility of the ear drum. The eardrum
must be extremely pliable so it can respond to the softest impulses.
But sometimes, over time, the eardrum stiffens and it takes a
stronger, more forceful sound wave to make it move. The audiologist
determines its flexibility by taking some air out of the air canal
(painless) and pumping some back in. As he does this, he measures
the minute movement of the eardrum.
3.
Word tests shed light on how well one understands normal conversation.
There is a difference between being able to hear words and being
able to understand them. The speech reception threshold
test determines the softest level at which an individual can hear
words. The speech recognition test determines how well
the individual can understand words spoken at normal volume. The
words include all the common phonemes (basic sounds) of normal
conversation.
4.
Interview Questions may include:
- health history
- history of ear
problems in general / which ear(s)
- noise exposure
history
- known hearing
loss / which ear(s)
- tinnitus
- vertigo
- family history
of hearing loss
Note:
For
a great deal more info on screening tests, including important
risk factors, visit the American Speech-Language-Hearing Association
at www.ASHA.org
This site has an enormous amount of very valuable information.
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Decibels
Sounds
are measured in decibels. Zero (0) decibels is the softest sound
a person with normal hearing can hear at least 50% of the time.
Here are decibel levels of everyday sounds:
0
Decibels Threshold
of hearing
10 Rustle
of leaves
20 Water
dripping
30 Whisper
40 Quiet
radio in room
50 Moderate
rainfall
60 Conversation,
dishwasher
70 Busy
traffic, vacuum cleaner
80 Alarm
clock
90 Lawnmower
100 Snowmobile,
chainsaw
110 Rock
music
120 Jet plane takeoff
The important
thing to know about decibels is this: if a sound increases
by 10 decibels, it doubles in loudness as we perceive it.
It sounds twice as loud to us. If a sound decreases
by 10 decibels, it seems as if the sound has been reduced to half
the volume.
Perceived
Change in Loudness by Decibel Level
This chart
shows the change in decibel sound level and
how it is perceived by the human ear:
+,
- 1 dB Not perceptible
+, - 3 dB Threshold of perception
+, - 5 dB Clearly noticeable
+, -10 dB Twice as loud or
1/2 as loud
+, -20 dB Four times as loud
or 1/4 as loud
+, -30 dB Eight times as
loud or 1/8 as loud
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Ranges of Hearing Loss
When we
speak of hearing loss, we mean threshold hearing points that are
higher than normal. If, for example, a person has a 45 dB loss
in the 4000 Hz range, it means that for him to be able to hear
a sound at that frequency or pitch, the sound must be at least
45 dB in loudness. He cannot hear sounds below that volume at
that frequency.
Normal
hearing.
Loss of 0 - 19 dB (decibels).
Mild
hearing loss. Loss of 20 - 39 dB.
Symptoms: Unable to hear soft sounds. Cannot hear
a whispered conversation in a quiet room. Can hear a
normal conversation in a quiet room but has difficulty in a
noisy environment
Moderate
hearing loss. Loss of 40 - 59dB.
Symptoms: Has considerable difficulty hearing a normal conversation
in a quiet room. If there is background noise, he will not be
able to understand many of the words, unless he lip reads.
Severe
hearing loss.
Loss of 60 - 89dB.
Symptoms: Cannot hear a conversation unless the speaker
speaks loudly.
Profound
hearing loss. Loss
of 90+ dB. Cannot understand speech even if the person
speaks very loudly. Can only hear very loud sounds such as a
chainsaw.
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Part
2: The Link between Audiograms, Hearing Loss, and
Speech Intelligibility.
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