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How We Hear

Hearing is one of the human body’s most extraordinary processes. A complex system of delicate and synchronous parts, it’s easy to take this critical sense for granted. To better understand why hearing loss happens, it’s important to first know how hearing works

It begins with sound
Sound begins with a vibration in the atmosphere. When something vibrates (whether it’s wind, a bell or a voice), it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the energy of the vibration through the air as a sound wave. That’s where your ear comes in.

Signs You May Need A Hearing Test

Sound waves are collected by the outer ear and directed along the ear canal to the eardrum. When the sound waves hit the eardrum, the impact creates vibrations, which, in turn, cause the three bones of the middle ear to move. The smallest of these bones, the stirrup, fits into the oval window between the middle and inner ear.
When the oval window moves, fluid in the inner ear moves, carrying the energy through a delicate, snail-shaped structure called the cochlea.
In the inner ear, thousands of microscopic hair cells are bent by the wave-like action of the fluid inside the cochlea. The bending of these hairs sets off nerve impulses, which are then passed through the auditory nerve to the hearing center of the brain. This center translates the impulses into sounds the brain can recognize, like words, music or laughter, for instance.
If any part of this delicate system breaks down, hearing loss can be the result.

Types Of Hearing Tests

Understanding the results of your hearing test has a huge impact on your ability to understand your hearing loss, how it impacts upon your hearing and communication abilities and your likelihood for success with hearing aids.

Audiometry – the ‘hearing beep test’ – how softly you can hear each pitch

First, you will have Air Conduction Testing which tells us how well you hear when sound has to travel all the way through the outer ear, middle ear, and the inner ear. This is when you wear headphones and press a button every time you hear a beep. We plot the responses to these beeps on a graph called an Audiogram, using X’s for the Left ear and O’s for the Right ear. This tells us your hearing thresholds – the softest sounds you can hear at a range of different pitches.

The further down the graph you see these markings, the worse your hearing is as it indicates we’ve had to turn the volume up for you to just hear it. ‘Normal hearing’ requires all the X’s and O’s to be at the top of the graph, above the 20dB line.

The graph also tells us how well you hear at different pitches, with Low-Frequency bass sounds on the left side of the graph, mid frequencies in the middle and High-Frequency treble sounds on the right side of the graph.

The second part of this Audiometry is Bone Conduction testing. This tells us how well you can hear when sound bypasses the outer ear and middle ear structures and travels directly through your skull to the inner ear (i.e. cochlea). This is when a bone conduction headband is placed on your head, directly behind your ear. Again, you press the button every time you hear the beep. We plot the responses to these beeps using brackets – and they represent the hearing sensitivity of your inner ear.

If bone conduction responses are the same as the air conduction thresholds, then we know that your middle and outer ear are fine and that the problem is within the inner ear. This is known as a sensorineural hearing loss – most common for age related hearing loss and industrial noise damage.

If bone conduction responses are significantly better than the air conduction thresholds then we know that something in the outer or middle ear is impairing the ability of sound to get to the inner ear and thereby causing the hearing loss – known as a conductive hearing loss.

Once an audiogram is complete we can overlay the pitch and volume of normal speech sounds to get a good idea of what you can hear and what you are missing. Unlike vision impairment, where everything diminishes, hearing loss commonly affects some pitches of hearing (sometimes severely) while other pitches remain totally normal. This is why people often say ‘I can hear fine, I just cant understand!’. If visual impairment were the same it would be like trying to read a book but rubbing out certain letters (e.g. t, s, p, ch, k… etc) while leaving other letters perfectly visible (o, u , a ….etc).

The Audiogram therefore tells us which sounds cannot make it from your outer ear to your brain, when spoken at a normal level, without the use of hearing treatment.

Interpreting the Audiogram

An audiologist will use Audiogram to describe your hearing loss according to

Conductive

Normal hearing for bone conduction scores (`{` & `}`), and a hearing loss for Air Conduction scores (X & O). Identifies a blockage, infection, or physical damage within the outer-ear or middle-ear

Sensorineural

Hearing loss (equally) for both air and bone conduction (i.e. nerve damage or degradation).

Mixed

Hearing loss for bone conduction score, and an even greater hearing loss for air conduction scores

The Severity of loss

The lower the scores fall on the Audiogram, the more severe the hearing loss.

0-20Normal Range
20 – 30Mild Loss
30 – 60Moderate Loss
60 – 90Severe Loss
90 +Profound Loss

The Slope of loss

  1. Flat loss – A hearing loss where hearing thresholds (scores) are relatively even across all frequencies. This is commonly of a conductive hearing loss.
  2. Sloping lossIncreasing degree of hearing loss at higher frequencies. This is the most common hearing loss and is caused by the ageing process and noise damage.

 “I can hear fine, I just can’t understand’

Low frequency speech sounds (125 Hz – 1000 Hz) are largely responsible for a person’s sense of the volume. High frequencies are responsible for clarity. Some of the high-frequency elements of speech include those made by words containing letters such as “f”, “ph”, “th”, “s” and “t”. These important high frequencies are usually the pitches that cant be heard which is precisely why people with hearing loss misinterpret what is said and complain that others are mumbling.

  1. Other Less common shapes include reverse slopes, cookie bites, corner audiogram
Hearing Service Program

Speech Recognition testing – how well you can understand audible speech

Word Recognition Testing is the best way to understand the severity of hearing loss. Using a term like moderate or severe or even using a percentage seems like an easy way to describe the severity of a hearing loss but it can be very misleading. For example, someone could have excellent low frequency hearing but a severe loss in their high frequencies. This would mean they have a normal 0% loss for low pitches but severe 80% loss for high pitches. The overall loss is neither normal nor severe and averaging it to 40% simply doesn’t make sense as it fails to accurately reflect the vastly different degrees of hearing at different pitches.

Word Recognition testing determines your ability to understand speech when presented at a comfortably audible level. The results give us a percentage score that gives us much more insight into what your actual hearing ability is.

To calculate the score, words are presented words at a volume that should be loud enough for you to hear but not too loud (based on your hearing thresholds from the Audiogram). You are asked to repeat the words while we score the percentage of those words that you get correct. This percentage represents your ability to comprehend speech without visual cues or context. Your word recognition score is therefore significantly more important to us than the Xs and Os of your audiogram. You can have bad hearing thresholds but have a great word recognition score or good hearing thresholds and a terrible word recognition score.

This will help us to understand how much success you should be able to expect with correctly programmed hearing devices. It tells us whether sound presented to your brain at a comfortably audible level will be clear or not. Hearing aids are designed to return audibility to sounds that would go unheard with a hearing loss. If programmed well, we would expect that you would perform close to your Word Recognition Scores in a quiet situation. In background noise, you should also perform somewhat better since in most cases, the return of high frequency sounds to your brain will improve your ability to separate speech from noise.

If you score well during this test, then you would expect to do well with hearing aids. If you score poorly, then you would expect to have limited benefit with hearing aids.

Three factors impact upon your word recognition score

Inner Hair Cell function

Hair cells within your inner ear bend in response to vibrations caused by sound pressure. When they bend an electrical signal is sent to your brain to be interpreted as meaningful sound. If these cells are damaged there’s no way for them to send the sound clearly to your brain – so you may be able to hear the sound but not clearly understand it due to the distorted signal sent to the nerve.

The Auditory Nerve

This transmits the impulses created by the hair calls to the brain. If something obstructs this pathway then the nerve impulses will be distorted by the time they reach the brain.

Brain Function

This transmits the impulses created by the hair calls to the brain. If something obstructs this pathway then the nerve impulses will be distorted by the time they reach the brain.

Hearing Loss Percentage – What does it mean

Using a percentage seems like an easy way to describe the severity of a hearing loss but it can be very misleading. A percentage loss of hearing (PLH) is designed for worker’s compensation and corresponds to the amount of hearing loss in pitches that are commonly damaged by exposure to loud noise. It usually does not accurately reflect how much hearing loss someone has nor the degree of communication trouble it causes.

For example, someone could have excellent low frequency hearing but a severe loss in their high frequencies. This would mean they have 0% loss for low pitches but 80% loss for high pitches. Averaging it to 40% doesn’t make any sense as it doesn’t accurately reflect the vastly different degrees of hearing at different pitches.

As such, the calculation is useful when used to assess and track the amount of hearing loss due to exposure to noise but not particularly effective in conveying the difficulty someone’s hearing loss is causing in daily life.

Tympanometry & Impedence Testing – the mechanics and pressure of the middle ear

Tympanometry is an objective test of middle ear function.

The job of the middle ear is to transmit sound from the ear canal to the inner ear. When sound waves hit the eardrum it causes it to vibrate. The eardrum will absorb some of the sound and send it through to the bones of the middle ear. The eardrum works best and absorbs sound most effectively when it is ‘floppy’ (i.e. compliant). If it is tight or rigid it will reflecting the sound back instead of absorbing it – dampening your hearing.

We test this using ‘Typmanometry’ during which we play a sound in the ear canal to see how much of it is reflected back as we change the pressure in the ear canal – it feels a bit like going up in a plane. Ideally, the eardrum will absorb the most amount of sound at normal room pressure (i.e. when we are not changing the pressure). This means your middle ear is working most effectively in normal daily conditions. If we have to add or subtract pressure to make the eardrum vibrate best then we know that the middle ear is not functioning optimally under normal conditions.  If the middle ear is full of fluid, the eardrum will be unable to move so sound will not travel through to the bones and will be reflected instead. If the pressure in the middle ear is less than normal room pressure then the eardrum will be sucked in and unable to effectively vibrate. Tympanometry is typically used to detect or rule out several things: the presence of fluid in the middle ear, a middle ear infection, a hole in the eardrum (perforation), or Eustachian tube dysfunction.

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