The Ears and Hearing

Auditory Function

The Ear

How We Hear

What Can Go Wrong

Effects of Hearing Loss

Degree of Hearing Loss

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The Ear

The external ear consists of the pinna and the ear canal. The pinna is the flap of skin attached to either side of our heads. It often causes concern as being either too large, not large enough, sticks out or is pinned to far back. The ear canal is 25 to 30mm long and ends at the eardrum. The eardrum as the name implies is a thin layer of skin stretched taught as a drum skin; it separates the external ear from the middle ear.

The Middle Ear

Behind the eardrum is a small air filled cavity, the middle ear. Within the middle ear cavity are three small bones (each about the size of a grain of rice) the malleus (hammer), the incus (anvil) and the stapes (stirrup) known collectively as the Ossicular Chain. The malleus is attached to the inner surface of the eardrum and the stapes is attached to the oval window of the inner ear. The incus connects these two bones. The Eustachian Tube connects the middle ear with the nose throat region and allows air to enter the middle ear.

The Inner Ear

The inner ear (cochlear) is a fluid filled bony cavity, shell shaped about the size of a pea. Within the inner ear is the Organ of Corti containing thousands of minute hair cells. These hair cells respond to the sounds relayed by the middle ear and transmit the signal to the brain via the eighth (auditory) nerve.

How We Hear

Sound waves travel down the external ear canal and set the eardrum in motion, these vibrations in turn pass along the Ossicular Chain. This mechanical energy passes into the inner ear via the joining of the stapes and the oval window of the inner ear.
These vibrations set the fluid of the inner ear in motion. Movement of the Basilar Membrane in the inner ear in conjunction with hair cells of the Organ of Corti, transform the mechanical energy into electrical stimulation corresponding to the original sound vibrations. This electrical stimulation travels up the eighth nerve into a specialised area (Auditory Cortex) of the brain, where it is interpreted.

What Can Go Wrong

There are two types of perpheral hearing loss. Well three, if you count a combination of the two. Well, four if you count a hearing loss which isn't really a hearing loss at all!
The two main forms of peripheral hearing loss are a conductive hearing loss associated with a external and /or middle ear problem and a sensori neural hearing loss associated with an inner ear and/or eighth nerve problem.
External ear conditions causing a conductive hearing problem can include atresia, where the ear canal is blocked by bone or tissue growth, excessive wax in the ear canal (this is not as common as people would think), obstruction and external ear infection.
Middle ear conditions causing a conductive hearing loss can include, middle ear infections, eustachian tube dysfunction, malformations of the middle ear, perforations of the eardrum and damage to the middle ear.

Most conductive hearing losses can be medically treated. The problem in relation to hearing is usually one of hearing loudly enough. A person can hear quite well if amplification is provided.

A mixed hearing loss combines in varying degrees elements of both a conductive hearing loss and a sensori neural hearing loss. For example a person may have a middle ear condition and a noise induced hearing loss at the same time. It would be expected the middle ear condition would respond to medical treatment while the sensori element would remain.
The fourth type of hearing loss is a functional hearing loss where an individual may knowingly or unknowingly feign a hearing loss for some motive such as compensation or attention.

The Effects of Hearing Loss

The degree of hearing loss, the age of onset and the type of hearing loss will all play a major role in determining the effects of any hearing loss.

The degree of hearing loss can be directly related to the degree of handicap. As a generalisation a child with an average hearing loss less than 70 decibels is likely to develop functional language. On the other hand, a child with an average hearing loss greater than 70 decibels is likely to develop vision as their primary channel for acquisition of language.

The age of onset is also important to subsequent speech and language development. In general the congenitally deaf or prelingually deaf child is retarded in language acquisition and eventual educational achievement. The postlingually deaf child is in a much better position with regard speech production and educational achievement. A parent can often find diagnosis of a hearing loss a lot easier to accept once satisfactory speech is evident.

The type of hearing loss also plays a major role in its effects. As mentioned earlier speech discrimination is usually normal in cases of purely conductive hearing loss while is present in varying degree with a sensori neural hearing loss. Generally, distortion is related to the degree of hearing loss so that the greater the loss the worse will be the distortion.

The effects of distortion has significant implications for the use of hearing aids. Conductive hearing losses will respond well to hearing aids as sounds only need to be made louder for the person to understand what is being said. However, with a sensori neural hearing loss it is possible to fit hearing aids to two individuals with identical hearing losses and with identical hearing aids only to find one works well and the other is of little value in comprehension. The reason being the difference in speech discrimination.

Degree of Hearing Loss

The size or degree of a hearing loss is measured in decibels (loudness) and across the frequency (pitch) range which is considered important for speech. The frequencies usually tested are 250, 500, 1000, 2000, and 4000 Hertz (quantification of frequency) and roughly corresponding to the sounds "ooh", "ah", "ee", "sh" and "s".
If a persons hearing is within the decibel (dB) scale for hearing (ISO) 0-20dB it is considered normal; 25-40dB is a mild loss; 45-70dB is a moderate loss; 75-90dB is a severe loss and 95dB+ a profound hearing loss. Hearing is measured in 5dB steps from 0 to 120dB.
As a general rule a person whose hearing loss is greater than 70dB (ISO) for all frequencies would not hear normal conversation.