HEARING TESTS AND AUDIOGRAMS

Hearing tests are designed to find out what you can and can’t hear, and audiologists are skilled at using a range of techniques to find out what your hearing is like. The following information explains what the most common of these tests are about and what the results mean.

How is hearing tested?

The most simple test of hearing ability is called 'pure tone audiometry' where you listen to a range of beeps and whistles (called pure tones) and indicate when you can hear them, maybe by pressing a button. The loudness of each tone is reduced until you can just hear the tone. The softest sounds you can hear (your hearing thresholds) are then marked on a graph called an audiogram.

When hearing is measured with pure tones presented through headphones, this measurement is called air conduction. The sounds go via the air, down the ear canal, through the middle ear, and to the very delicate organ of hearing in the inner ear—the cochlea.

The sensitivity of the cochlea can also be tested by placing a small vibrator on the mastoid bone behind the ear and again measuring the softest sounds that can be heard.

Sounds presented this way travel through the bones of the skull to the cochlea and hearing nerves, bypassing the middle ear. This type of testing is called bone conduction.

The air conduction and bone conduction hearing levels on the audiogram can tell us a lot about where the hearing problem is.

What exactly is an audiogram?

Look at the blank audiogram graph below.

Along the top of the graph the numbers range from 125 to 8000. These numbers refer to frequencies, or different pitches of sounds.

Frequency is expressed in terms of the number of cycles per second, or Hertz. The higher the number, the higher the pitch of the sound. For example, 250 Hertz (250 Hz) sounds like middle C on the piano, while the high-pitched ringing of the telephone is about 3000 Hz.

Normal, young, healthy human ears can actually hear frequencies as low as 20 Hz and as high as 20,000 Hz, but we only test hearing in the range of 250 Hz to 8000 Hz, as most sounds of speech occur in this frequency range.

Loudness or level of sounds is measured in units called decibels. Zero decibels (0 dB) does not mean ‘no sound’. It is just extremely soft. Conversational voice level is around 65 db, and 120 db is very, very loud—about as loud as a jet taking off when you are standing 25 meters away. The figures along the side of the graph are hearing levels in decibels.

Air conduction thresholds for the right ear (that is, the softest sounds the right ear can hear at each frequency) are marked as an ‘O’, and the left ear is an ‘X’ on the audiogram.
Bone conduction thresholds are marked on the audiogram as ? (better ear), [(right ear) or] (left ear).

What does the audiogram mean?

Your audiogram is a ‘picture’ of your hearing. It indicates how much your hearing varies from normal and, if there is a hearing loss, where the problem might be located in the hearing pathway.

If the hearing thresholds obtained by bone conduction are the same as the air thresholds, this indicates no blockage of sound in the outer or middle ear. We could safely assume the hearing loss is caused by a loss of sensitivity in the cochlea or hearing nerve. This type of loss is called sensorineural (pronounced sen-sorry-new-rol) hearing loss.

An Example of a mild to severe sensorineural hearing loss in both ears.

Many things can damage the delicate hearing nerves. Exposure to excessive noise can cause a sensorineural hearing loss, and so can the ageing process. There is rarely any medical treatment for sensorineural hearing losses and so the impairment is permanent.

If the bone conduction hearing thresholds are normal, but there is a loss of hearing for air-conducted sounds, this is called a conductive hearing loss. This means that the cochlea is normal and healthy, but there is some blockage to sound in the middle or outer ears.

Middle ear infection is a common cause of a conductive hearing loss in young children. Outer and middle ear blockages can often be corrected by medical or surgical treatment.

An Example of a mild cunductive hearing loss in both ears.

It is possible to have both a sensorineural and a conductive hearing loss (e.g. if a person has a noise caused impairment and a perforated ear drum). This is called a mixed hearing loss.

An Example of a moderate to profound mixed hearing loss.

Tympanometry

This is not a hearing test, but a test of how well the middle ear system is functioning and how well the eardrum can move.

A small rubber tip is placed in the ear and a little air is pumped into the outer ear canal. If there is a problem in the middle ear it may show up on this test. For example, if there is very little movement of the eardrum, it may indicate fluid behind the drum as a result of a middle ear infection.

Often the results of tympanometry indicate the location of the blockage that is causing the hearing loss and if medical treatment may help.

Speech perception testing

The ability to hear speech is a function of 2 abilities.

• The ability to detect the sounds of speech.
• The ability to understand speech.

The audiogram shows how much sound someone can detect, but does not tell us how clearly speech can be heard by the hearing impaired person. Predictions can be made from the degree and type of hearing loss, but to measure a person’s speech discrimination—how well they can understand speech—special tests are used. For example, words may be presented (usually at different levels of loudness) which the hearing impaired person has to repeat.

A person who has a problem understanding the words, even when they are loud enough, may find the benefit provided by a hearing aid will be limited. People who have a sensorineural hearing loss often fall into this category. Generally speaking, the greater the hearing loss, the poorer the speech discrimination.