Tests of ear function

There are many different methods of assessing hearing and ear function. Some of the most commonly used methods are described below.

• Audiometry
• Air Conduction
• Bone Conduction
• Conductive hearing loss
• Sensori neural hearing loss
• Mixed hearing loss
• Tympanometry
• Speech Standardised Audiometry
• Evoked Response Audiometry (ERA)
• Otoacoustic Emissions

Audiometry

The most widely used test to assess hearing is Pure Tone Audiometry in which an audiometer generates pure tone signals of frequency 125Hz, 250Hz, 500Hz, 1,2,4 and 8kHz at variable intensities ranging from –10 dB to +120 dB usually in steps of 5dB.

Signals of decreasing intensity at each frequency are presented to the person tested, from a level the person can hear to find the point at which they fail to hear. The hearing threshold levels are usually plotted on a graph or audiogram with sound intensity (dB) on the y (vertical) axis and the frequency (Hz) along the x (horizontal) axis. Standard symbols are used to denote right (o) and left (x) ears for air conduction and right ([ or Δ) or left (] or Δ) ears for bone conduction.

The hearing threshold is defined as the quietest sound heard by the person when being tested. A normally hearing person would expect to have a threshold of 20dB or better and this represents no hearing loss on the audiogram.

It should be noted that audiometry is a subjective test of hearing.

Normal audiogram

Air Conduction

Air Conduction assesses the function of both the conduction (outer and middle ear) and sensorineural (cochlea and auditory nerve) components of the ear. To measure air conduction (AC), the person wears headphones and the signal passes by air conduction through the outer and middle ear. It is then transmitted to the inner ear, auditory nerve and auditory cortex of the brain.

Bone Conduction

Bone Conduction (BC) assesses the function of the cochlea and auditory nerve. To measure bone conduction, the signal stimulates the cochlea directly by the application of the vibratory stimulus to the skull.

Using these two measures, the type of hearing loss can be classified into conductive, sensorineural or mixed type.

Conductive hearing loss

If there is a problem in the external or middle ear (conductive hearing loss) the AC threshold will be less than the BC threshold because the person will hear better by bone than air conduction (See diagram below). This is called the air bone gap (ABG). The greater the air conduction loss, the greater will be the ABG.

Audiogram showing conductive hearing loss in the left ear

Sensorineural hearing loss

If there is a problem with the cochlea or the auditory nerve, the AC and BC thresholds will be the same (See diagram below).

Audiogram showing sensori-neural hearing loss in the right ear

Mixed hearing loss

Mixed hearing loss is a reduction in hearing of both AC and BC. (See diagram below).

Tympanometry

This is an objective test used to assess middle ear function.

Speech Standardised Audiometry

This is when standardised pre-recorded lists of words or sentences are presented at different intensities. The volume at which half the words can be correctly repeated is recorded.

Evoked Response Audiometry (ERA)

This is an objective measure of the electrical activity of the auditory nerve in response to differing sound levels.

Otoacoustic Emissions

This is an objective measure of cochlear function. It is especially useful in assessing the hearing of newborns.