In its simplest form a hearing test (audiogram) is a method of investigation to assess the sensitivity of a person's subjective hearing threshold by presenting them with short duration pure tones of differing frequencies until they can no longer hear them.

Screening Audiometry is a fairly simple form of Clinical Audiometry that can be performed by non-audiologists such as school nurses or health visitors to assess whether the subject/patient has acceptable hearing thresholds. If they do not then they should be re-tested and if they still fail to obtain an acceptable hearing threshold then they should be referred for clinical audiometry.

Screening Audiometry

Sound Transmission

The function of the ear is to convert airborne sound waves into action potentials in the auditory nerves for onward transmission to the cortex. The tympanic membrane responds to the sound pressure waves by moving in and out, thus acting as a resonator reproducing the vibrations of the sound source. When the sound stops the drum almost stops immediately being very critically damped. The movement of the drum is transmitted to the malleus, which rocks on its axis and transmits the movement through its short process to the incus. Movement of the incus is transmitted to the head of the stapes in such a way that the stapes footplate moves to and fro, like a door, hinged at the posterior edge of the oval window. The ossicles thus act as a lever system, converting the resonant vibrations of the drum into movements of the stapes footplate against the perilymph fluid in the cochlea.

This system increases the sound pressure that arrives at the oval window in two ways. Firstly there is a slight leverage advantage, due to the arrangement of the ossicles that increases the force 1.3 times. Secondly the area of the tympanic membrane is much greater than that of the oval window. These combined effects produce a transformation ratio of 22:1.

In Summary:

• Sound waves impinge on the drum causing it to vibrate.
• Ossicles vibrate as a unit.
• Stapes moves in and out of the oval window.
• Sound waves are transmitted along scala vestibula in the perilymph.
• Short waves (high frequencies) act at the base of the cochlea.
• Long waves (low frequencies) act at the apex of the cochlea.
• The wave is transmitted across scala media to scala tympani.
• Waves travel along scala tympani.
• Effect of wave on round window causes it to bulge outwards.
• Wave motion distorts Reissner’s membrane and basilar membrane thus stimulating the organ of hearing (Organ of Corti)
• Impulses from hair cells are then transmitted along the cochlea nerve.

Equipment

To perform a hearing test an audiometer is required. These come in many shapes and sizes but in its simplest form the audiometer consists of:

• An oscillator producing pure tones of a frequency that can be varied, usually from 125 - 8000 Hz in one half-octave steps.
• An attenuator, which alters the intensity of the sound, delivered usually in 5dB steps (volume).
• A speaker that delivers the sounds to the ear (headphones).

Normally other controls will include, an interrupter, which is a noiseless switch that allows the sound to be presented, and a selector switch to deliver the sound in either the left earphone or right earphone. These are the basic controls of the simplest audiometer and are only adequate for screening purposes. Clinical audiometers usually have two channels and are able to 'mask' (see below) and additionally have a bone conductor.

All Audiometric equipment comply with (BS) EN 60645, calibrated in accordance with (BS EN) ISO 389 and Audiometric Test Methods performed under (BS EN) ISO 8253-1:2010 standards, these are all European standard too.

Clinical Audiometry - Pure Tone Audiogram (Hearing test)

In the clinical situation additional features are essential if more sophisticated testing is to be undertaken. One of these includes masking or a spectrum of noise that can be delivered to the ear not under test in order to prevent it from receiving loud signals intended for the test ear. Usually masking is available as wide band where the sound consists of all frequencies and is always used in speech audiometry, or narrow band where the noise frequencies are filtered to include only those frequencies around the test frequency. This latter type of masking is usually used in pure tone testing where the frequency of the noise automatically alters with the frequency of the test signal. A masking attenuator similar to a pure tone attenuator and graduated in the same way governs the masking output.

A Bone conductor is essential if cochlea function is to be assessed. This consists of a vibrator on a headband that can be placed on the mastoid bone. Pure tones are directed through the conductor as 'Force' rather than sound pressure. Masking for bone conduction is usually through an insert earphone or headphone and again narrow band noise is used.

Pure Tone Testing

The purpose of pure tone testing is to determine the quietest possible sound that the patient can hear at a variety of frequencies. With this in mind the test technique merely achieves this end in the quickest and most accurate way. Most patients tested can be done so in a set pattern and valid results will be obtained, however no two individuals are alike or will respond in the same way to a method of testing, this particularly applies to children, elderly patients and malingerers. For this reason the test technique below is purely as a guide that can be adapted to suit a particular situation.

Method:     Instruct the patient as to what is required of him. Explain that he will hear a series of sounds, each and every time that he hears the sound he is to signal that he has heard it. Sometimes the sounds will be loud and sometimes very faint but every time he hears the sound he is to indicate. One ear will be tested at a time, the better ear first. With children the above method may have to be repeated more than once, it is no use beginning the test until the subject knows what is expected. Fit the headphones over the ears ensuring that the pinna is closed completely with the red earphone over the right ear. If the subject wears spectacles remove them.

Technique:     The tones are first presented at an intensity that the patient can hear well in order to give a little practice in the technique. The intensity is then reduced in 10dB steps until the subject ceases to signal that he can hear the sound. We can assume at this stage that the hearing threshold has been passed. The sound intensity is then increased in 5dB steps until the subject again signals that he has heard it. Again the sound is decreased in 10dB steps and increased in 5dB steps. This procedure is repeated until the subject consistently responds to one intensity and nothing at a lower intensity. This value can then be recorded as the hearing threshold for that one particular frequency. The next frequency is then tested in the same way, although it is not now necessary to give too loud a starting signal, as the subject should now be aware of what is happening. If the subject fails to hear this starting frequency increase the intensity by 20dB.

Protocol:     Normally testing starts at 1000Hz and progresses to the highest frequency then drops to 500Hz and progress down to the lowest frequency. The better ear is always tested first, if neither is better as a matter of course the right ear is tested first. Tone presentation should never be at regular intervals, even the most honest subject finds it difficult to resist the temptation to signal when he thinks a tone is due if the tester has been presenting the tones at regular intervals.

As pointed out earlier the individual patient will dictate the method of testing, for instance a very young child will rapidly get bored listening to hundreds of sounds. It is far better to get reliable results of three frequencies on each ear than a complete audiogram that is hopelessly inaccurate at all frequencies. So a better method would be to test one ear at 1000Hz then change ears, then test 3000Hz in both ears and so on. Elderly patients are usually tested easily when a slower sequence of presentations is used. Their responses are not as quick as younger patients and rapid tone presentations should be avoided. They usually like to be quite sure that they have heard the tone before signalling so it may be necessary to repeat the tone a couple of times at the same intensity.

Psychogenic hearing losses (non-organic or hysterical) can sometimes be very difficult to detect but usually the tester should get some idea that all is not as it should be in the normal routine test. Again as in testing children it is no use recording threshold levels that are untrue without making some comment on the bottom of the audiogram. Some of the signs that all is not well include inconsistent responses or responding to the initial tone and using this as their threshold and not responding to any lower intensity. Certain types of patient seem to be prone to psychogenic losses particularly adolescent girls, unhappy servicemen (usually junior ranks) and service personnel due to leave the Forces. Another group is children but particularly those who have been subjected to a lot of audiometric testing and seem to thrive on the extra attention that a hearing loss brings. Often there is no apparent reason for psychogenic loss. Sometimes the patient will produce illogical responses because he does not know what is expected of him.