Audiometry

Audiometry (from Latin: audīre, "to hear" and metria, “to measure") is a branch of Audiology and the science of measuring hearing acuity for variations in sound intensity and pitch and for tonal purity, involving thresholds and differing frequencies.^[1] Typically, audiometric tests determine a subject's hearing levels with the help of an audiometer, but may also measure ability to discriminate between different sound intensities, recognize pitch, or distinguish speech from background noise. Acoustic reflex and otoacoustic emissions may also be measured. Results of audiometric tests are used to diagnose hearing loss or diseases of the ear, and often make use of an audiogram.

Hearing assessment

Apart from testing hearing, part of the function of audiometry is in assessing or evaluating hearing from the test results. The most commonly used assessment of hearing is the determination of the threshold of audibility, i.e. the level of sound required to be just audible. This level can vary for an individual over a range of up to 5 Decibels from day to day and from determination to determination, but it provides an additional and useful tool in monitoring the potential ill effects of exposure to noise. Before carrying out a hearing test, it is important to obtain information about the person’s past medical history, not only concerning the ears but also other conditions which may have a bearing on possible hearing loss detected by an audiometric test. The hearing loss is usually bilateral, but variations in each ear can also be observed. Wax in the ear can also cause hearing loss, so the ear should be examined to see if syringing is needed; also to determine if the eardrum has suffered any damage which may reduce the ability of sound to be transported to the cochlea.

Types

• Subjective Audiometry
  • 1. Pure tone audiometry
  • 2. Speech audiometry
  • 3. Bekesy audiometry
• Objective Audiometry
  • 4. Acoustic immittance audiometry/ Tympanometry
  • 5. Evoked response audiometry
  • 6. Otoacoustic emissions (DPOAE & TEOAE)
  • 7. In situ audiometry: a technique for measuring not only the affliction of the person's auditory system, but also the characteristics of sound reproduction devices, in-the-canal hearing aids, vents and sound tubes of hearing aids.^[2][3]

Process

The audiometric test can be carried out using automatic or manual audiometers, but the essential test procedure is the same:

• The subject is asked to remove anything which might upset the test results, e.g. spectacles, earrings, hearing aids.
• The ear canals are inspected for any wax buildup or other obstruction.
• Instructions are given about the test procedure and how the subject is required to indicate whether he/she can just hear or cannot hear a certain sound (the sound level may be increased from a very low level or reduced from a high level).
• Headphones are fitted carefully over the ears and the test is then carried out on each ear.

First a threshold test is undertaken to determine the lowest audible level, in which each ear is subjected to sound at a mid-range pitch or frequency of 1 kHz at varying levels of loudness (within a set decibel range) from low to high and then high to low. The procedure is repeated several times so that an average threshold can be derived from the test. Thresholds can vary due to slight changes in the procedures adopted in setting up the test, e.g. variation of the position of the earphone on the ear.

Following this pre-check, both of the subject’s ears are tested through a range of frequencies(usually 0.5, 1, 2, 3, 4, 6 and 8 kHz) and hearing level recorded for each frequency, again via a series of sound exposures. From these an average result can be determined.

Errors and accuracy

When the test is completed, a second threshold check should be carried out to see that no errors have crept in during the test. Both threshold checks should agree within a maximum of 10 dB. If they do not, a re-test must be performed.

The accuracy of audiometry can be affected by four main factors:

• Technical limitations – how accurately the frequency and the hearing levels are determined.
• Learning effect – the first ear tested sometimes appears worse than the second one since the individual becomes more proficient at detecting the threshold.
• Headphone fit – some of the variation in threshold measurement has been attributed to differences in the location of the headphones, which in turn affect the detection of the threshold.
• Background noise – audiometric tests should be carried out in a sound-proof chamber to eliminate external sounds from influencing the test.

A further complication of audiometric testing is that it is subjective and relies on the cooperation of the subject. If the subject is unable or unwilling to co-operate with the test then unrepresentative results will be obtained.

The technique described above enables a comparison of the threshold of hearing of the individual undergoing audiometry with a reference value at a range of octave band frequencies (125, 250, 500, 1000, 2000, 4000, 8000 Hz). From this data a pictorial representation, an audiogram, of hearing loss at various frequencies is produced.

See also

• Audiometer
• Equal-loudness contours
• Audiography
• Otoacoustic emission
• Pure tone audiometry
• Evoked potential

References