Pure-tone audiometry is the main hearing test used to identify hearing threshold levels of an individual, enabling determination of the degree, type and configuration of a hearing loss and thus providing a basis for diagnosis and management. Pure-tone audiometry is a subjective, behavioural measurement of a hearing threshold, as it relies on patient responses to pure tone stimuli. Therefore, pure-tone audiometry is only used on adults and children old enough to cooperate with the test procedure. As with most clinical tests, standardized calibration of the test environment, the equipment and the stimuli is needed before testing proceeds (in reference to ISO, ANSI, or other standardization body). Pure-tone audiometry only measures audibility thresholds, rather than other aspects of hearing such as sound localization and speech recognition. However, there are benefits to using pure-tone audiometry over other forms of hearing test, such as click auditory brainstem response (ABR). Pure-tone audiometry provides ear specific thresholds, and uses frequency specific pure tones to give place specific responses, so that the configuration of a hearing loss can be identified. As pure-tone audiometry uses both air and bone conduction audiometry, the type of loss can also be identified via the air-bone gap. Although pure-tone audiometry has many clinical benefits, it is not perfect at identifying all losses, such as ‘dead regions’ of the cochlea and neuropathies such as auditory processing disorder (APD). This raises the question of whether or not audiograms accurately predict someone's perceived degree of disability.
Pure-tone audiometry procedural standards The current International Organization for Standardization (ISO) standard for pure-tone audiometry is ISO:8253-1, which was first published in 1983. The current American National Standards Institute (ANSI) standard for pure-tone audiometry is ANSI/ASA S3.21-2004, prepared by the Acoustical Society of America. In the United Kingdom, The British Society of Audiology (BSA) is responsible for publishing the recommended procedure for pure-tone audiometry, as well as many other audiological procedures. The British recommended procedure is based on international standards. Although there are some differences, the BSA-recommended procedures are in accordance with the ISO:8253-1 standard. The BSA-recommended procedures provide a "best practice" test protocol for professionals to follow, increasing validity and allowing standardisation of results across Britain. In the United States, the American Speech–Language–Hearing Association (ASHA) published Guidelines for Manual Pure-Tone Threshold Audiometry in 2005.
Variations There are cases where conventional pure-tone audiometry is not an appropriate or effective method of threshold testing. Procedural changes to the conventional test method may be necessary with populations who are unable to cooperate with the test in order to obtain hearing thresholds. Sound field audiometry may be more suitable when patients are unable to wear earphones, as the stimuli are usually presented by loudspeaker. A disadvantage of this method is that although thresholds can be obtained, results are not ear specific. In addition, response to pure tone stimuli may be limited, because in a sound field pure tones create standing waves, which alter sound intensity within the sound field. Therefore, it may be necessary to use other stimuli, such as warble tones in sound field testing. There are variations of conventional audiometry testing that are designed specifically for young children and infants, such as behavioral observation audiometry, visual reinforcement audiometry and play audiometry. Conventional audiometry tests frequencies between 250 Hertz (Hz) and 8 kHz, whereas extended high-frequency audiometry tests in the region of 8 kHz-16 kHz. Some environmental factors, such as ototoxic medication and noise exposure, appear to be more detrimental to high frequency sensitivity than to that of mid or low frequencies. Therefore, extended high frequency audiometry is an effective method of monitoring losses that are suspected to have been caused by these factors. It is also effective in detecting the auditory sensitivity changes that occur with aging.
Cross hearing and interaural attenuation
When sound is applied to one ear the contralateral cochlea can also be stimulated to varying degrees, via vibrations through the bone of the skull. When the stimuli presented to the test ear stimulates the cochlea of the non-test ear, this is known as cross hearing. Whenever it is suspected that cross hearing has occurred it is best to use masking. This is done by temporarily elevating the threshold of the non-test ear, by presenting a masking noise at a predetermined level. This prevents the non-test ear from detecting the test signal presented to the test ear. The threshold of the test ear is measured at the same time as presenting the masking noise to the non-test ear. Thus, thresholds obtained when masking has been applied, provide an accurate representation of the true hearing threshold level of the test ear. A reduction or loss of energy occurs with cross hearing, which is referred to as interaural attenuation (IA) or transcranial transmission loss. IA varies with transducer type. It varies from 40 dB to 80 dB with supra-aural headphones. However, with insert earphones it is in the region of 55 dB. The use of insert earphones reduces the need for masking, due to the greater IA which occurs when they are used (See Figure 1). Air conduction results in isolation, give little information regarding the type of hearing loss. When the thresholds obtained via air conduction are examined alongside those achieved with bone conduction, the configuration of the hearing loss can be determined. However, with bone conduction (performed by placing a vibrator on the mastoid bone behind the ear), both cochleas are stimulated. IA for bone conduction ranges from 0-20 dB (See Figure 2). Therefore, conventional audiometry is ear specific, with regards to both air and bone conduction audiometry, when masking is applied.
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