Pure-Tone Audiometry and Thresholds

Pure-tone audiometry is the core behavioural test of hearing sensitivity. It presents single-frequency tones at controlled levels and finds the threshold, the softest level at which a listener reliably detects each tone, separately for air conduction and bone conduction. The results are plotted on an audiogram that describes the degree and configuration of hearing loss across the speech frequencies.

Definition

Pure-tone audiometry determines hearing thresholds for pure tones across a range of frequencies, by air conduction and bone conduction, and displays them on an audiogram to quantify the degree, configuration, and type of hearing loss.

Scope

This entry covers the threshold concept, the air- and bone-conduction routes, the standard bracketing procedure for finding thresholds, the role of masking, and how the audiogram is read to classify loss as conductive, sensorineural, or mixed. It is a reference description of the method, not instructions for clinical interpretation in an individual.

Core questions

• What is the softest level at which each test frequency is reliably detected?
• How does the air-bone gap distinguish conductive from sensorineural components?
• When and how must the non-test ear be masked to avoid crossover?
• How is the shape of the audiogram used to classify the loss?

Key concepts

• Auditory threshold
• Air conduction and bone conduction
• Air-bone gap
• Audiogram
• Hearing level (dB HL)
• Bracketing (down-10, up-5) procedure
• Masking and central masking
• Pure-tone average

Mechanisms

Tones are delivered through earphones for air conduction and through a bone vibrator on the mastoid or forehead for bone conduction. The threshold is found by a bracketing procedure in which level is lowered after each response and raised after each non-response until a criterion proportion of responses defines threshold; this clinical method derives from the work of Carhart and Jerger (1959), and the underlying adaptive logic of stepping toward a criterion point connects to transformed up-down psychoacoustic methods (Levitt 1971). Air conduction tests the whole pathway, while bone conduction bypasses the outer and middle ear to estimate cochlear sensitivity; a gap between them (the air-bone gap) signals a conductive component. Because a loud signal can cross the skull to the other cochlea, the non-test ear is masked with noise when the two ears differ enough to risk crossover. Calibration of the audiometer in decibels hearing level references thresholds to normal-hearing reference values (ASHA 2005).

Clinical relevance

The pure-tone audiogram is the reference description of an individual's hearing sensitivity and the starting point for characterizing the type and degree of loss. It is widely used to document hearing for monitoring, screening, and research. This entry explains how thresholds are measured and displayed; it is not a basis for individual diagnosis or treatment decisions.

Epidemiology

Pure-tone audiometry is the most widely used clinical hearing test and the standard against which hearing loss is defined epidemiologically, for example in surveillance of noise-induced and age-related hearing loss and in occupational hearing-conservation testing.

History

Calibrated electric audiometers made systematic threshold measurement possible in the early twentieth century, and standardized clinical procedure was consolidated by Carhart and Jerger's 1959 description of a preferred bracketing method. Psychoacoustic refinement of adaptive threshold tracking followed (Levitt 1971), and professional guidelines later codified manual pure-tone audiometry and masking practice (ASHA 2005).

Key figures

• Raymond Carhart
• James Jerger
• Harry Levitt

Seminal works

• carhart-jerger-1959
• levitt-1971

Frequently asked questions

What is a hearing threshold?

It is the softest sound level at which a listener reliably detects a tone, determined by lowering and raising the level around the point where detection becomes inconsistent.

Why are both air conduction and bone conduction tested?

Air conduction tests the whole pathway, while bone conduction estimates cochlear sensitivity by bypassing the outer and middle ear; a difference between them (the air-bone gap) indicates a conductive component.

Why is masking sometimes needed?

A sufficiently loud tone can cross the skull to the opposite cochlea, so noise is presented to the non-test ear to prevent it from responding and giving a misleading threshold.