An audiogram is only as good as the technique that produced it. This module covers the test environment, the equipment and its calibration, and the threshold-seeking method that turns a patient’s responses into a reliable set of numbers.

The test environment

Audiometry measures the quietest sound a person can detect, so the room must be quiet enough not to mask those faint tones. Testing is carried out in a sound-treated booth whose background noise is low enough that it does not raise the measured thresholds. Without an adequately quiet room, normal low-level thresholds simply cannot be obtained, and the audiogram will falsely suggest a mild loss.

Transducers

Air conduction is tested with earphones and bone conduction with a vibrator placed on the skull. Two kinds of earphone are in common use.

Supra-aural earphones sit on the pinna. They are robust and familiar but provide only moderate isolation between the ears.
Insert earphones sit a foam tip inside the ear canal. They give better isolation between the ears, reduce the chance of a collapsing canal, and attenuate background noise.

The choice of transducer matters well beyond comfort: it changes how much sound crosses the head to the opposite ear, and so it changes when masking is needed — the subject of its own module.

Calibration

A tone delivered at, say, “20 dB HL” only means something if the audiometer is calibrated so that 20 dB HL corresponds to a defined sound level for that transducer and frequency. Calibration is therefore not optional housekeeping — it is what makes the dB HL scale meaningful and lets results be compared between visits and between centres.

Trainee Calibration is governed by published standards. The international standard for the methods of pure-tone air- and bone-conduction audiometry is ISO 8253-1¹¹; in the United States the corresponding standard is ANSI/ASA S3.21.¹² Equipment is checked daily by a listening and biological check, and calibrated formally against these standards at defined intervals — typically annually, and after any repair.

The decibel scales

Three decibel scales appear in audiometry and confusing them is a common source of error.

dB SPL (sound pressure level) is the physical scale, referenced to a fixed pressure.
dB HL (hearing level) is the clinical scale. It is built so that 0 dB HL at every frequency represents the average threshold of normal-hearing young adults. This is the scale plotted on the audiogram, and it is the reason the normal line is flat across the chart even though the ear is not equally sensitive at every pitch.
dB SL(sensation level) is referenced to a particular person’s own threshold. A tone at 20 dB SL is 20 dB above that individual’s threshold. Several special tests, including SISI, are specified in dB SL.

The threshold-seeking method

The threshold is defined as the lowest level at which the patient responds to the tone on at least half of the presentations. The method used to find it across the audiological community is the modified Hughson–Westlake procedure — the original ascending technique of Hughson and Westlake¹⁰ as refined by Carhart and Jerger.⁹

TraineeIn practice the procedure works as a “down 10, up 5” staircase. Each frequency usually starts with a clearly audible tone. After a response the level drops by 10 dB; after no response it rises by 5 dB. Threshold is taken as the lowest level producing responses on at least two of three (or three of five) ascending trials. ^9,11 Short tone bursts are used rather than a continuous tone, because a brief tone against silence is easier to detect and avoids adaptation.

Order of testing

Testing usually begins with air conduction in the better ear, at 1000 Hz — a mid-frequency the patient hears well and which gives reliable, repeatable responses — then moves to the higher frequencies and finally the lower ones, with 1000 Hz re-tested as a reliability check. Bone conduction is then tested. Comparing the two completed curves is what allows the loss to be classified, as the next module sets out.

Clinician Several technique errors produce characteristic false patterns. A collapsing ear canal under supra-aural earphones mimics a high-frequency conductive loss and is one reason to favour insert earphones. Tactile responses to a bone vibrator at high intensities — the patient feeling rather than hearing the stimulus — can masquerade as residual bone-conduction hearing in a profound loss. And failing to mask when masking is indicated produces a shadow curve: the apparent threshold of a poor ear is really the better ear responding to cross-heard sound. Each of these is best caught by suspecting it in advance, which is why technique and interpretation cannot really be separated.