Hearing Aid Selection and Fitting

Selecting and fitting a hearing aid is the clinical process of choosing an appropriate device and then programming it so that its amplification matches an individual's hearing loss. A prescriptive formula translates the audiogram into target gain at each frequency and level, and probe-microphone (real-ear) measurement verifies that the device actually delivers those targets in the person's ear canal.

Definition

Hearing aid selection and fitting is the process of choosing a device and programming its gain and output to prescriptive targets derived from a person's hearing thresholds, then verifying the result with real-ear measurement.

Scope

This topic covers the rationale for prescriptive fitting, the main prescription methods, and the role of real-ear verification in confirming and adjusting amplification. It describes the methodology and evidence behind fitting; it is reference-educational and does not provide individualized fitting instructions or settings.

Core questions

• How is an audiogram translated into a target amount of amplification at each frequency?
• Why is the gain in the ear canal verified rather than assumed from the device settings?
• What do prescription methods such as NAL-NL2 aim to optimise?

Key concepts

• Prescriptive targets
• NAL-NL2 prescription method
• Real-ear (probe-microphone) measurement
• Real-ear-to-coupler difference
• Loudness normalisation versus speech intelligibility goals
• Verification and fine-tuning

Mechanisms

A prescription method maps measured hearing thresholds to target gain at multiple frequencies and input levels. The NAL family of procedures, including NAL-NL2, derives targets aimed at maximising speech intelligibility while keeping overall loudness at or below that of a normal-hearing listener, and incorporates empirical adjustments such as different preferred gain for new versus experienced users (Keidser, 2011). Because ear-canal acoustics vary between people, the gain a device produces at the eardrum cannot be assumed from its settings; a probe microphone placed in the ear canal measures the actual amplified signal so the fitting can be matched to target and adjusted (Cox, 1990; Dillon, 2012). This verification step is the mechanism by which a prescription is converted from a theoretical target into a confirmed real-ear result.

Clinical relevance

Prescriptive fitting and verification are the methods that link a person's audiogram to the amplification they actually receive, and they underpin much of the evidence on hearing aid outcomes. Understanding them supports critical reading of fitting studies and of claims that devices are 'fitted to the audiogram.' This entry describes the methodology and is not a basis for fitting or programming a device for any individual.

History

Early fitting relied on comparative trials and rules of thumb. Australian National Acoustic Laboratories and other groups developed explicit prescription formulae from the 1970s onward, culminating in nonlinear procedures such as NAL-NL1 and NAL-NL2 for compression devices. In parallel, probe-microphone real-ear measurement matured in the 1980s and 1990s, making it possible to verify in the ear canal that prescribed targets were met, and it remains the reference standard for fitting verification.

Debates

What should a prescription optimise?

Prescription methods differ in whether they aim primarily to normalise loudness or to maximise speech intelligibility at a comfortable loudness; the NAL approach prioritises intelligibility while constraining overall loudness, and the balance between these goals remains a design choice with practical consequences for the user.

Key figures

• Gitte Keidser
• Harvey Dillon
• Robyn Cox

Seminal works

• keidser-2011
• cox-1990

Frequently asked questions

What is a real-ear measurement?

It is a verification procedure in which a thin probe microphone is placed in the ear canal alongside the hearing aid to measure the amplified sound actually reaching the eardrum, so the clinician can confirm it matches the prescribed target rather than relying on the device's settings alone.

What is NAL-NL2?

NAL-NL2 is a prescription procedure developed by the National Acoustic Laboratories that converts an audiogram into target gain for nonlinear (compression) hearing aids, aiming to maximise speech intelligibility while keeping loudness comfortable and incorporating empirical adjustments for factors such as listening experience.