Module 4

Prescription & Fitting

From the audiogram to a target: the prescriptive formulae that set how much gain to give at each frequency and level, and the workflow that applies them.

From audiogram to target

A prescription translates the audiogram into a gain target for each frequency and input level. The historical intuition is the half-gain rule — provide roughly half the hearing loss as gain — but modern procedures are far more sophisticated, prescribing different gain for soft, medium and loud inputs to restore audibility while preserving comfort and loudness relationships [1].

Degree of loss (high-frequency threshold) — 70 dB HL

The target gain follows the loss — more gain where hearing is poorer. Shown with the simple half-gain rule; real procedures (NAL-NL2, DSL) also vary gain by input level. Illustrative.

NAL-NL2 and DSL

Two evidence-based families dominate. NAL-NL2 (National Acoustic Laboratories) aims to maximise speech intelligibility at a comfortable overall loudness, and is the common default for adults[2]. DSL m[i/o](Desired Sensation Level) aims to place a wide range of speech within the listener’s audible range and is the standard for children, where maximising audibility for language learning is paramount[3]. Both prescribe by frequency and level; the choice reflects the patient and the goal.

The fitting workflow

A defensible fitting is a sequence: select an appropriate device and coupling; enter the audiogram and a validated prescription; and then verify the real output against the targetrather than trusting the manufacturer’s “first-fit”, which often deviates substantially from any prescription [8]. Fine-tuning then addresses the patient’s reports — but always from a verified, on-target starting point, the subject of the next chapter.