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Hearing aid fitting and verification: prescriptions, real-ear measures, RECD, the test box and bilateral fitting
A good fitting has three steps: choosing a target from a prescription (NAL-NL2 or DSL), verifying that the aid really reaches the target in the person's ear (real-ear measures, or RECD and the test box), and measuring outcome in daily life. In children every step is tighter, because the canal is small and they cannot report.
Why it matters in the exam
Prescriptions: DSL was built with children in mind and targets audible speech at every input level, with age-appropriate RECD values — so it is the common choice for children; NAL-NL2 is common in adults and aims at maximal intelligibility at a comfortable overall loudness. Real-ear measures with a probe microphone near the eardrum: REUR/REUG — the natural response of the open canal (this is where the canal resonance is seen); REAG — the gain with the aid working; REIG = REAG minus REUG. The RECD is the difference between the SPL a signal produces in the canal and the SPL it produces in a 2cc coupler; in infants it is large, because the canal is small. The RECD is measured once and the fitting verified in the test box, because a long measurement is hard in an infant.
The test box (a 2cc coupler, per ANSI S3.22) checks gain, frequency response, OSPL90, distortion, internal noise and battery drain — but not functional gain (the difference between aided and unaided sound-field thresholds) and not the true performance of an open fit, which depends on the canal. The UCL is used to set maximum output. An infant diagnosed with permanent loss is fitted within about a month of diagnosis (per JCIH). In a symmetrical 50 dB loss, a single aid gives up localisation, summation and hearing in noise, and risks auditory deprivation in the unaided ear; bilateral fitting does not suit binaural interference — when the poorer ear degrades the better ear's performance. A 50 dB conductive loss usually needs more gain and gives good clarity (a normal dynamic range), compared with a 50 dB sensorineural loss with recruitment. In children the priority is a better signal-to-noise ratio (a remote microphone).
Key points
- DSL for children; NAL-NL2 for adults.
- REIG = REAG minus REUG; REUR shows resonance.
- RECD = canal minus coupler; large in infants.
- Test box: not functional gain and not an open fit.
- Bilateral in most cases; not with binaural interference.
Summary table
| Measure or principle | Definition | Use |
|---|---|---|
| DSL v5 | Speech audibility, values by age | The preferred prescription for children |
| NAL-NL2 | Maximal intelligibility at comfortable loudness | Common in adults |
| REUR / REUG | Open-canal response | Shows canal resonance |
| REIG | REAG minus REUG | The gain the aid added beyond the natural ear |
| RECD | Canal SPL minus 2cc coupler SPL | Large in infants; verify in the box |
| Test box | Gain, OSPL90, distortion, noise, battery | Not functional gain and not an open fit |
| Bilateral versus unilateral | Localisation, summation, release from noise | Not suited to binaural interference |
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Sources
- American Academy of Audiology, 2013 — Clinical Practice Guidelines: Pediatric Amplification
- Keidser, Dillon, Flax, Ching & Brewer, Audiology Research, 2011 — The NAL-NL2 prescription procedure
- Scollie et al., Trends in Amplification, 2005 — The Desired Sensation Level multistage input/output algorithm (DSL v5)
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.