CI Atlas · Audiological Evaluation · Module 14

14Real-ear measurement & aided verification

Fitting a hearing aid is a prescription, and like any prescription it has to be verified against what actually reaches the patient — not what the manufacturer's software predicts. Real-ear measurement does exactly that: a slender probe tube slipped close to the eardrum records the true sound pressure the device delivers, vent leaks and ear-canal resonances and all, and that measured output is matched to an evidence-based target for soft, average and loud speech. This objective check has replaced the older behavioural method of functional gain, which was at the mercy of soundfield calibration, and it is doubly important in children, whose small ears turn the same setting into far more sound pressure. Above all, verification has to come first: only an aid proven to be on target lets a poor result be blamed on the ear rather than the fitting. This module covers verifying amplification.

TProbe-microphone measurement

Probe-microphone (real-ear) measurement places a silicone probe tube ~5 mm from the eardrum and records the actual SPL the device delivers — capturing earmould, vent and ear-canal-resonance effects that a 2-cc coupler misses. In-situ on-ear measures are preferred over simulated coupler measures.[2020]

Probe-microphone real-ear measurement records the actual SPL a hearing aid delivers ~5 mm from the eardrum, capturing earmould and vent effects a 2-cc coupler misses. Aided output is matched within ±5 dB of an evidence-based prescriptive target — NAL-NL2 or DSL 5.0 — for soft, average and loud speech. REM is now the standard of care because it is objective and needs no behavioural response; it must precede any aided speech test so that poor performance can be blamed on the ear, not an under-fit aid. Schematic.

CPrescriptive targets

Aided output is matched within ±5 dB of an evidence-based prescriptive target — NAL-NL2 or DSL 5.0 — for calibrated speech at 50/60/70 dB SPL (soft/average/loud). Severe and profound losses tend to prefer ~10 dB more gain than the basic prescription, and the aim is audibility of soft speech across the frequencies.

CWhy REM replaced functional gain

The older alternative, functional gain (the unaided-minus-aided soundfield threshold difference), is vulnerable to soundfield-calibration error — two aids must differ by ≥15 dB on soundfield thresholds to be judged significantly different. REM is now the standard of care because it is objective, needs no behavioural response, covers all frequencies, and avoids that calibration problem. Aids are also checked against ANSI S3.22 tolerances with a biologic listening check.

CChildren & the RECD

Children require paediatric DSL 5.0 targets (greater gain and output) and an individually measured (or age-corrected) RECD, because a small ear develops more SPL for the same output. Aided thresholds worse than 25–30 dB HL mean the child misses low-level speech; elevated aided thresholds despite a correct target reflect the limited dynamic range of severe loss, not a fitting error, and flag risk of language delay and possible implantation. Verification must precede any aided speech test so a poor result can be attributed to the ear, not an under-fit aid — the candidacy logic of the next chapter depends on it.[2020]

The real-ear-to-coupler difference bridges what a hearing aid produces in a test coupler and what it actually delivers in the patient's ear. By Boyle's law, a smaller cavity develops a higher pressure for the same volume velocity — so a newborn's tiny ear canal turns the same coupler output into much higher SPL at the eardrum. Assuming an adult RECD would over-fit a baby and risk loudness/discomfort, so the RECD must be measured individually (or age-corrected) for valid paediatric targets. Schematic.

Real-ear measurement verifies the aid against a prescriptive target, and the two main formulae embody different goals. NAL-NL2 aims to maximise speech intelligibility at a comfortable loudness — efficient gain, favoured for adults. DSL v5.0 aims to make a wide range of speech audible, prescribing more gain and a wider output — preferred for children, who must overhear language to learn it. Matching real-ear output to the chosen target (the dashed line shows the alternative) is the verification step that functional gain could never deliver. Illustrative; schematic.

Case 10.14 · Verify before you blame the ear

A child makes poor progress with hearing aids. Before concluding the aids are inadequate and moving toward implantation, the team reviews the fitting.

What verification step is essential first?

Self-assessment — Module 142 questions

Question 1 · Trainee

What does probe-microphone real-ear measurement verify?

Question 2 · Clinician

Why must verification precede aided speech testing or a hearing-aid trial?

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