Chapter 8 — Real-ear measurement and verification

Probe-tube mics, REIG / REAR, NAL-NL2 and DSL targets.

A hearing aid programmed using only the patient’s audiogram and the manufacturer’s coupler-gain specification is fitting blind. The manufacturer measures coupler gain in a standardised 2 cubic-centimetre coupler; the patient’s actual ear canal has a volume of 0.5–2 mL, a different acoustic compliance, a different end termination (the live tympanic membrane has very different impedance from the rigid coupler microphone), and an entirely different transfer function from the eartip to the eardrum. Coupler gain and real-ear gain typically differ by 5–15 dB across the audiometric range — and the difference is patient-specific.

Real-ear measurement (REM) — placing a thin probe-tube microphone in the patient’s ear canal to measure the actual SPL at the eardrum — is the only way to know what the device is actually delivering. Best-practice guidelines from the American Academy of Audiology, the American Speech-Language-Hearing Association, and the Pediatric Working Group recommend REM-verified fittings as the standard of care for every hearing-aid fitting; clinical surveys, however, have consistently found that less than 30% of US audiologists routinely verify with REM. The gap between recommended and actual practice represents one of the largest preventable sources of underperforming hearing aids.

This chapter develops REM as the bridge between the audiogram (Ch 2) and the prescription targets driven by the hearing-aid algorithms of Ch 7. Three lessons:

• 8.1 The probe-tube microphone and the REUR — what gets measured, how, and the real-ear-unaided response that establishes the baseline.
• 8.2 REIG and prescription targets — the real-ear insertion gain, NAL-NL2 and DSL v5 targets, and the ±5 dB match criterion.
• 8.3 Verification protocol and clinical practice — the standard verification workflow, common deviations from target, and how to fix them.