9.5 Sound-Field Verification, Functional Gain & Aided Testing
Key Takeaways
- Functional gain is the unaided sound-field threshold minus the aided sound-field threshold at each frequency, measured through a calibrated loudspeaker typically at 1 metre and 0 degrees azimuth.
- Sound-field thresholds must use frequency-modulated warble tones or narrow-band noise because steady pure tones create standing waves that shift the level at the ear by 10-15 dB with small head movements.
- Functional gain overestimates real-world gain in wide dynamic range compression instruments because it is measured at threshold, where WDRC applies maximum gain — real-ear measurement at 55/65/75 dB SPL is the professional standard instead.
- Functional gain is also limited by ambient-noise floor effects, expansion and adaptive noise reduction attenuating steady stimuli, inability to isolate one ear, and test-retest variability of ±5 dB or more.
- Sound-field verification remains the correct objective method for cochlear implants, bone-anchored and middle-ear implants, aural atresia or severe stenosis, contraindicated probe-tube placement, and for demonstrating aided versus unaided word recognition to patients and families.
9.5 Sound-Field Verification, Functional Gain & Aided Testing
[!IMPORTANT] Domain 4 of the NBC-HIS blueprint opens with a competency that is easy to overlook if you have modernised entirely to probe-microphone verification: "Identify the appropriate adjustment given objective verification outcome results (e.g., sound field aided/unaided discrimination, sound field aided/unaided warble tones or narrow band noise)." Sound-field verification is explicitly tested. You need to know how to run it, how to read it, and why real-ear measurement supplanted it.
What Sound-Field Testing Measures
In sound-field (free-field) testing the stimulus is delivered through a calibrated loudspeaker rather than an earphone, with the patient seated at a fixed distance and azimuth — commonly 1 metre at 0° azimuth, directly in front. The patient is tested twice: unaided, then aided with the instrument in place and programmed.
Functional Gain
Functional gain is the difference between the unaided and aided sound-field thresholds at each frequency:
If a patient detects a 2000 Hz warble tone at 60 dB HL unaided and at 35 dB HL aided, functional gain at 2000 Hz is 25 dB.
Why Warble Tones and Narrow-Band Noise — Never Steady Pure Tones
This is the classic exam point. A steady pure tone radiating into a small, hard-walled test room produces standing waves: reflections interfere with the direct sound, creating pressure peaks and nulls spaced through the room. Move the patient's head a few centimetres and the level at the ear can change by 10-15 dB, so a pure-tone sound-field threshold is not reproducible.
The fix is to spread the stimulus energy across a band so that peaks and nulls average out:
- Frequency-modulated (warble) tones sweep continuously around a centre frequency.
- Narrow-band noise presents a band of noise centred on the test frequency.
Both yield stable, repeatable sound-field thresholds. Sound-field thresholds are also referenced to their own calibration values, so a sound-field dB HL is not interchangeable with an earphone dB HL.
Aided and Unaided Discrimination
The blueprint also names sound-field discrimination testing: word recognition measured unaided and then aided, at a conversational level (commonly 50-65 dB HL) and often repeated in competing noise at a specified signal-to-noise ratio. This does not verify gain — it demonstrates functional benefit and generates the concrete numbers that drive counselling ("your word score went from 48% unaided to 84% aided in quiet, but only to 60% in restaurant noise, which is why we are also discussing a remote microphone").
Reading the Result and Making the Adjustment
Aided sound-field thresholds should place the long-term average speech spectrum within audibility — as a working target, aided warble-tone thresholds roughly in the 20-30 dB HL range across 250-4000 Hz for a mild-to-moderate loss, shaped to the patient's configuration.
| Sound-Field Finding | Likely Cause | Appropriate Adjustment |
|---|---|---|
| Aided thresholds good at 500-1000 Hz, poor (≥ 45 dB HL) at 2000-4000 Hz; patient reports speech is "muffled" | Insufficient high-frequency gain, occluded wax guard, underpowered receiver, or excessive feedback-manager gain reduction | Inspect and replace the wax guard, verify receiver power is adequate for the loss, then increase high-frequency gain and re-verify |
| Little or no functional gain at any frequency, but the aid passes electroacoustic analysis in the test box | Coupling failure in the ear — vent too large, dome too small, slit leak, or shallow seating | Correct the physical fit first: larger dome, closed or reduced vent, remake or modify the mold. Adding gain into a leak produces feedback, not audibility |
| Aided thresholds better than expected, but the patient still reports discomfort with loud sounds | Threshold-level testing tells you nothing about the upper end | Verify MPO/OSPL90 against the measured UCL; functional gain cannot detect an output problem |
| Aided thresholds appear excellent, yet the patient reports poor benefit for conversation | Nonlinear processing artefact — WDRC applies maximum gain at threshold | Re-verify with real-ear measurement at multiple input levels (55/65/75 dB SPL); do not credit threshold-based gain to conversational speech |
| Unaided and aided word recognition are both poor and essentially unchanged | Retrocochlear or central involvement, or a poor cochlear substrate | Re-examine word recognition and rollover, counsel expectations, and consider referral or implant candidacy discussion — more gain will not fix discrimination |
Why Real-Ear Measurement Is the Preferred Method
Functional gain was the standard before probe-microphone systems became affordable. It has real, testable limitations:
- Floor effects. You cannot measure an aided threshold below the ambient noise floor of the test room, and a patient with near-normal low-frequency hearing may show 0 dB of functional gain in the lows simply because the unaided threshold was already at the floor — not because the aid provides no low-frequency gain.
- Nonlinear processing invalidates it. Wide dynamic range compression applies its greatest gain to the softest inputs. A threshold-level tone therefore receives far more gain than conversational speech does, so functional gain systematically overestimates real-world gain in a WDRC instrument.
- Expansion and noise reduction work against you. Expansion deliberately reduces gain for very low-level inputs, and adaptive digital noise reduction can classify a steady warble tone or narrow-band noise as noise and attenuate it — both underestimate the aid's performance on speech.
- It cannot isolate an ear. In the sound field, the better ear responds. Verifying one ear at a time requires removing or masking the other, which is cumbersome and imprecise.
- Poor test-retest reliability — repeat measurements commonly vary by ±5 dB or more, which is the same order as the adjustment you are trying to verify.
- It is slow. Two full threshold searches at multiple frequencies take far longer than a probe-microphone run.
Real-ear measurement solves all six: it measures in the patient's own ear canal, at speech-like input levels, with the aid processing a speech stimulus, one ear at a time, in minutes. This is why probe-microphone verification against a validated prescriptive target is the professional standard of practice.
When Sound-Field Testing Is Still the Right Tool
Sound-field verification is not obsolete. It remains the appropriate — sometimes the only — objective method when a probe tube cannot be placed or would be meaningless:
- Cochlear implant recipients — there is no acoustic signal in the canal to measure, so aided sound-field thresholds and aided word recognition are the standard outcome measures.
- Bone-conduction devices and osseointegrated systems (BAHA/Ponto), middle-ear implants, and bone-conduction softbands — output bypasses the ear canal entirely.
- Aural atresia or severe canal stenosis, where no probe tube can be safely seated.
- Active drainage, recent otologic surgery, or a perforation, where inserting a probe tube is contraindicated.
- Patients who cannot tolerate a probe tube — some children, some patients with dementia or severe canal sensitivity.
- Demonstrating benefit to the patient and family, where aided-versus-unaided word recognition in quiet and in noise is far more persuasive than a target-match curve the patient cannot interpret.
[!CAUTION] Two traps. First, if a vignette gives you a patient whose hearing aid is functioning normally on electroacoustic analysis but shows no functional gain, the fault is in the acoustic coupling to the ear, not in the device — chase the vent, dome, and seal. Second, if a vignette offers both real-ear measurement and functional gain as options for a conventional air-conduction fitting, real-ear measurement is the better answer; functional gain becomes correct only when the scenario blocks probe-tube placement or removes the acoustic pathway.
Why must sound-field threshold testing use frequency-modulated (warble) tones or narrow-band noise rather than steady pure tones?
A patient fitted with wide dynamic range compression receiver-in-canal instruments shows 35 dB of functional gain at 2000 Hz on aided sound-field warble-tone testing, yet reports that conversational speech is still not clear enough. Electroacoustic analysis is within manufacturer specification. What is the MOST likely explanation?
A patient uses a bone-anchored hearing system on the right side following surgery for aural atresia. You need to objectively verify the device's performance. What is the MOST appropriate verification method?