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?
Warble tones and narrow-band noise spread energy across a band, averaging out the standing-wave peaks and nulls that make steady pure-tone sound-field thresholds unreproducible.
Steady pure tones cannot be calibrated in a sound field, so no reference equivalent threshold values exist for loudspeaker presentation.
Hearing aid feedback cancellation systems can only be disabled for modulated signals, so pure tones would trigger entrapment artifacts.
Warble tones excite a wider region of the basilar membrane, which raises the patient's sensitivity and produces better aided thresholds.
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?
The functional gain measurement is invalid because standing waves inflated the unaided threshold.
Functional gain is measured at threshold, where WDRC applies its maximum gain, so it overestimates the gain actually available for conversational-level speech.
The receiver is underpowered for the loss, which electroacoustic analysis cannot detect.
A 35 dB functional gain exceeds the prescriptive target, so the instrument is over-amplifying and should be reduced.
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?
Real-ear insertion gain measured with a probe tube placed within 5 mm of the tympanic membrane.
Aided and unaided sound-field thresholds using warble tones or narrow-band noise, supplemented by aided word recognition.
ANSI S3.22 electroacoustic analysis in a 2cc coupler using the HA-2 coupler with 25 mm of tubing.
Speech mapping with the ISTS stimulus presented at 55, 65, and 75 dB SPL.
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