REM Fitting & Verification — Quick Reference

Purpose: ensure the REM procedure culminates in the optimal fitting for the patient.

External loudspeaker on each REM system capable of generating a variety of input signals.
Ear-level probe module includes:
- Reference microphone: monitors/calibrates soundfield output; maintains signal intensity at measurement point.
- Retention cord: stabilizes reference microphone position.
- Probe tube: measures signal arriving to the tympanic membrane (TM).
- Probe microphone: measures sound from the probe tube.

Type I: Pure tone signals swept across frequencies; used to measure MPO (maximum output).
- Higher output than speech signals; DFS signals attenuate Type I when activated.
- Does not reflect compression or channel interactions.
Type II: Complex, speech-like signals; broadband with random frequencies at varying intensities.
- Mimics speech; supports verification of prescriptive targets for output & frequency response.
- Unpredictable amplitude changes can obscure responses to spectral shapes; standardized Type II signals preferred for digital tech.

Calibrated signals for repeatable verification:
- LTASS: Long-Term Average Speech Spectrum (10 s) to represent speech energy over time.
- ISTS: International Speech Test Signal (6 female talkers, multiple languages).
- ICRA: Distorted speech signal (largely unintelligible).
- Speechmap / Speech-STD / Live signals also used in practice.

Measured speech envelope shows dynamic range arriving at the TM; valleys to peaks ~ $30\ \text{dB SPL}$ .
To maximize SII, entire speech envelope must be above threshold; LTASS is averaged over time.

LTASS is frequency-dependent and varies with:
- Vocal effort (mid-frequency LTASS changes).
- Microphone position (azimuth affects high-frequency LTASS).
- Language (low-frequency LTASS changes).
LTASS is recorded for $10\ \text{s}$ to compute the average; different intensities yield different LTASS.
Individual LTASS differs from standardized LTASS.

Substitution method (soundfield equalization):
- A sound level measurement microphone placed at subject’s position; calibration stored as reference.
- Limitations: head/body movement reduces precision; location changes reduce accuracy.
Modified pressure methods:
- Concurrent equalization: reference mic monitors signal during testing; automatic recalibration every $10\ \text{s}$ .
- Stored equalization: probe module calibrated once per patient; stored for fitting; avoids leakage contamination with open domes.
- Limitation: head movement can affect final recording.

Contamination occurs when amplified output leaks from the ear; the reference mic detects it and lowers REM input, underestimating output.
Stored equalization helps prevent this contamination.

Probe tube extends the probe microphone; all three (reference mic, probe mic, probe tube) are calibrated to account for intensity differences.
Calibration aims to make the probe tube acoustically invisible by aligning tube/tip with reference mic during calibration.

On-ear calibration setup prompts insertion depth and proximity to TM; aim for within $5\ \text{mm}$ of TM.
Different insertion methods exist:
- Otoscopic method: may cause bump/pull; less precise.
- Constant depth method: use fixed distance from intertragal notch to TM (e.g., male ~ $30\ \text{mm}$ , female ~ $28\ \text{mm}$ ; pediatric adjustments).
- Acoustic method: insert while monitoring high-frequency notch; target within $5\ \text{mm}$ of TM; ensure notch no longer drags gain at high frequencies (e.g., 6 kHz) by the end.
Recommended approach: combine constant depth and acoustic insertion techniques for accuracy.

Typical reference depths (approximate):
- Adult male: ~ $30\ \text{mm}$ from intertragal notch to TM.
- Adult female: ~ $28\ \text{mm}$ .
- Pediatric variations apply by age group.
Insertion guides often show marks at $15\ \text{mm}$ , $10\ \text{mm}$ , and $5\ \text{mm}$ to assist depth monitoring.

Geometric method (used for uncooperative patients): place probe along outer ridge; tip extends 3$-$5\ \text{mm} beyond earmold tip; depth depends on canal length.
Distance from eardrum must be within $5\ \text{mm}$ to avoid HF attenuation and standing waves.

Standing waves occur if probe tube is > $5\ \text{mm}$ from the TM, causing HF attenuation and underestimation.
Within $5\ \text{mm}$ prevents HF attenuation and provides accurate SPL across frequencies.

Working distance (distance between patient and speaker): $18''\text{ to }36''$ (approx. 46 cm to 91 cm).
Recommended testing distance: at least $2\times\text{WD}$ (i.e., $36''\text{ to }64''$ ) from reflective surfaces.
Ambient room noise: at least $10\ \text{dB}$ lower than REM signal to minimize interference.

Azimuth impacts LTASS reliability:
- 0° azimuth (direct in front): most reliable.
- 45°: acceptable for some situations.
- 90°: leads to significant LTASS variability/errors.
Vertical alignment: speaker should be level with the ear to accurately measure high-frequency output.

Otoscopic method: insert with visualization; may cause discomfort and inaccuracy.
Constant depth: measure from intertragal notch to TM; target ~2$-$5\ \text{mm} from TM for accuracy.
Acoustic method: monitor high-frequency notch during insertion; final position within $5\ \text{mm}$ of TM.
Best practice: combine constant depth with acoustic insertion for robust results.

Function of each ear-level probe module part; output targets for binaural benefit ( $\leq 15\ \mathrm{dB}$ ).
Type I vs Type II signals and their roles in verification.
LTASS calculation, factors affecting LTASS, and interpretation.
Calibration methods (substitution, concurrent equalization, stored equalization) and contamination issues.
Probe mic, probe tube calibration and achieving acoustic transparency.
Probe insertion techniques (constant depth, acoustic, geometric) and the importance of staying within $5\ \text{mm}$ of the TM to avoid HF attenuation.
ANSI working distances, patient azimuth, and setup considerations for reliable REM results.