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IEC 61669:2001 โ Electroacoustics: Measurement of Hearing Aid Performance
💡 Standard Role: IEC 61669:2001 establishes uniform methods for measuring the electroacoustic performance of air-conduction hearing aids, providing essential test protocols for manufacturers, audiologists, and regulatory compliance testing worldwide.
1. Scope and Measurement Framework
IEC 61669:2001 specifies measurement methods for characterizing the electroacoustic performance of hearing aids that deliver amplified sound to the ear via air conduction. The standard covers all types of air-conduction hearing aids including behind-the-ear (BTE), in-the-ear (ITE), in-the-canal (ITC), completely-in-the-canal (CIC), and receiver-in-canal (RIC) devices. The primary measurement parameters include full-on gain (FOG), output sound pressure level (OSPL90), frequency response characteristics, total harmonic distortion (THD), equivalent input noise, and battery current consumption.
The standard defines two principal measurement environments: the 2 cm³ coupler (IEC 60318-5) which simulates the residual ear canal volume for hearing aid testing, and the free-field measurement setup for determining hearing aid directional characteristics and reference test gain settings.
⚠ Measurement Standardization: While IEC 61669 provides international standards, note that national variants exist — notably ANSI S3.22 (USA) which has historically differed in some test signal specifications and tolerances. Modern revisions have sought harmonization.
2. Key Measurement Parameters and Methods
2.1 OSPL90 and Full-On Gain
The OSPL90 test measures the maximum output sound pressure level produced by the hearing aid when driven by a 90 dB SPL input — the saturation output. This is the single most important safety-related parameter, as excessive output can cause further hearing damage. The Full-On Gain (FOG) measurement determines the maximum acoustic gain of the device with the volume control at its maximum setting.
Test Protocol: Input: 90 dB SPL swept sine (for OSPL90)
Measurement: 2 cm³ coupler with calibrated microphone
Frequency range: 200 Hz — 8 kHz (extended to 16 kHz for modern devices)
Key metric: OSPL90 should not exceed 132 dB SPL (safety limit per IEC 60118-0)
Measurement: 2 cm³ coupler with calibrated microphone
Frequency range: 200 Hz — 8 kHz (extended to 16 kHz for modern devices)
Key metric: OSPL90 should not exceed 132 dB SPL (safety limit per IEC 60118-0)
2.2 Frequency Response and Reference Test Gain
The frequency response curve maps the hearing aid’s gain as a function of frequency, typically measured at one-third octave intervals across the 200 Hz to 8 kHz range. The Reference Test Gain (RTG) is defined as a gain setting 15 dB below the FOG at 1.6 kHz, providing a standardized reference for comparing distortion and noise measurements across devices.
| Parameter | Symbol | Input Signal | Measured Range | Typical Tolerance |
|---|---|---|---|---|
| Output SPL (saturation) | OSPL90 | 90 dB SPL sweep | 100 — 140 dB SPL | ±3 dB |
| Full-on gain | FOG | 50 dB SPL sweep | 20 — 80 dB | ±5 dB |
| Frequency response | FR | 50-70 dB SPL sweep | 200 — 8000 Hz | ±3 dB |
| Total harmonic distortion | THD | 70 dB SPL at 500/800/1600 Hz | 0.1 — 10% | ±1% |
| Equivalent input noise | EIN | None (self-noise) | 15 — 35 dB SPL | ±3 dB |
| Battery current | Ibat | 60 dB SPL (1 kHz) | 0.5 — 5 mA | ±0.1 mA |
3. Engineering Design Insights for Hearing Aid Testing
From an engineering perspective, the practical challenges of hearing aid measurement include:
- Acoustic leakage control: The seal between the hearing aid’s sound outlet and the coupler is critical — even small leaks significantly affect low-frequency measurements (below 500 Hz). The standard specifies calibration procedures and leakage verification methods.
- Directional microphone testing: Modern hearing aids with dual-microphone beamforming require free-field measurements in an anechoic environment with a defined sound field (typically diffuse or frontal incidence). The polar response pattern must be measured at minimum in the horizontal plane.
- Digital processing delay: Modern digital hearing aids introduce group delay from analog-to-digital conversion, DSP processing, and digital-to-analog conversion. While not directly specified in the 2001 edition, group delay above 10 ms can cause audible artifacts (comb filtering) when the hearing aid user also hears their own voice through bone conduction.
✅ Best Practice: For accurate and repeatable measurements, precondition hearing aids at the reference test gain for 5 minutes before recording data. This stabilizes the automatic gain control (AGC) circuits and avoids transient startup effects that can distort the frequency response measurement by up to 6 dB.
4. Worked Example: BTE Hearing Aid Measurement
A typical behind-the-ear hearing aid measurement sequence per IEC 61669 proceeds as follows: (1) Mount the hearing aid in the 2 cm³ coupler with appropriate tubing and earhook adapter. (2) Measure OSPL90 by sweeping from 200 Hz to 8 kHz at 90 dB SPL input. (3) Set the volume control to full-on and measure FOG. (4) Reduce gain to reference test gain level. (5) Measure THD at 500, 800, and 1600 Hz. (6) Measure equivalent input noise by recording output with acoustic input at minimum (anechoic chamber). (7) Record battery current under standard test conditions.
❓ Q1: What is the difference between OSPL90 and OSPL90-in-situ?
A: OSPL90 is measured in a standard 2 cm³ coupler. OSPL90-in-situ uses a probe microphone in the real ear canal to measure the actual output reaching the eardrum. The in-situ value can differ by 5-15 dB depending on ear canal geometry and venting.
❓ Q2: How does the standard address wireless hearing aid features (telecoil, Bluetooth)?
A: The 2001 edition primarily addresses acoustic input measurements. Wireless features are covered in later amendments and related standards (IEC 60118-4 for telecoils, IEC 60118-13 for wireless interference).
❓ Q3: What are common sources of measurement variability?
A: The most significant sources are coupler placement variation, earhook/tubing differences, acoustic seal quality, ambient noise (particularly for EIN measurements below 25 dB SPL), and insufficient preconditioning time for AGC circuits.
❓ Q4: How does the 2 cm³ coupler relate to real-ear acoustics?
A: The 2 cm³ coupler approximates the average adult residual ear canal volume. Real-ear measurements typically show higher output above 2 kHz due to the ear canal resonance (around 3-4 kHz), which the coupler does not fully replicate. Real-ear-to-coupler difference (RECD) correction factors are used to relate the two.
