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IEC 62104: Characteristics of Headphones and Earphones — Measurement Methods
A comprehensive technical examination of IEC 62104, the international standard defining characteristics and measurement methods for headphones and earphones. This article covers frequency response measurement using artificial ears, impedance characterization, sensitivity determination, and total harmonic distortion testing, offering practical engineering insights for electroacoustic design and quality assurance.
1. Introduction and Scope
IEC 62104, officially titled “Characteristics of headphones and earphones,” was developed by IEC Technical Committee 100/TA 20 (Audio, video and multimedia systems and equipment). The standard defines the essential electroacoustic characteristics that should be specified for headphones and earphones, together with standardized methods of measurement. Although the standard has been withdrawn and its content incorporated into IEC 60268-7, it remains a critical reference for understanding headphone measurement methodology and transducer characterization.
The standard applies to all types of headphones and earphones, including supra-aural (on-ear), circumaural (over-ear), and intra-concha (in-ear) designs. It covers both wired and wireless configurations, though the measurement methods focus primarily on the electroacoustic transducer performance independently of the signal transmission mechanism. The standard also addresses measurement conditions for active noise-cancelling headphones, specifying that the noise cancellation circuitry must be active during testing with the reference noise signal applied.
Design Insight: The coupler/artificial ear used for headphone measurements is the single greatest source of measurement variability. A change from IEC 60318-1 (ear simulator) to IEC 60318-2 (coupler) can shift the measured frequency response by 3-5 dB at 8 kHz due to differences in acoustic impedance. Always specify which coupler standard was used when publishing headphone specifications.
2. Key Electroacoustic Characteristics
IEC 62104 defines a comprehensive set of characteristics that fully describe the performance of a headphone or earphone. These are grouped into three main categories: electrical, electroacoustic, and mechanical.
2.1 Frequency Response
The frequency response is the most widely cited headphone characteristic and is defined as the sound pressure level (SPL) developed in the artificial ear as a function of frequency, measured at a constant input voltage or power. IEC 62104 specifies that the measurement be performed using a swept sine signal or pink noise with a bandwidth of at least 20 Hz to 20 kHz. The frequency response is typically normalized to the SPL at 1 kHz and expressed in decibels relative to that reference.
The standard requires that measurements be conducted at three distinct input levels corresponding to low (70 dB SPL), medium (85 dB SPL), and high (100 dB SPL) listening levels, with the medium-level measurement being the primary reference. This multi-level approach reveals nonlinear behaviors such as compression in the transducer’s magnetic circuit or suspension compliance variations that are not apparent from a single-level measurement.
| Parameter | Condition | Notes |
|---|---|---|
| Coupler Type | IEC 60318-1 (ear simulator) or -2 (coupler) | Depends on headphone style |
| Measurement Signal | Swept sine or pink noise | 20 Hz – 20 kHz minimum |
| Input Levels | 70, 85, 100 dB SPL (nominal) | Three levels for linearity check |
| Clamping Force | 4.5 ± 0.5 N (circumaural) | Affects low-frequency seal |
| Ambient Noise | < 20 dB SPL (A-weighted) | Measured in anechoic environment |
| Warm-up Time | ≥ 30 minutes at rated input | Stabilizes voice coil temperature |
2.2 Impedance and Sensitivity
The electrical impedance of headphones is measured as a function of frequency and provides insight into the transducer’s electromechanical behavior. The standard specifies measurement over the frequency range of 20 Hz to 20 kHz at a voltage level of 0.1 to 1.0 V RMS. The nominal impedance is defined as the minimum magnitude of the impedance curve above the fundamental resonance frequency. For dynamic (moving-coil) transducers, the impedance typically shows a peak at the fundamental resonance frequency (Fs), which is an important parameter for enclosure design and equalization.
Engineering Note: The impedance peak at resonance can be 2-5 times the nominal impedance for dynamic headphones. This has critical implications for amplifier design: a headphone amplifier must be capable of delivering its rated output voltage into this peak impedance without distortion. For a 32 Ω nominal headphone with a 150 Ω peak, the amplifier must maintain its voltage swing into a 150 Ω load at the resonance frequency.
2.3 Total Harmonic Distortion
IEC 62104 specifies total harmonic distortion (THD) measurement at multiple frequency points, including both low frequencies (where distortion is typically highest due to suspension nonlinearities) and mid-band frequencies. The standard requires THD to be reported at 90 dB SPL and 100 dB SPL, with the measurement bandwidth limited to 20 kHz to exclude ultrasonic artifacts. The distortion analysis typically includes the 2nd through 5th harmonics, with higher-order components being weighted more heavily in the subjective perception of sound quality.
| Frequency | Premium Class | Standard Class | Basic Class |
|---|---|---|---|
| 100 Hz | < 0.5% | < 1.0% | < 3.0% |
| 500 Hz | < 0.3% | < 0.5% | < 1.5% |
| 1 kHz | < 0.2% | < 0.5% | < 1.0% |
| 5 kHz | < 0.3% | < 0.5% | < 1.5% |
| 10 kHz | < 1.0% | < 2.0% | < 5.0% |
3. Measurement Methodology and Best Practices
The measurement methodology prescribed by IEC 62104 emphasizes reproducibility. The headphones must be carefully positioned on the artificial ear to achieve a repeatable acoustic seal, particularly important for circumaural designs where leakage at low frequencies dramatically affects the measured response. The standard specifies a clamping force of 4.5 ± 0.5 N for circumaural headphones, measured using a force gauge positioned between the earpads at the point of contact with the artificial ear.
For in-ear monitors and earphones, the standard specifies the use of an ear simulator conforming to IEC 60318-4 (formerly IEC 60711), which incorporates a precisely calibrated acoustic impedance approximating that of the human ear canal. The earphone must be inserted to a standardized depth using the specified ear tip type and size. Variations in insertion depth of as little as 1 mm can cause SPL variations of 5 dB or more above 4 kHz due to standing wave formation in the residual ear canal volume.
Key Insight: Temperature and humidity significantly affect headphone measurements. The polymer materials used in diaphragms and suspensions change compliance with temperature at approximately 0.5%/°C. IEC 62104 specifies measurement conditions of 23 ± 2 °C and 45 ± 10% relative humidity. Allow at least 30 minutes for the device under test to stabilize in these conditions before beginning measurements.
4. Frequently Asked Questions
Q: What is the difference between IEC 62104 and IEC 60268-7?
A: IEC 62104 was the original standard dedicated to headphone and earphone characteristics and measurement methods. It has been withdrawn and its content incorporated into IEC 60268-7 (Sound system equipment — Part 7: Headphones and earphones), which is the currently active standard. IEC 60268-7 includes additional material on wireless headphone testing, active noise cancellation measurement, and updated references to ear simulator standards.
Q: Why does my headphone’s measured frequency response differ from the manufacturer’s published curve?
A: Differences commonly arise from three sources: (1) coupler type — manufacturers often use a different artificial ear than third-party reviewers; (2) measurement position — even 2 mm of repositioning can change high-frequency response by 3 dB; (3) sample variation — unit-to-unit variation in mass-produced headphones can be 2-3 dB in the high frequencies due to diaphragm thickness tolerances.
Q: How is active noise cancellation (ANC) measured under IEC 62104?
A: While the original 62104 edition had limited ANC coverage, the successor standard IEC 60268-7 specifies that ANC performance is measured by applying a diffuse-field noise signal (pink noise from 20 Hz to 5 kHz) at 80 dB SPL from a known sound source, then measuring the residual noise inside the artificial ear. The noise reduction is calculated as the difference between the external noise level and the internal residual level as a function of frequency.
Q: What is the significance of the 300 Hz to 3 kHz band for headphone measurements?
A: This frequency range corresponds to the primary spectral content of speech and is the region where the human ear is most sensitive to distortion. IEC 62104 places special emphasis on measurement accuracy in this band because small deviations in frequency response (±2 dB) are perceptible as changes in vocal timbre. The standard specifies tighter tolerances for measurement equipment in this range (±0.5 dB SPL accuracy).
