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IEC 62585 โ Corrections for Free-Field Response of Sound Level Meters
A sound level meter reading is only as accurate as its frequency response correction. IEC 62585 defines the methods for determining these corrections, accounting for microphone diffraction, windscreen effects, and the use of couplers and electrostatic actuators — ensuring traceable, repeatable measurements in free-field conditions.
💡 Why it matters: Without proper free-field corrections, sound level meter measurements can differ from the true sound pressure by 2–6 dB at high frequencies — a significant error in noise exposure assessment and product noise compliance testing.
1 🏷 Scope and Application
IEC 62585:2012 applies to sound level meters conforming to IEC 61672 (the primary performance standard for sound level meters). It specifically addresses the measurement chain from the microphone diaphragm to the instrument output, providing correction values that enable:
- Conversion from the pressure response (measured using a coupler or actuator) to the free-field response
- Correction for the effects of diffraction around the microphone and the instrument body
- Compensation for frequency response deviations of the microphone in free-field conditions
- Corrections when windscreens and other accessories are attached
The standard covers two fundamental scenarios: using the sound level meter with a sound calibrator (the most common field practice) and using it with a comparison coupler or electrostatic actuator (laboratory methods).
2 🧮 Correction Determination Methods
2.1 The Free-Field vs. Pressure Response Problem
When a sound level meter is calibrated using a coupler (pistonphone or sound calibrator), the microphone is exposed to a uniform pressure field. However, in actual use the microphone is placed in a free field where sound waves arrive predominantly from one direction. The microphone and the instrument body cause diffraction and scattering of the incident sound waves, altering the pressure at the diaphragm compared to the free-field pressure that would exist in the absence of the instrument.
The difference between the free-field pressure at the microphone position and the actual diaphragm pressure is frequency-dependent and can be significant:
| Frequency (Hz) | Typical Free-Field Correction for 1/2″ Microphone (dB) | Typical Free-Field Correction for 1/4″ Microphone (dB) | Source of Error |
|---|---|---|---|
| 125 | 0.0 | 0.0 | Negligible diffraction |
| 500 | 0.0 | 0.0 | Negligible diffraction |
| 1000 | 0.0 | 0.0 | Negligible diffraction |
| 2000 | 0.1 | 0.0 | Onset of diffraction |
| 4000 | 0.4 | 0.1 | Diffraction peak |
| 8000 | 1.5 | 0.3 | Strong scattering |
| 12500 | 2.0–3.0 | 0.8 | Resonance effects |
| 16000 | 3.0–5.0 | 1.2 | Complex scattering |
⚠️ Note: Smaller microphones (1/4″ vs. 1/2″) exhibit smaller free-field corrections because their dimensions are smaller relative to the acoustic wavelength. For high-frequency measurements above 10 kHz, using a 1/4″ microphone is strongly recommended.
2.2 Determination Using a Sound Calibrator
When a sound calibrator is used, the method of substitution (IEC 62585, Clause 12) is applied:
- The sound level meter is placed in a free-field environment (typically an anechoic chamber)
- A reference sound source generates pure tones at discrete frequencies across the instrument’s range
- The device under test is removed and replaced with a reference measurement microphone at the same position
- The difference between the reading of the sound level meter and the reference microphone at each frequency defines the correction
2.3 Comparison Coupler and Electrostatic Actuator Methods
For laboratory use, the standard describes two alternative procedures:
- Comparison coupler method (Clause 13): A specialized coupler with known acoustical properties is used to compare the sound level meter microphone against a reference microphone. The method covers frequencies up to several kHz.
- Electrostatic actuator method (Clause 14): An electrostatic grid is placed close to the microphone diaphragm to apply a known electrostatic force, simulating sound pressure. This method works up to very high frequencies (often 20 kHz+), providing corrections across the full frequency range.
✅ Recommendation: For field calibration, use the sound calibrator method with corrections provided by the manufacturer. For type-approval testing, the electrostatic actuator method provides the most comprehensive frequency response characterization.
3 📊 Engineering Design Insights and Practical Considerations
3.1 Windscreen Effects and Corrections
Windscreens are essential for outdoor measurements but introduce their own frequency response deviations. IEC 62585 requires that corrections for windscreens be determined at frequencies up to 2 kHz (at minimum). The correction depends on:
- Windscreen diameter (larger windscreens have more effect at lower frequencies)
- Material density and porosity
- Mounting configuration
| Windscreen Type | Diameter (mm) | Typical Correction at 125 Hz (dB) | Typical Correction at 1 kHz (dB) | Typical Correction at 8 kHz (dB) |
|---|---|---|---|---|
| Standard foam ball | 90 | -0.1 | -0.2 | -0.8 |
| Large foam ball | 120 | -0.2 | -0.3 | -1.2 |
| Multi-layer wind shield | 150 | -0.3 | -0.5 | -2.0 |
3.2 Uncertainty Analysis
The standard’s focus on traceability requires a thorough uncertainty budget. The main contributors to the overall uncertainty of the determined corrections include:
- Reference microphone calibration uncertainty (typically ±0.2 dB)
- Positioning accuracy of the microphone relative to the reference (repeatability)
- Environmental conditions (temperature, pressure, humidity)
- Electrical measurement chain noise floor
- Anechoic chamber performance (cut-off frequency, background noise)
🚨 Caution: The combined standard uncertainty in free-field corrections typically ranges from 0.3 dB at low frequencies to 1.0 dB at high frequencies. Using corrections without a stated uncertainty invalidates the traceability chain.
3.3 Practical Implications for Noise Compliance Testing
In product noise testing per ISO 3744 or ISO 3745, the measured sound pressure levels at each microphone position must be corrected for the free-field response of each sound level meter. Failing to apply these corrections — particularly at frequencies above 2 kHz — can lead to:
- Underestimation of tonal noise components (e.g., fan blade-passing frequencies)
- Incorrect determination of A-weighted sound power levels
- Non-reproducibility between different measurement laboratories
Frequently Asked Questions
Q1: Are the corrections in IEC 62585 already applied in modern sound level meters?
Most modern Class 1 sound level meters store free-field correction curves in their firmware. However, these factory corrections are typically averaged across production batches. For highest accuracy, IEC 62585 recommends that individual microphones be characterized — particularly for type-approval and forensic measurements.
Q2: What is the difference between pressure response and free-field response?
The pressure response is measured by placing the microphone in a uniform pressure field (e.g., using a coupler) where the sound pressure is the same everywhere. The free-field response is what the microphone measures when placed in a propagating sound wave, including diffraction and scattering effects from the instrument body.
Q3: Can I use the same correction for all angles of sound incidence?
No. The free-field correction is defined for a specific angle of incidence (typically 0° — grazing incidence for the standard configuration). At other angles, frequency response may differ significantly. Some standards require measurements at multiple angles with separate corrections.
Q4: How often should free-field corrections be re-determined?
IEC 62585 recommends re-determination whenever the microphone or preamplifier is changed, after any mechanical shock, or at intervals prescribed by the quality system (typically annually for accredited laboratories).
