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IEC TS 62370: Electroacoustics — Instruments for the Measurement of Sound Intensity
Specifications, calibration methods, and engineering practice for sound intensity measurement instrumentation
IEC TS 62370, published in 2017, is a Technical Specification that specifies requirements for instruments used in the measurement of sound intensity. Sound intensity measurement is a powerful acoustic engineering technique that differs fundamentally from conventional sound pressure measurement: instead of measuring the scalar acoustic pressure at a point, sound intensity measures the vector quantity describing the flow of acoustic energy per unit area (in W/m²). This vector nature allows engineers to determine the direction of sound propagation, identify the location of noise sources, and calculate sound power in situ without requiring specialized acoustic environments like anechoic chambers or reverberation rooms.
Sound intensity is defined as the time-averaged product of instantaneous sound pressure and the corresponding particle velocity component in the direction of propagation. IEC TS 62370 covers instruments using the “p-p” (pressure-pressure) method, where particle velocity is approximated using two closely spaced pressure microphones and the finite-difference approximation of Euler’s equation. This technique was pioneered in the 1970s and standardized internationally to enable reliable, repeatable sound power determination in real-world environments.
Instrument Specifications and Performance Requirements
The standard classifies sound intensity instruments into two grades based on measurement precision: Class 1 (precision) and Class 2 (survey). Class 1 instruments must meet tighter tolerances on the pressure-residual intensity index and the frequency response flatness, making them suitable for laboratory-grade measurements and compliance testing against noise regulations. Class 2 instruments are intended for field surveys and diagnostic applications where the highest precision is not required.
Key performance parameters specified in IEC TS 62370 include: the pressure-residual intensity index (delta_pI0), which characterizes the ability of the instrument to discriminate between sound pressure and intensity components at low frequencies; the phase mismatch between microphone channels, which is the dominant error source at low frequencies; the frequency range of operation, typically from 20 Hz to 6.3 kHz for general-purpose probes but extendable to 10 kHz or higher for specialized probes with smaller microphone spacing; the dynamic range in terms of sound intensity level (dB re 1 pW/m²); and the calibration stability over time and environmental conditions.
| Parameter | Class 1 | Class 2 | Test Method |
|---|---|---|---|
| Pressure-residual intensity index (at 250 Hz) | >= 15 dB | >= 10 dB | Coupler test with equal pressure on both microphones |
| Phase mismatch between channels | <= 0.3 deg at 1 kHz | <= 0.6 deg at 1 kHz | Electrostatic actuator or reciprocity calibration |
| Frequency range | 20 Hz – 6.3 kHz | 50 Hz – 5 kHz | Frequency response measurement |
| Level linearity error | <= 0.5 dB over 60 dB range | <= 1.0 dB over 50 dB range | Insert voltage or pistonphone method |
| Calibration drift (1 year) | <= 0.3 dB | <= 0.5 dB | Annual recalibration check |
The most critical error source in sound intensity measurement is phase mismatch between the two microphone channels at low frequencies. At frequencies below 200 Hz, the phase difference between the two microphone signals becomes very small (proportional to 1/f), and any residual phase mismatch in the measurement system can dominate the intensity reading. A phase mismatch of just 0.1 degree at 50 Hz can introduce an error exceeding 10 dB in the measured intensity for typical sound fields. Regular phase calibration using an electrostatic actuator or a specialized phase calibration coupler is essential for reliable low-frequency measurements.
Calibration Methodology and Engineering Design Insights
IEC TS 62370 specifies a multi-tier calibration approach that reflects best practices in acoustic metrology. Field calibration (performed before and after each measurement session) verifies the system with a sound calibrator at one or more frequencies. Laboratory calibration (performed annually or after any repair) provides full characterization of the pressure-residual intensity index, frequency response, phase response, and level linearity over the full operating range. The standard specifies that the field calibration should achieve an uncertainty of typically +/- 0.2 dB for sound pressure level measurements, while laboratory calibration targets +/- 0.1 dB for the reference conditions.
The engineering practice of sound intensity measurement involves several critical procedures. The scan method (moving the probe systematically over a measurement surface) is commonly used for sound power determination and noise source identification in situ. The standard provides guidance on scanning velocity, measurement time per area element, and spatial averaging techniques. For sound power determination, the measurement surface must enclose the source completely, and the intensity must be measured on this surface at sufficient spatial sampling density — typically 10-20 measurement positions per octave band wavelength for statistically reliable results. The negative partial power indicator, where the measured intensity on some surface segments points inward rather than outward, is a valuable diagnostic tool that reveals absorption, flanking transmission, or reactive field components that would otherwise bias the sound power estimate.
ISO 9614-1 (discrete point method) and ISO 9614-2 (scanning method) provide the standardized procedures for sound power determination using sound intensity measurements. IEC TS 62370 complements these standards by specifying the instrument performance requirements needed to achieve the stated measurement uncertainties. When applied together, these standards enable sound power determination in situ with typical expanded uncertainties (k=2) of 1-3 dB for most practical situations, making them invaluable for field testing of machinery, appliances, and industrial equipment where traditional reverberation room methods would be impractical or impossible.
| Application | Measurement Objective | Typical Probe Configuration | Key Challenge |
|---|---|---|---|
| Sound power determination in situ | Determine noise emission of machinery in its operating environment | 50 mm or 12 mm spacer | Background noise rejection, reactive near field |
| Noise source identification | Locate dominant noise sources on complex machinery | 12 mm spacer (high freq) or 50 mm (low freq) | Spatial resolution vs. low-frequency sensitivity trade-off |
| Sound transmission loss of partitions | Measure transmission loss of walls, windows, panels in buildings | 12 mm spacer | Flanking transmission, structural coupling |
| Acoustic material absorption | Determine sound absorption coefficient in situ | 12 mm or 50 mm spacer | Surface impedance matching, edge diffraction |
| Vehicle pass-by noise | Identify tire noise, engine noise, wind noise contribution | 6 mm or 12 mm spacer | Wind-induced noise on probe, Doppler shift |
Q1: What is the difference between sound intensity and sound pressure measurement?
A: Sound pressure is a scalar quantity measured at a single point. Sound intensity is a vector quantity that quantifies the magnitude and direction of acoustic energy flow. This allows sound intensity measurements to reject background noise and identify the direction of noise sources, which is impossible with conventional sound pressure measurements alone. Sound intensity also enables direct determination of sound power without requiring specialized acoustic test environments.
A: Sound pressure is a scalar quantity measured at a single point. Sound intensity is a vector quantity that quantifies the magnitude and direction of acoustic energy flow. This allows sound intensity measurements to reject background noise and identify the direction of noise sources, which is impossible with conventional sound pressure measurements alone. Sound intensity also enables direct determination of sound power without requiring specialized acoustic test environments.
Q2: Why is the microphone spacer size important in the p-p probe?
A: The spacer determines the distance between the two microphones, which affects the frequency range and sensitivity of the probe. A 50 mm spacer is suitable for low-frequency measurements (down to 20 Hz) but has limited high-frequency response due to spatial aliasing. A 12 mm spacer extends the high-frequency range (up to 10 kHz) but reduces low-frequency sensitivity. A 6 mm spacer is used for very high frequencies (up to 20 kHz) in specialized applications. The choice of spacer involves a fundamental trade-off between low-frequency sensitivity and high-frequency spatial resolution.
A: The spacer determines the distance between the two microphones, which affects the frequency range and sensitivity of the probe. A 50 mm spacer is suitable for low-frequency measurements (down to 20 Hz) but has limited high-frequency response due to spatial aliasing. A 12 mm spacer extends the high-frequency range (up to 10 kHz) but reduces low-frequency sensitivity. A 6 mm spacer is used for very high frequencies (up to 20 kHz) in specialized applications. The choice of spacer involves a fundamental trade-off between low-frequency sensitivity and high-frequency spatial resolution.
Q3: How often should a sound intensity probe be calibrated?
A: Field calibration (using a sound calibrator) should be performed before and after each measurement session. Full laboratory calibration, including phase mismatch characterization and pressure-residual intensity index verification, should be performed at least annually or after any event that could affect probe performance (mechanical shock, disassembly, exposure to harsh environments). For high-precision Class 1 measurements, some laboratories recommend semi-annual full calibration with quarterly phase verification.
A: Field calibration (using a sound calibrator) should be performed before and after each measurement session. Full laboratory calibration, including phase mismatch characterization and pressure-residual intensity index verification, should be performed at least annually or after any event that could affect probe performance (mechanical shock, disassembly, exposure to harsh environments). For high-precision Class 1 measurements, some laboratories recommend semi-annual full calibration with quarterly phase verification.
Q4: Can sound intensity measurement be performed outdoors?
A: Yes, but with caveats. Wind-induced noise on the probe microphones is the primary limitation for outdoor measurements. Windscreens are essential, and measurements should be avoided when wind speeds exceed 5 m/s. Temperature gradients and atmospheric turbulence can also affect measurement accuracy. Despite these challenges, outdoor sound intensity measurement is widely used for environmental noise assessment, wind turbine noise characterization, and pass-by vehicle noise testing.
A: Yes, but with caveats. Wind-induced noise on the probe microphones is the primary limitation for outdoor measurements. Windscreens are essential, and measurements should be avoided when wind speeds exceed 5 m/s. Temperature gradients and atmospheric turbulence can also affect measurement accuracy. Despite these challenges, outdoor sound intensity measurement is widely used for environmental noise assessment, wind turbine noise characterization, and pass-by vehicle noise testing.
