Precision Procedures for Verifying the Inverse Square Law According to ISO 26101 — Traversing directions, microphone positioning, and measurement intervals in detail —
08/11/2025
MFAC / MSAC
Introduction: Verifying the Inverse Square Law Is the Key to Chamber Certification
For an anechoic or semi-anechoic chamber to be considered ISO-compliant, the inverse square law (–6 dB per doubling of distance) must be verified.
The basic procedure is outlined in Annex A of ISO 3745:2012, with detailed steps provided in ISO 26101:2012.
Key Elements of ISO 26101: Six Requirements for Accurate Measurement
1. Sound Source Position
The test source should generally be placed near the center of the room or 1 meter above the floor.
2. Microphone Traversing Directions
At least five traverses are required, covering central surfaces, corners, and opposing boundaries.
3. Starting Point for Measurement
Start at 1/4 wavelength of the lowest frequency.
(e.g., for 100 Hz: approx. 0.85 m)
4. Measurement Intervals
| For frequencies below 1 kHz | intervals no greater than 1/10 of the wavelength |
| For frequencies above 1 kHz | intervals no greater than 25 mm (strictly specified) |
5. Maximum Measurement Distance
Up to 1/2 wavelength of the lowest frequency
6. Noise Type-Specific Methods
| Broadband noise | use pink noise |
| Narrowband sources | measure using continuous tone (pure tone) sweeps |
Visualization: Plotting Distance vs. Sound Pressure Level
The measured results are plotted as sound pressure level vs. distance, and evaluated based on how closely they follow the ideal –6 dB slope.
Large deviations may indicate inadequate absorption or undesired reflections in the environment.
Sonora’s Approach: 12-Sided Source + Precision Traversing System
At Sonora, we employ HBK’s dodecahedron speaker system and automated microphone traversing rigs to carry out precision verification under ISO 26101 standards.
- Measurement intervals: 2.5 mm to 5 mm
- Over 30 measurement points per test
- Real-time slope calculations for automatic pass/fail judgment
Conclusion: ISO 26101 Is the Benchmark for Modern Anechoic Testing
The role of inverse square law verification has become increasingly important since the ISO 3745 era.
At Sonora, we have built a robust verification framework based on ISO 26101, providing reliable performance certification for both new and retrofitted chambers.