Overview
ISO 17497-2:2012 - "Acoustics - Sound‑scattering properties of surfaces - Part 2: Measurement of the directional diffusion coefficient in a free field" specifies a standardized free‑field method to measure the directional diffusion coefficient of surfaces. The diffusion coefficient quantifies the uniformity of the reflected polar distribution (how evenly a surface scatters sound) and complements the random‑incidence scattering coefficient measured in reverberation rooms (ISO 17497‑1). ISO 17497‑2 is intended to inform manufacturers, specifiers and acousticians about the quality of deliberately or accidentally diffusing surfaces and to support validation of geometric room‑acoustic models.
Key technical topics and requirements
- Measurement principle: Acquire polar responses of reflected sound pressure levels using a controlled source and microphone array; extract reflected sound and compute the directional diffusion coefficient from polar data.
- Directional and normalized coefficients:
- Directional diffusion coefficient (dθ,φ) - value for a single source position (0 to 1).
- Normalized directional diffusion coefficient (dθ,φ,n) - test specimen normalized to a reference flat surface to remove finite‑panel effects.
- Random‑incidence diffusion coefficient (d) - average/weighted combination across representative source positions.
- Measurement geometry:
- Semicircular (single‑plane) or hemispherical (multi‑plane) microphone layouts centered on the reference point.
- Receiver angular resolution: Δθ ≤ 5° and Δφ ≤ 5°.
- Far‑field guidance: at least 80% of receiver positions outside the specular zone; example distances given (source to reference ≈ 10 m, receiver radius ≈ 5 m).
- Frequency range: One‑third‑octave bands covering 100 Hz to 5 000 Hz (or scaled frequencies for physical scale models using ratio 1:N).
- Measurement environment: Qualified anechoic chamber or large non‑anechoic space using Annex A qualification methods; options for two‑dimensional boundary measurements or three‑dimensional goniometers.
- Reporting: Clause 9 defines test report requirements, including geometry, source/receiver positions, frequency bands and diffusion coefficient results.
Practical applications and users
Who uses ISO 17497‑2:
- Acoustic product manufacturers (diffusers, architectural panels) for product characterization and quality control.
- Acoustic consultants and designers evaluating surface treatments in concert halls, studios, industrial spaces and reverberation chambers.
- Researchers and developers validating and calibrating geometric room‑acoustic models and diffuser designs.
- Standards bodies and test labs implementing reproducible free‑field scattering tests.
Practical benefits:
- Provides a quantitative measure of diffusion quality (directional uniformity) rather than only scattering amount.
- Supports design decisions where spatial distribution of reflected energy affects speech intelligibility, clarity and sound‑field uniformity.
Related standards
- ISO 17497‑1 - Measurement of the random‑incidence scattering coefficient in a reverberation room.
- IEC 61260 and ISO 266 - referenced for one‑third‑octave band definitions and preferred frequencies.
Keywords: ISO 17497-2, directional diffusion coefficient, sound‑scattering properties, free‑field measurement, acoustic diffuser testing, goniometer, anechoic chamber, one‑third octave.