Overview
ISO 9053-2:2020 - Acoustics: Determination of airflow resistance - Part 2: Alternating airflow method specifies an alternating‑airflow test technique for measuring the airflow resistance of porous materials used in acoustic applications. The method complements ISO 9053‑1 (static flow method) by using a low‑frequency sinusoidal volume flow (typically 1–4 Hz, e.g. 2 Hz) generated by a piston to create pressure modulation across a specimen. Measured sound pressure differences between a closed cavity and the specimen‑terminated cavity are used to compute airflow resistance, specific airflow resistance and airflow resistivity.
Key topics and technical requirements
- Principle: A piston produces an alternating volume flow into an air cavity; the difference in cavity sound pressure with and without the specimen yields the airflow resistance.
- Definitions: Airflow resistance R (Pa·s·m−3), specific airflow resistance Rs (Pa·s·m−1) and airflow resistivity σ (Pa·s·m−2) are defined and used for reporting.
- Frequency and excitation: Piston frequency f shall be in the 1–4 Hz range and accurately measured; piston stroke amplitude and resulting RMS volumetric flow are part of the measurement.
- Required equipment:
- Alternating‑flow generator (sinusoidal piston)
- Sound level meter or narrow‑band sound pressure measuring device
- Vessel with air cavity, measurement cell and airtight termination
- Static pressure and frequency measurement devices
- Thickness measurement for test specimens
- Specimen requirements: Guidance on homogeneity, shape, dimensions and number of specimens is included.
- Measurement and analysis: Measurement of sound pressure levels for both terminated and airtight conditions, calculation of airflow resistance, and treatment of uncertainty.
- Annexes: Normative/informative annexes cover effective ratios of specific heats, acoustic modelling, uncertainty calculation and airflow resistance of perforated supports.
Practical applications and users
- Applications:
- Characterizing porous acoustic absorbers, sealing materials, and sound‑absorbing panels where dynamic (alternating) flow behaviour matters
- Quality control, material selection and performance specification for acoustic engineering and product development
- Research into visco‑inertial effects and low‑frequency behaviour of porous materials
- Who should use it:
- Acoustic test laboratories and manufacturers of acoustic materials
- Product engineers and R&D teams designing absorbers, foams, fibrous media and composite acoustic systems
- Standards bodies and conformity assessment organizations requiring standardized airflow resistance data
- When to prefer Part 2: Use the alternating airflow method when the material exhibits significant visco‑inertial behaviour at low frequencies or when dynamic response (rather than static flow) better represents in‑service acoustic conditions. Note: ISO 9053‑1 addresses static flow measurements.
Related standards
- ISO 9053‑1 - Determination of airflow resistance using static flow (complements Part 2)
- ISO/IEC Guide 98‑3 (GUM) - Uncertainty of measurement (referenced for uncertainty treatment)
Keywords: ISO 9053-2:2020, alternating airflow method, airflow resistance, airflow resistivity, acoustics testing, porous materials, piston method, sound pressure measurement.
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