ISO 7235:2003

Overview

ISO 7235:2003 - Acoustics defines laboratory measurement procedures for ducted silencers and air-terminal units, covering: insertion loss, flow noise (regenerated sound), total pressure loss, and transmission loss (frequency-band results). The standard specifies the substitution method and laboratory arrangements (reverberation rooms, anechoic terminations, substitution ducts) for ambient-temperature tests. It applies to silencers used in ventilating, air‑conditioning, air‑intake/exhaust and similar systems, but not to reactive silencers for motor vehicles. ISO 7235:2003 notes that laboratory results may differ from in‑situ measurements (see ISO 11820 for on‑site testing).

Key Topics and Requirements

• Insertion loss (dB): Determined by the substitution method - compare sound power with the test object and with a non‑absorbing substitution duct.
• Transmission loss (dB): Measured for air‑terminal units, derived from reverberation‑room measurements and substitution duct theory.
• Flow noise / Regenerated sound: Measure sound power level (frequency bands) produced by airflow through the silencer (preferably measured via a reverberation room connected by a transmission element).
• Total pressure loss and coefficient (ζ): Measure total pressure upstream and downstream under flow; report pressure loss and calculate loss coefficient using face velocity and air density.
• Test facilities & instrumentation: Requirements for reverberation rooms, test ducts, anechoic terminations, excitation equipment, flow measurement devices and acoustic instrumentation (referenced standards for precision).
• Measurement conditions: Laboratory (ambient temperature), with and without airflow; guidance on when airflow affects insertion loss (notable above typical design velocities).
• Ancillary guidance: Annexes provide specifications for sound‑field excitation, transmission elements, duct walls, band conversions, large silencers, longitudinal attenuation, and test arrangements.

Applications and Who Uses It

• Silencer manufacturers: Verify product acoustic performance, provide data for datasheets.
• Acoustical test laboratories: Perform reproducible, comparable lab tests of insertion loss, flow noise and pressure loss.
• HVAC and ventilation designers: Select and size silencers and air‑terminal units based on measured insertion loss and pressure penalties.
• Specifiers and certification bodies: Compare products across labs and assess compliance with acoustic performance requirements. Practical uses: sizing silencers for ventilation systems, predicting regenerated noise downstream of silencers, estimating pressure losses for fan selection, and producing standardized acoustic data for procurement.

Related standards

• ISO 11820 (in‑situ measurements), ISO 3741 / ISO 3746 (sound power in reverberation rooms), ISO 5167‑1, ISO 5221, ISO 9614‑3, IEC sound level and calibrator standards, plus octave‑band filter standards (IEC 61260).