ISO 5136:2003

Overview

ISO 5136:2003 - "Acoustics - Determination of sound power radiated into a duct by fans and other air-moving devices - In-duct method" specifies a laboratory method to measure the sound power emitted by ducted fans and similar air-moving equipment into an anechoically terminated duct. The standard covers measurements on the inlet and/or outlet side, for steady, broadband, narrowband and discrete-frequency noise, and under air temperatures from −50 °C to +70 °C. Test ducts are typically circular with diameters 0.15 m to 2 m (informative annexes H and I address smaller and larger ducts).

Key topics and technical requirements

• Test configuration: measurements in an anechoically terminated circular duct; clear definition of test, terminating and intermediate ducts.
• Microphone and windscreen: use of sampling tubes or microphone shields to suppress turbulent pressure fluctuations. Maximum mean flow velocities at the microphone head: foam ball 15 m/s; nose cone 20 m/s; sampling tube 40 m/s.
• Signal-to-noise: for aerodynamic sources (boxes, dampers, silencers) the sound-to-turbulence signal-to-noise ratio must be at least 6 dB.
• Flow conditions: guidance on flow straighteners and swirl - the method assumes swirl ≤15° where applicable; Annex J provides a swirl measurement example.
• Frequency analysis: one‑third‑octave band measurements and procedures for A-weighted conversion (Annex C).
• Uncertainty and calibration: clauses on measurement uncertainty, instrumentation, system calibration and required reporting.
• Annexes: practical guidance for anechoic terminations, sampling tube design, test methods for small/large ducts, and evaluation of anechoic performance.

Applications and users

ISO 5136 is used by:

• Acoustic test laboratories and certification bodies performing fan noise ratings.
• HVAC and fan manufacturers for product development, quality control and noise data specification.
• Acoustic consultants and R&D teams assessing noise from ducted systems (air handlers, dust collectors, ducted furnaces, silencers).
• Regulatory bodies and procurement teams requiring standardized sound power data for ducted equipment.

Typical uses:

• Generating comparable sound power data for product catalogs and noise labeling.
• Supporting noise prediction in duct networks and building services engineering.
• Verifying performance against noise criteria and regulatory limits.

Related standards

• ISO 5801 (fan performance / airflow conditions) - often used together with ISO 5136.
• ISO 7235 (flow-generated noise measured via reverberation room method) - alternative for some attenuator/fan combinations.
• Reverberation/free-field and intensity standards for free-space sound power: ISO 3741 / 3743 / 3744 / 3745 / 3746 / ISO 9614.
• Relevant IEC standards for instrumentation: IEC 60651, IEC 60942, IEC 61260.

Keywords: ISO 5136, in-duct method, acoustics, sound power, ducted fans, anechoic termination, sampling tube, fan noise testing.