ISO 13347-3:2004

Overview

ISO 13347-3:2004 - "Industrial fans - Determination of fan sound power levels under standardized laboratory conditions - Part 3: Enveloping surface methods" defines laboratory procedures to determine airborne sound power levels of industrial fans using enveloping surface measurement techniques. The standard is limited to airborne noise (vibration is not measured) and is intended for controlled laboratory conditions rather than in-situ field surveys. It supports repeatable, comparable fan noise data for product development, specification and noise-control planning.

Key topics and technical requirements

• Scope & limitations
  • Applies to industrial fans defined in ISO 5801 and ISO 13349.
  • Vibration effects are not measured; field conditions rarely meet laboratory requirements.
• Installation types (per ISO 5801)
  • Type A: free inlet, free outlet
  • Type B: free inlet, ducted outlet
  • Type C: ducted inlet, free outlet
  • Type D: ducted inlet, ducted outlet (casing breakout noise can be determined)
• Acoustic environment
  • Tests follow room conditions specified in ISO 3744; can be indoor or outdoor.
  • Prefer large spaces for high flowrate fans to avoid turbulent microphone placement.
  • Outdoor tests: wind ≤ 3 m/s recommended; temperature and relative humidity variations ≤ ±5%.
  • Environment correction K (Annex A) should be ≤ 2 dB per one‑third octave band (practical tolerance noted).
• Measurement surfaces & methods
  • Enveloping surfaces: rectangular parallelepiped, sphere/hemisphere, small-radius hemisphere - microphone positions are defined on a hypothetical surface surrounding the source (or ending on a reflecting plane).
  • Enables determination of open-inlet/open-outlet sound power and, with end-reflection corrections, approximate in-duct sound power for small fans.
• Test rig and instrumentation
  • Ducting, anechoic terminations, flow measurement and flow control follow ISO 5136 and ISO 5801 guidance.
  • Noise from flow control devices must be at least 10 dB below fan noise; drive/transmission contributions must be checked (impeller substitution test and background corrections for differences 6–10 dB).

Practical applications and users

• Who uses it: fan manufacturers, acoustic test laboratories, HVAC engineers, environmental/noise consultants, certification bodies and R&D teams.
• Why use it: to obtain reproducible fan sound power levels for product data sheets, comparative selection, noise control design and prediction of installation sound pressure levels.
• Practical tips: use qualified acoustic labs, select the appropriate enveloping surface for source geometry, and apply end‑reflection corrections when in‑duct testing is impractical.

Related standards

• ISO 3744 (sound power - free field over reflecting plane)
• ISO 5136 (in‑duct sound power)
• ISO 5801 (fan performance testing)
• ISO 13347-1 (general overview), ISO 13347-2, ISO 13347-4
• ISO 10302, ISO 13349

Keywords: ISO 13347-3, industrial fans, fan sound power level, enveloping surface methods, fan noise testing, laboratory acoustic testing, ISO fan standards.