A sound power measurement technique optimised for low-speed fan tones

Air-moving devices such as fans are routinely characterised experimentally in terms of their sound power, and sound quality which depends on both broadband and tonal noise levels. The International Organization for Standardization method 5136, widely used in industry and academia, estimates sound power in third-octave bands radiated into a duct. Since the International Organization for Standardization method is not designed for tone measurements and overlooks the unevenness of the modal power distribution, an extended method has been developed and implemented based on mode decomposition. The ‘two-port’ source model is formulated to include higher-order modes and applied for the first three modes which require six independent stationary measurements on each side or ‘port’. The resulting experimental rig is much shorter than the International Organization for Standardization rig and does not require anechoic terminations. Both methods have been used to characterise the same fan, and the results compared to understand tone measurement shortfall of the International Organization for Standardization rig.