Experimental comparison of standardized sound power measurement procedures for fans

Abstract

Three standardized measurement procedures for determination of the sound power radiated from a fan inlet or outlet were compared experimentally: free field or enveloping surface method over a reflecting plane (ISO 3744, DIN 45 635 Part 1), reverberation room method (ISO 3741/2, DIN 45 635 Part 2), and in-duct method (ISO 5136, DIN 45 635 Part 9). The results of the former two agree quite well in the frequency range above 80 Hz. The differences below that limit are due to the measurement uncertainties of both test codes. In the frequency range of plane wave propagation, the in-duct method yields higher levels than the other two as the result of the reflection of sound waves at the fan openings; the differences can be described by using the end reflection characteristics of a flanged duct end. In the higher order duct mode frequency range, the in-duct levels are lower than the free field and reverberation room levels. The level differences are frequency dependent with an average amount of about 3 dB on the fan inlet side and about 5 dB on the outlet side.