Free-field sound power levels of open-inlet/open-outlet fans and comparison with in-duct measurements

Abstract

Sound power levels were measured for an axial-flow fan and a centrifugal fan, both having 600-mm-diam impellers. One-third-octave-band sound power levels were measured by microphones at essentially free-field locations and in the test ducts for two installation configurations: "1… an open inlet and an anechoic outlet duct and "2… an open outlet and an anechoic inlet duct. Two parallelepiped ''rectangular box'' measurement surfaces and a hemispherical measurement surface were used for free-field tests. The requirement in ISO 3744 that the environmental correction be not more than 2 dB was not satisfied at some frequencies in the frequency range of interest. On average, the smallest environmental corrections were observed for the smallest measurement surface. Differences, as great as 4 dB, between the free-field sound power levels were largest at frequencies for which the environmental correction was also large. For plane sound waves, in-duct sound power levels were greater than the free-field sound power levels. At frequencies where higher-order-mode sounds propagated through the test ducts, sound power levels determined by the in-duct method were less than those determined by the free-field method. This paper discusses reasons for the differences between the sound power levels by the two methods. © 1995 Institute of Noise Control Engineering.

On Sound Power Determination in Flow Ducts

“…For engineering grade accuracy measurements, the environmental correction has to be K )2 dB over the entire frequency range of interest. In a European round-robin test, it was found not possible to satisfy this criterion for all frequencies (see references [42,43]). In the latter study, values of as much as K "4 dB were observed at certain frequencies, despite the fact that the environmental test conditions were favourable, considering practical situations.…”

Acoustic scaling of an axial fan with non-uniform inlet flows

“…The sound powers are measured using a 1/2" microphone flush-mounted on the duct. According to Neise (1995), in the frequency range of higher-order mode sound propagation, the induct sound power levels were lower than the freefield sound power levels. Table 3 gives the measured sound power indices of the rotational speed for each case of non-uniform inflow conditions.…”