Design of terminating ducts for fan noise testing (Inter-Noise 2000)

Résumé et principaux résultats

Inter-Noise 2000 - Nice (France) - Août 2000

A new ISO Standard (ISO 13347) is being prepared by Technical Committee ISO/TC117/WG2 Industrial Fans - Acoustics, which describes different methods for the determination of fan sound power levels under standardized laboratory conditions. The measurement procedures cover the following fan installations: free inlet/free outlet, free inlet/ducted outlet, ducted inlet/free outlet, ducted inlet/ducted outlet. The sound levels are measured either in the duct, according to a procedure described in ISO 5136, or outside the duct. A duct in which the sound pressure is to be measured for determination of the in-duct sound power is called a "test duct". Such a duct has to be terminated with a good anechoic termination to minimize duct end reflections which contaminate the pressure signals measured in the duct. On the other end, a duct in which no sound measurements are to be made is called a "terminating duct". This duct still needs to be fitted with an anechoic termination in order to specify a standardized acoustic load impedance, as it is now well known that the sound power level radiated by a fan depends on the acoustic impedance of the ducts connected to the fan. The performance of the anechoic termination of a terminating duct does not need to be as high as that of a test duct. Standard specifies that below 160 Hz the maximum permissible pressure reflection coefficients for a terminating duct is twice higher than those of a test duct.
This paper presents results of tests carried out for defining "simplified" anechoic terminations for terminating ducts, which are less expensive and bulky than true anechoic terminations. This simplified termination is a set of simple cylindrical silencers in series with stepped outer diameters. It is shown that only two silencers suffice to conform to the reflection coefficients of the standard for duct diameters between 400 and 800 mm and a single silencer is suitable above 800 mm. The influence of the termination configuration on the fan sound power level measured on the side opposite to the terminating duct is also shown. This paper could be cited as a reference in the new standard.

Commissions

Ventilateurs

Thème

Ventilateurs

Mots-clés

Acoustique, Ventilateurs

Auteurs

GUEDEL Alain