DANSK DSF/FPREN ISO 11691 PDF

1.1 General This document specifies a laboratory substitution method to determine the insertion loss without flow of ducted, mainly absorbent, circular and rectangular silencers, as well as other duct elements for use in ventilating and air-conditioning systems. NOTE 1 – Laboratory measurement procedures for ducted silencers with superimposed flow are described in ISO 7235. This document is applicable to silencers where the design velocity does not exceed 15 m/s. As the method does not include self-generated flow noise, this document is not suitable for tests on silencers where this type of noise is of great importance for the evaluation of the silencer performance. As most silencers, particularly in offices and dwelling, have design velocities below 15 m/s this document can often be a cost efficient alternative to ISO 7235. The insertion loss determined according to this document in a laboratory will not necessarily be the same as the insertion loss that will be obtained in an installation in the field. Different sound and flow fields in the duct will yield different results. In this document the sound field will be dominated by plane wave modes. As this document requires regular test ducts, the results may include some flanking transmission via structural vibrations in the duct walls, that sets an upper limit to the insertion loss that can be determined. NOTE 2 – ISO 7235 gives methods for determining this limit. This document is intended to be used for circular silencers with diameters of 80 mm to 2000 mm or rectangular silencers with cross-sectional areas within the same range. 1.2 Measurement uncertainty Due to lack of knowledge the determination of the uncertainty of results obtained from measurements according to this document does not comply with ISO/IEC Guide 98-3[2]. Exact information on the precision of the method cannot be given at this time. Therefore this document is denoted a survey standard. However, due to a simpler and more narrowly defined test set-up it is estimated that this method will have a standard deviation of reproducibility, σR, as defined in ISO 5725-2, which is equal to or better than that of ISO 7235.