BS EN ISO 11691:2009

1.1 General This International Standard 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 International Standard 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 International Standard is not suitable for tests on silencers where this type of noise is of great importance for the evaluation of the silencer performance. The insertion loss determined according to this International Standard 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. As this International Standard 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 International Standard is intended to be used for circular silencers with diameters of 80 mm to 2 000 mm or rectangular silencers with cross­sectional areas within the same range. 1.2 Measurement uncertainty Exact information on the precision of the method cannot be given at this time. Therefore this International Standard is denoted a survey standard. lnterlaboratory tests are necessary for the determination of the standard deviation of reproducibility, σR, of the method (relevant terms and methods are given in ISO 5725-1). It is, however, estimated that this method will have a σR which is comparable to that of ISO 7235. See table 1. Table 1 - Estimated values of the standard deviation of reproducibility Midband frequencies of one-third-octave band Hz Standard deviation of reproducibility, σR dB 50 to 1 250 1 600 to 10 000 2 3