ISO 11691:2020

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    Laboratory measurement procedures for ducted silencers with superimposed flow are described in ISO 7235[5].
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[5].
The insertion loss determined according to this document in a laboratory is not necessarily the same as the insertion loss obtained in an installation in the field. Different sound and flow fields in the duct yield different results. In this document, the sound field is dominated by plane wave modes. Due to the use of regular test ducts, the results can include some flanking transmission via structural vibrations in the duct walls that sets an upper limit to the insertion loss that can be determined.
This document is intended to be used for circular silencers with diameters of 80 mm to 2 000 mm or for rectangular silencers with cross-sectional areas within the same range.