1.1 This International Standard specifies measurements on silencers in situ. It is applicable to measurements on silencers in practical applications for acoustic analysis, acceptance tests and similar evaluations. Results obtained in accordance with this International Standard cannot be compared to performance data obtained from laboratory measurements on ducted silencers in accordance with ISO 7235, partly because of different test conditions (such as sound field distribution, flow, temperature and mounting conditions) and partly because of different definitions. Depending on the method used, the measurement is either of insertion loss Dis, or transmission loss Dts. The measurement method depends upon the type of silencer and the installation conditions (e.g. insertion loss measurements must be carried out for blowdown silencers). NOTE 1 The subscripts denote the practical application of the silencer and the particular installation and operating conditions: “s?? stands for “in situ??, “t?? for transmission, and “i?? for insertion. Additional characteristic quantities, which could include measurements taken using artificial sound sources or measurements taken to determine the directivity of sound propagation from the silencer, may be agreed upon in accordance with this International Standard. 1.2 This International Standard is applicable to silencers which are installed either as a whole or in the form of individual baffles in the propagation path of sound (e.g. openings of ducts) originating from a sound source (machine, building, plant such as a gas turbine generator, scrubbing plant, cooling tower, heating ventilation and air conditioning (HVAC) plant, exhaust stack, air intake duct, weapon, internal combustion engine, compressor, etc.); all types of passive silencers (absorptive, reactive, reflection and blowdown silencer); active silencers (involving amplifiers and loud-speakers) as far as the insertion loss of passive silencers is equivalent to the off/on conditions of active devices; and other measures or means of effecting acoustic attenuation in air or other gases (e.g. components installed in ducting, louvres, grilles and deflector hoods). Additionally, this International Standard is applicable to the determination of the effect of cleaning or refurbishing silencers. This International Standard is not applicable to closed high-pressure systems (e.g. silencers in closed pipes) since measurements of structure-borne sound are not anticipated. 1.3 Quantities to be measured include the following: sound pressure levels in octave bands with centre frequencies at least from 63 Hz to 4 kHz and, if possible and required, from 31,5 Hz to 8 kHz or in one-third-octave bands with centre frequencies from 50 Hz to 5 kHz and, if possible and required, from 25 Hz to 10 kHz at a point or points on the source side of a silencer, at a point or points on the receiver side of a silencer; static and dynamic pressures, flow velocities and temperatures at selected positions. Operating data to be determined include flow rate, pressure and speed, which define the operating conditions of the machine or plant to be silenced.