BS EN ISO 7235:2009

1   Scope This International Standard specifies methods for determining

— the insertion loss , in frequency bands, of ducted silencers with and without airflow,

— the sound power level, in frequency bands, of the flow noise (or regenerated sound) generated by ducted silencers,

— the total pressure loss of silencers with airflow, and

— the transmission loss , in frequency bands, of air-terminal units.

The measurement procedures are intended for laboratory measurements at ambient temperature.
  Measurements on silencers  in situ  are specified in [ISO 11820](/search?q=ISO%C2%A011820). 
It is to be noted that the results determined in a laboratory according to this International
  Standard will not necessarily be the same as those obtained  in situ  (installation), as different sound and flow fields will yield different results.
  For example, the pressure loss will be lower under laboratory conditions than  in situ , but will be comparable between different laboratories. 
This International Standard is applicable to all types of silencer including silencers
  for ventilating and air-conditioning systems, air intake and exhaust of flue gases,
  and similar applications. Other passive air-handling devices, such...