1.1 ,This guide is intended to provide a simple and consistent procedure for the in-situ field measurement of in-duct sound pressure levels in large low pressure industrial air ducts, such as for gas turbines or fans, where considerations such as flow velocity, turbulence or temperature prevent the insertion of sound pressure sensors directly into the flow. This standard guide is intended for both ambient temperature intake air and hot exhaust gas flow in ducts having cross sections of four (4) square meters, or more.