ISO 2151:2004 specifies methods for the measurement, determination and declaration of the noise emission from portable and stationary compressors and vacuum pumps. It prescribes the mounting, loading and working conditions under which measurements are to be made, and includes measurement or determination of the noise emission expressed as the sound power level under specified load conditions and the emission sound pressure level at the work station under specified load conditions. It is applicable to compressors for various types of gases, oil-lubricated air compressors, oil-flooded air compressors, water injected air compressors, oil-free air compressors, compressors for handling hazardous gases (gas compressors), compressors for handling oxygen, compressors for handling acetylene, high pressure compressors (over 40 bar/4 MPa), compressors for application at low inlet temperatures (i.e. below 0 °C), large compressors (over 1 000 kW input power), portable and skid-mounted air compressors, and rotary positive displacement blowers and centrifugal blowers and exhausters in applications of 2 bar/0,2 MPa or less. It is not applicable to compressors for gases other than acetylene having a maximum allowable working pressure of less than 0,5 bar/0,05 MPa, refrigerant compressors used in refrigerating systems or heat pumps, nor to hand-held portable compressors.