This International Standard specifies the acoustical performance requirements for reference sound sources: ⎯ temporal steadiness (stability) of the sound power output, ⎯ spectral characteristics, ⎯ directivity. Temporal steadiness is defined in terms of the standard deviation of repeatability (see 5.2). The performance requirements on directivity index can only be verified in a hemi-anechoic room (see 5.5.) The spectral characteristics can be verified in either a hemi-anechoic room or a reverberation test room from measurements of the frequency band sound power levels according to this International Standard (see 5.4). This International Standard also specifies procedures for providing level calibration data and uncertainty on a sound source intended for use as a reference sound source in terms of its sound power level under reference meteorological conditions as defined in Clause 4 in octave and in one-third octave bands, and with frequency weighting A. This International Standard is titled as a calibration standard even though the method is conducted in a test laboratory and the level calibration results are not directly traceable to national standards of measure in a strict metrological sense. Test laboratories performing this method, and users of the calibration data that results from this method, are not expected to meet all requirements normally associated with a calibration laboratory. NOTE – ISO/IEC 17025 specifies different requirements for the competence of testing and calibration laboratories respectively. Laboratories testing reference sound sources would typically comply with the requirements for testing laboratories but not necessarily with those for calibration laboratories. This International Standard specifies methods to calibrate reference sound sources not only in a free field over a reflecting plane but also in reverberation test rooms at different distances from the boundary surfaces. For the position of the reference sound source on one reflecting plane, the two different test environments mentioned above are considered equivalent for frequency bands above or equal to 200 Hz. Below 200 Hz some systematic differences may occur (see 11.2). For frequencies below 100 Hz an alternative calibration method using sound intensity is given. The sound source may either be placed directly on the floor or mounted on a stand to be used at a certain elevation above the floor. According to this International Standard stand-mounted sources have to be calibrated in reverberation test rooms. Floor-mounted sources can be calibrated both in hemi-anechoic and reverberation test rooms. For floor-mounted sources in hemi-anechoic rooms, this International Standard is valid only for sources whose maximum vertical dimension is less than 0,5 m and whose maximum horizontal dimension is less than 0,8 m. According to this International Standard only floor-mounted reference sound sources may be used when carrying out measurements on a measurement surface. For reference sound sources to be used or calibrated under reverberant conditions, no such restrictions on maximum dimensions apply.