This International Standard specifies the acoustical performance requirements for reference sound sources: — temporal steadiness and repeatability of the sound power output, — spectral characteristics, — directivity index. The stability of the sound power output and the directivity index, for those sources where directivity is needed, are normally only determined in connection with pattern evaluation of the reference sound source. Because of the directivity measurements (for an exception see 5.5), pattern evaluations can only be performed in a hemi-anechoic environment. For regular verification measurements, only the frequency band sound power levels are normally determined. In this case measurements may be made in either hemi-anechoic or reverberant conditions. This International Standard also specifies procedures for calibrating a sound source intended for use as a reference sound source in terms of its sound power level under the reference condition that the characteristic impedance of air (ρc) is equal to 400 Ns/m3 in octave and in one-third-octave bands, and with frequency weighting A. Different procedures are specified for pattern evaluation and verification. NOTE Reference sound sources may also be used for measurements in one-half-octave bands, e.g. for ISO 9295. However, under these circumstances the stability and reproducibility limits stated in this International Standard will not apply. This International Standard specifies methods to calibrate reference sound sources not only in a free field over a reflecting plane but also in reverberation 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 100 Hz. Below 100 Hz the measurement uncertainties are significantly different (see Table 1). This International Standard is applicable to a sound source which is intended for use as a reference sound source. 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. For floor-mounted sources, 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.