Most room acoustic parameters are derived from the shape and slope of the measured energy decay curve, regardless of the actual sound level. However, several parameters, such as Sound Strength 𝐺𝐺 and Speech Transmission Index STI, are based on the actual sound pressure level at the receiver position and can only be derived using a calibrated measuring system. The ISO 3382-1 standard describes two methods for calibrating a measuring system in well-known environments: a Reverberation or an Anechoic chamber. Using these methods, it is not necessary to know the actual sound pressure levels involved in the parameters, but the relative difference from a reference value. In both cases, the equipment is first calibrated in the test chamber. Afterwards, the calibrated equipment is moved to the room under testing, for the actual measurements. Any settings and parts of the equipment used in the calibration chamber must be identical to those used for the field measurements. This restriction may lead to errors in the measured results, due to accidental or necessary changes in the measurement setup. An effective solution has been presented by
Christensen et al., by adding an extra step on the calibration procedure. The method is called Twostep calibration and it registers the levels at the calibration chamber and compensates for them in case they have been changed in the room to be tested.

The proposed method removes the necessity of having strictly fixed equipment between the test chambers and in the ordinary room, but it does not remove the requirement of using a reverberation or anechoic chamber for the first part of the calibration. Given that these chambers are typically very difficult to access, calibration remains generally a tedious process for many acousticians that wish to perform a fast measurement of 𝐺𝐺 and STI. This paper presents a procedure where the acoustician can use calibration data from a reference source, provided that they use a ‘copy’ of that reference in the measured room. The presented method is inherently the Two-step calibration method, in which the first step in the calibration chamber does not need to be repeated by the acoustician. The proposed procedure could be applied to any commercially available source that is produced in series, provided that its directivity pattern remains the same across all copies. In that sense, sound sources with multiple loudspeaker drivers such as dodecahedrons may not suit the method. Typically, the drivers have varying sensitivities, and thus varying output levels. The overall directivity pattern of the source is affected by the sensitivity of each driver, so it becomes more uncertain with many drivers. Conversely, if the source consists of a single driver, the directivity pattern will remain generally the same, while the overall level can be adjusted by the two-step calibration method. In this paper, the proposed method is evaluated using several copies of Odeon Omni, which is a single-driver portable source. The pre-calibration is first performed for all copies in a reverberation chamber, and the measurement of 𝐺𝐺 in an auditorium is investigated using the pre-calibrated copies.