B. Calibration Process
A SLM is calibrated by comparing the sound pressures measured by a standard microphone and a target SLM in the same sound field. The standard microphone is calibrated in accordance with the standard reciprocity calibration method [7]. Before calibrating the SLM, both free sound field and pressure sound field should be calibrated. The two sound fields’ calibration processes are similar and the only difference between them is the position where to place the standard microphone. In free sound field calibration the standard microphone is located 1 meter away from the loudspeaker in the anechoic chamber while in pressure sound field calibration the standard microphone is inserted into the active coupler. Therefore we take free sound field calibration as an example to describe the processes. Firstly, the standard microphone is placed in front of the loudspeaker just 1 meter away from it in the anechoic room and the centres of standard microphone and loudspeaker