Signal Calibration
Output levels of the signals were calibrated at the reference point which was 30 cm away from the loudspeaker using a sound level meter (Norsonic, Lierskogen, Norway) both in SPC and OR by a step of 10dB. Procedure of calibration was illustrated in our previous paper, and the averaged background noise level was 20.59 A-weighted dB in SPC and 48.46 A-weighted dB in OR(9).