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).