PHYSICAL FACTORS 



255 



should be localized at the same virtual angle. The solid lines of Fig. 6 

 show a stage layout used to test this hypothesis with the 2-channel 

 system. All points on each line have a constant ratio of distances to 



C-SCHANNELS 

 0°-3 CHANNELS 



LEFT 



CENTER 

 MICROPHONES 



Fig. 6 — Pick-up stage contour lines of constant apparent angle. 



the microphones. The resulting direct sound differences in pressure 

 expressed in decibels and the corresponding calculated apparent angles 

 are indicated beside the curves. The apparent angles were calculated 

 for an observing position on a line midway between the two loud speak- 

 ers but at a distance from them equal to the separation between them. 

 The microphones were turned face up at the height of the talker's 

 lips to eliminate quality changes caused by changing incidence angle. 

 It was found that a caller walking along one of these lines maintained 

 a fairly constant virtual angle. For caller positions far from the 

 microphones the observed angles were somewhat greater than those 

 computed. For highly reverberant conditions, the tendency was 

 toward greater calculated than observed angles. Reverberation also 

 decreased the accuracy of localization. 



A change of relative channel gain caused a change in virtual angle 

 as would be expected from loudness difference considerations. For 

 instance, if the caller actually walked the left 3-db line, he seemed to 

 be on the 6-db line when the left channel gain was raised 3 db. Many 

 of the effects of moving about the pick-up stage could be duplicated 

 by volume control manipulation as the caller walked forward and 

 backward on the center path. With a bridged center microphone 

 substituted for the two side microphones similar effects were possible 

 and, in addition, the caller by speaking close to the microphone could 

 be brought to the front of the virtual stage. 



