B2.1 Localization cues. Drag the speaker or move the slider between −180° and +180° while watching the wavefront arrive at each ear. When the source is off-center, sound must travel further to one ear than the other. The extra time this takes is the interaural time difference (ITD). There is also an interaural intensity difference (IID) due to shadowing by the head. Now select the Sound checkbox and put on headphones; you should hear the sound move back and forth as you move the speaker. Next, listen with IID or ITD alone. Which is more effective?