Fig. 1.

Acoustic properties of a spoken trilled <r>, low-pass–filtered at 800 Hz, to focus on temporal fine structure ITD. (A) Spatial configuration obtained by virtual acoustics. Direct sound presented from −30° (left leading), together with two delayed copies from the right (+65°; 4-ms delay and +130°, 8-ms delay). The signal has a negative direct-to-reverberant ratio [−4 dB; mean ILD: +3 dB (right louder); mean ITD: +140 µs (right leading)] and an even larger amplitude weighted mean ITD (+252 µs). It might, therefore, come as unexpected that informal tests revealed for all 10 participants a localization of the sound toward the left hand side, similar to the direct sound presented without reverberation. The envelope slope weighted mean ITD (−197 µs) appears to be a better indicator for perceived localization. (B) Acoustic signals at the left and right ear. The regular modulation pattern with ∼45-ms cycle duration is evident. Because the ITD cannot be identified by eye at this scale, the sign of the ITD is indicated by color-coding the Hilbert envelope (pale blue for left-leading ITDs and pale red for right-leading ITDs while within the human physiological range of ±700 μs). The ITD is consistently left-leading at the beginning of each cycle but right-leading during the peak or end of the modulation cycle. (C) A magnified three-cycle segment from B is replotted together with the ITD (green). In this resolution, the fine-structure phase is visible. ITDs outside of the physiologic range occur during incoherent segments and are not plotted. (D) The same segment as in C but without any reflections; only the sound directly from the source arrives at the ears. A constant left-leading ITD is visible.