Binaural recordings of 400 Hz tones with 25 Hz sinusoidal amplitude modulations (0-100% depth) played from a loudspeaker in my dining room. Analyses show that ITD and ILD again resemble ‘ideal’ values before recent reflections arrive. Values then drift and fluctuate due to combinations of the modulations, acoustic reflections and background sounds. Modulations prompt contributions to spatial hearing from cues arising throughout the tones.
Interaural time differences (ITDs) computed across zero-crossings after passing each recording through a 400 Hz auditory filter (circles in upper panel). Red and blue lines in the lower panel show right and left amplitude envelopes, respectively.
- • ITDs fluctuate broadly prior to when each pure-tone starts (before 0 ms).
- • Values then contract for ~20 ms as amplitude rises, as would occur in a quiet, anechoic, setting.
- • Values next begin to drift and fluctuate after ~20 ms, due to modulations, acoustic reflections and background sounds.
When compared with recordings of pure-tones lacking modulations (greens lines):
- • ITD are seen to skew positively as amplitude declines but skew negatively as amplitude inclines. Deep amplitude throughs therefore prompt substantial increases in ITD variation over time.
- • ITDs coinciding with amplitude peaks attain values closer to those observed at each tone’s onset (before ~20 ms), as would occur in a quiet, anechoic, environment.
Binaural recordings for 0° azimuth, cropped 1 second before and after the recorded tone’s onset. As when listening to pure-tones without modulations over headphones, a listener is likely to hear a single compact auditory ‘image’, from a frontal direction, that remains in roughly the same position when channels for the left and right ears are reversed.
channels reversed
That cues throughout these modulated tones contribute substantially to spatial hearing is evident when recordings are again cropped so that they start 50 ms after each recorded tone’s onset. In contrast to pure-tones without modulations, listeners will likely hear auditory ‘images’ comparable to those of un-cropped recordings (above).
channels reversed