Greetings to All, First, thanks to Dave and Haigel for your insights and responses to my recent post. I know little about insects or arachnids, and had no idea scorpions have a keen sense of sound-source direction. This suggests the equivalent of an interaural time delay, but as it would apply to vibrotacticle sensation at scorpions’ feet or pectines. This is an educated guess because I can’t image much difference in motion intensity in the short distance between feet, particularly if the vibration was produced by a distant source. Speaking of interaural time delays, I have a concern or suggestion as to how educators teach the concepts of phase and time delay. In the speech and hearing sciences (as well as recording sciences), it is often taught that time delay and phase delay are intimately related (sometimes to the point of making phase and time seem synonymous). When talking about a complex signals (what most of the world consists of), this creates confusion. Wouldn’t it be better to teach, at least with sound in mind, that time-delay is akin to distance, not phase? Moving a loudspeaker back, say 10 meters, is equivalent to a 30 ms (approx.) time delay that is independent of frequency. All frequencies are phase delayed, but the amount of phase shift is different for every frequency for a given time delay (perhaps the source of confusion among students). But here’s something a little more interesting: If phase delay between ears were responsible for our ability to discern sound-source direction, then shifting the phase (apart from time delay) should provide a directional cue. Actually, I haven’t tried presenting a 1100 Hz tone to both ears with one side phase reversed and both ears receiving the onset of tone at the same time (i.e. appropriate phase shift for frequency/head dimension, but no time delay). In air, the 1100 Hz tone is “shifted” 180 degrees over 6 inches, which is approximately the distance between ears. Conversely, I can time delay the tone so that it is presented to one ear 450 usec ahead of the opposite ear, but the initial phase of the respective presentations are same for both ears. This is time delay without phase delay. The perception or sense of direction to phase delay versus (solely) time delay is not the same. I guess I bring this up because I’ve overheard discussions where someone is attempting to explain ITDs cues by saying they’re the same as a sound’s phase delay between the ears. Furthermore, I continue to see online blogs where students are asking for a clear (not textbook-written) explanation of the difference between phase and time delay. Admittedly, none of the above addresses surround sound or Ambisonics (or does it??), but I do wish to ask if anybody has listened to broadband sounds that have been shifted 90 degrees (relative to the opposite ear). Such a phase shift is no trivial task for broadband sounds. It is, of course, trivial to take a pure tone and delay it 90 degrees using a wave editor, time delay, or an all-pass filter tuned such that x frequency will be shifted pi/2 radians. I suppose a fixed, broadband shift could be accomplished via a Hilbert transformation or careful design of two sixth-order filters combined to provide the requisite phase shift over a wide frequency range (the latter would work in real time, though this isn’t requisite for a demo). I don’t know whether arbitrary phase shifts (apart from simple polarity reversal) result in image blurring or whatever, particularly under earphones where wave superposition in the sound field doesn’t occur. Note that this is hugely different from time delay because all frequencies are being shifted 90 degrees regardless whether there is or isn’t a time delay. It’s also different from all-pass filters where phase shift is also frequency dependent (and usually 90 degrees at one and only one frequency). Has anybody out there built or listened to a circuit with a fixed, 90-degree phase shift across the audible or speech frequencies? Again, the complex reference signal would be presented to one ear, and the processed signal presented to the opposite ear. Could be interesting, or could be totally uneventful. At least it would demonstrate the differences between time delay and phase shift. Thanks for reading. Best to All, Eric Eric L. Carmichel Cochlear Concepts -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20130205/b8be14b7/attachment.html> _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
