10 ms·

I guess that makes sense considering we use the delay of sounds reaching each ear separately to determine the direction sound comes from. Assuming the speed of

• smcl 5y ago
  That's a nice estimation, but I would have imagined direction is down to whether a sound is more muffled or quiet in one ear than the other.
  • Griffinsauce 5y ago
    That doesn't need to be the case when there are reflections.
    • smcl 5y ago
      I'm not sure I follow
      • jacquesm 5y ago
        A reflection can easily be louder than the original sound, but the longer time-of-flight allows the ear to distinguish between the two. That's how even in a hall with echoing walls you can still pinpoint a soundsource with relatively high accuracy.
        • datameta 5y ago
          The first chapter of The Sound Book by Trevor Cox goes into great detail about this (but without getting deep into the math of it). Well worth the read for those interested in the acoustics of architecture and our perception of how it modifies the soundscape.
          • jacquesm 5y ago
            Another interesting one is 'the acoustical foundations of music'. ISBN 0393090965
  • whiddershins 5y ago
    No it is difference in phase.
    • jacquesm 5y ago
      Only for LF.
  • vitus 5y ago
    To piggyback on the sibling comments: If you only determined direction based on relative volume between your ears, you wouldn't be able to distinguish between sounds in front vs behind you.
    • smcl 5y ago
      Yep that's why I added "muffled" (i.e. not 100% clear). Maybe it's worth clarifying that I wasn't trying to correct the commenter, just adding my original (naive? wrong?) belief about how this worked :)
      • Ensorceled 5y ago
        Since you sound sincere here ... I read your comment as disagreeing with and correcting the original comment.
        • smcl 5y ago
          Oops, it wasn't intended! I think maybe the "but" in my comment makes it sound that way?
          • Cederfjard 5y ago
            Yeah, to me it reads ”that’s a nice theory you have, but now I’ll tell you mine, which I think is the correct one”.
            • smcl 5y ago
              I just liked how they related the speed of sound to the distance between ears to explain the (possibly less than) ~0.5ms thing, and how it was nicer than the naive explanation I had previously assumed
    • ryandvm 5y ago
      You also wouldn't be able to do the cocktail party trick. Your brain's ability to ignore everything that isn't of the desired ITD phase shift is what allows you to selectively ignore most of the noise in the room and focus on a single conversation.
  • mattkrause 5y ago
    It's a combination of both. The relevant keywords are "Interaural Time Difference" (ITD; this phenomenon) and "Interaural Intensity (or Level) Difference" (IID/ILD; i.e., volume). In fact, there are a few other mechanisms too. The shape of the pinna (external ear) does some filtering that allows you to distinguish sounds that produce identical ITD/IID. The neuroscience of this is really fascinating, and the circuits have been worked out pretty well.
• jacquesm 5y ago
  We do much better than that, we use a combination of phase (arrival time of the sound-front if you wish at low frequencies) and amplitude to determine direction. Good article on this: http://alumni.media.mit.edu/~araz/sss/Sound_Localization.html http://alumni.media.mit.edu/~araz/sss/Sound_Localization.htm...
  • sudosysgen 5y ago
    Don't forget the transformation of the signal by the head, chest and outer ear! It's quite amazing.
  • jcims 5y ago
    Yeah I'm thinking the relative phasing of the group delay is essentially having a comb filter effect on the audio, and the difference is perceived qualitatively rather than anything approximating a time delay. For anyone not familiar, here's an example of comb filtering, where reflections interfere and you get wobbles in your frequency response (shown in an fft mid video). https://www.youtube.com/watch?v=Amj4UevyRfU https://www.youtube.com/watch?v=Amj4UevyRfU
• lebuffon 5y ago
  It's more involved than just left/right delay. We localize sound in the vertical direction and front and back as well. This is accomplished by the delays created between direct into the ear sound and reflections off the outer ear folds. These reflections create "comb" filters in the audio spectrum which we learn to associate with direction. Its remarkable. A test to prove this was so was to fill the outer ear with plasticine and perform localization tests on subjects. They could not localize sound in that condition. The early work as I recall was at the Heinrich Herz institute in the mid 1970s. I am suspicious that part of what this article is reporting is due to phase cancellation effects causing similar filtration that people can hear as timbre change rather than actually detecting the time delay. (Source: My recording engineering final paper)
  • EForEndeavour 5y ago
    Now I really want some plasticine / modeling clay to attach to parts of my pinnae and experiment with my ability to localize sounds. Have video games already emulated the spectral effects of sound direction for players using headphones, or even speaker systems with known spatial distribution? I can imagine modulating the sound coming from an in-game object to match its perceived source direction to its location relative to the player.
    • kd5bjo 5y ago
      The problem is that each person's pinnae have a unique geometry, which makes the notches of the comb filter lie at different frequencies for everyone. As far as I know, there's no good way to determine this other than a direct measurement, which requires specialist equipment.
      • nitrogen 5y ago
        Video games, VR, and other headphone systems do use generalized HRTFs for directional audio. Edit: I'm currently working on a series of videos to explain sound direction and perception. Some of the code I wrote for my demos is or will be on GitHub.
      • 5y ago
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• robwwilliams 5y ago
  The headline is odd. There are populations of neurons in the auditory system—the medial superior olive—that receive input from both ears. Even in humans these MSO neurons are exquisitely sensitive to binaural differences. This is how the owl catches the mouse at night. In some species delays of 10-20 microseconds can be detected and encoded by MSO neurons even tracking up to frequencies well above 40 kHz. This is amazing when you realize that neurons cannot fire at a rate above 1 kHz. Phase locking and ensemble encoding is used. For example, adult mice have small heads, and ear separation is merely 5-7 mm; yet this is sufficient to locate the position of an ultrasonic squeak generated by a mouse pup at 40 kHz. This is a computational feat that requires extreme temporal precision in binaural auditory processing across comparatively noisy wetware (transduction noise, phase locking error, synaptic release noise, conduction velocity smear, dendritic integration in MSO). And doubly impressive given that the brain has no “given” time base or oscillator to define a compute cycle. We must all build and refine our own internal set of pseudo-clocks for sensory and motor systems, in order to define the cumulative temporal context in which we are embedded. This is crucial for the mouse to quickly avoid the talons of the owl. More on timing in brain: @robwilliamsiii (see pinned tweet).
  • randlet 5y ago
    Thanks for this super interesting comment. The complexity of life never ceases to boggle the mind.
    • sam_goody 5y ago
      It is my understanding that what really happens is far more impressive. 1. When the sound is coming from, say, the back left, the difference between the ears is far less than the width of the head. 2. It is not that the brain hears on one side "later", it is that the brain is constantly hearing two different things, what is registered on the right and what is on the left, simultaneously. Since there are often sounds coming from both directions at once, there is no easy way to match the sounds up. Instead the subconscious has to hold each of the incoming sounds for some fractions of a second, and then combine the different sounds (with smell, etc) while matching up the different parts, using the differences to account for direction. Since this per-force takes time, the subconscious meanwhile sends a "image" to the consciousness with the assumed incoming data based on whatever had happened before [not on what is happening now], while processing the current data to predict what image will be sent to the consciousness during the next incoming round. This means we perceive a world created by the subconscious's guess of what should be happening now, based on past input. And the current input is held for processing. And STILL, the mouse is able to avoid the owl!
• amelius 5y ago
  > in half a millisecond it travels about 17 cm, which, I would guess, is larger than the average distance between our ears What is the speed and variability of neural signals traveling through the brain?
• frankus 5y ago
  I remember back in the late 90s I had a BeBox and there was this cool nodes-and-connections audio program where you could add various effects. My friend mentioned this delay effect and so we rigged up a graph where one stereo channel was delayed by some number of milliseconds. The effect was pretty uncanny. Wearing headphones, the sound seemed to be coming exclusively from the non-delayed side, until you removed that headphone and the delayed side was clearly still playing.
  • out_of_protocol 5y ago
    That's built-in echo-cancelling module, allowing you to easily getting correct direction to the true source of sound
    • nitrogen 5y ago
      AKA the Haas effect or precedence effect, making the first sound to arrive the most important for localization, even if echoes are louder.
• deleted 5y ago
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• jasonwatkinspdx 5y ago
  "Detect" for experiments like this is almost certainly an ABX test. That is you get to listen to samples A, B, and X repeatedly and are asked to decide whether X is A or B.