Stereophonic Hearing

Yesterday I had a clear illustration of how the brain determines the direction of a noise. I was listening to a podcast in ear buds when my wife asked me something. I took one bud out and talked to her for a while as the podcast continued to chatter in my left ear. Then the cordless phone rang. And I kept spinning around, trying to hear what direction the phone was in, but I couldn’t! You need two ears to pinpoint direction just as you need two eyes for stereoscopic vision.

Comments

  1. #1 Jonathan Lubin
    September 7, 2009

    Right you are. The most infuriating thing about having my hearing go south is that I can no longer tell where sounds are coming from.

  2. #2 Marcus B
    September 7, 2009

    Made me think of this most awesome demonstration of stereophonic hearing: http://www.youtube.com/watch?v=IUDTlvagjJA

    You will need earphones/buds.

  3. #3 DeafScientist
    September 8, 2009

    Welcome to my world. I was born with a roughly 130 dB loss in one ear (and a 50-55 dB loss in the other). You get daft things like, as a kid, asking your parents what room in the house they’re in, only to get “here” in reply. To which I’d reply “where’s here?”

    Naturally, I try keep a mental note of where I stuck the cell phone. That sort of thing becomes automatic after a while.

    There’s some very interesting research on stereophonic hearing. It’s amazing the whole thing works, given how slow neurons are.

  4. #4 Martin R
    September 8, 2009

    I wonder if a person with only one working ear can learn to determine direction by turning their head while the noise goes on. That way you’d at least be able to hear when your head is between the source of the noise and your working ear.

  5. #5 DeafScientist
    September 8, 2009

    I’ve seen this explained at least two different ways. My explanation is a bit tentative as I’m short on time to check my understanding of this right now.

    One explanation is that your head gives a physical screening effect for shorter-wavelength sounds at least. My impression is that it’s a bit limited in that this doesn’t work well for sounds that are weaker (it’s not as an effective way of picking location so a better “signal” is needed than for binaural cues) or for lower frequency sounds (longer wavelengths “reach around” the head, so to speak). In my anecdotal experience, it’s also less effective for sounds from further away, possibly because of echo effects of walls, etc. confusing the source of the sound. Obviously the sound has to be continuous, as you say, as you need to move your head around and “sample” directions to get anything more that the vaguest notion of where the sound is coming from. (Your body can act as a screen too, ditto for holding a hand to your ear.)

    A trickier explanation is that your outer ear (the bit sticking out on the side of your head!) is able to be used to locate the source of the sound to at least some degree. I’ll defer to better sources than me for that. It’s something I mean to learn some time and come to think of it I’ll probably make it one of the early articles of my blog when I finally get the thing up. Perhaps this works by inferring if the sound is received directly into the ear channel, or bounced off the outer ear through detecting if there are “echoes” from the received sound bouncing off the outer ear.

    I suspect the reality is a mixture of the two effects and fully understanding it might end up being quite complex and subtle!

    I find it tricky to locate ringing sounds, as it takes quite a while “sampling” rings at several positions to deduce the location of the sound. In my country, the telephone companies offer up to 8 rings before they transfer the caller to my message, so I don’t have a lot of leeway for getting it right! (The default is just 3 and this reminds me it’s probably still three at my new address… another job to do…) It practice is better to remember where the infernal phone “got to”.

  6. #6 Leif
    September 9, 2009

    When hunting I allways use earmuffs with microphones to:
    1/ protect my hearing when shooting (sound is cut at 60dB)
    2/ have the opportunity to raise the volume and thus hear oncomming animals as early as possible.

    The problem is that the microphones points ahead, so my hearing is directed forward. The microphones are stereophonic but I´m almost without hearing backwards.

  7. #7 Leif
    September 9, 2009

    -> DeafScientist
    At my kids preschool there are two kids with coclear implants. They got the implants one at a time, so they went from deaf to mono to stereo….

  8. #8 Martin R
    September 9, 2009

    Leif, I get a vague feeling that as a hunter, you aren’t quite 100% a Maglemose reenactor. (-;