How do we tell where sounds are coming from?

14 October 2012

Interview with

Owen Brimijoin, MRC Institute of Hearing Research, Glasgow

Chris -   Pleasure.  Let's go back to Owen because Owen, one of the things that was mentioned when Alan was talking to Soren about hearing aids was this whole concept of binaural hearing, how you compare what the right or left side of the brain is getting from each ear.  You work on this.  How important is this for us when we're just going about our daily business?

EarOwen -   It's very important because it's actually one of the few rock solid cues that you have to determine where a sound is coming from.  There are 3 basic ways you can do it, 2 of them do involve comparing the information at the 2 ears.  If you have a sound say, on your right, it will be louder in your right ear, and it will be quieter in your left ear in a small part because it's further away, but mostly because the head is in the way, it creates an acoustic shadow, its said.  And then similarly, it takes longer, it takes longer time for the sound to pass to the further ear and the brain is exquisitely sensitive to these subtle differences in timing.  At the greatest, they're really only about a half a millisecond. 

Chris -   Jonathan Manning has got in touch with a lovely question.  It made me laugh, but then think.  So maybe he deserves some Ig Nobel nomination for this.  He says, "Can people with massive heads locate sounds better or more quickly than people with smaller heads?"

Owen -   Right, yes.  I'd  have to say - I've never seen a study entitled, "Sound localisation ability as a function of head size" but I have to say, yes so if your sensors are further apart, then the cues will be larger.  They'd be exaggerated.

Chris -   What about front to back?

Owen -   Right, that's where the 3rd cue comes in, the spectral shaping properties of your ear.  Everyone's got different folds and different ridges in their outer ears and sounds coming from different directions kind of bounce off it in different ways and essentially, give it a different timbre. Just as you could tell the difference between an oboe and a violin playing the same note, so too can you tell the difference between the sound that's ahead, and behind because it just sounds different.  That's a way to kind of get around that problem that sound in front arrives at the 2 ears at the same time, but it also arrives at the ears at the same time if it's directly behind.

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