You want to go out, but you can't find your phone, what do you do?
You get someone to call it!
Ah ha, here it is, I'm an idiot.
But how exactly does your brain figure that out?
Cause your eyes have kinda got it easy.
Light bounces off everything you can see and it hits your retina as the same shape as the
scene in front of you.
So if you can see your phone, you kindof automatically know where it is relative to everything else.
But to hear where something is, all you've got is sound waves.
Concentric ripples in the air, coming from somewhere out there in 3D space.
Now even if we collapse it into 2D, its as if, and I've stolen this analogy from a Canadian
psychologist called Al Bregman, there is a great big lake and out on the lake there are
boats and ducks and jet skis, and you've got two tiny little channels coming off the lake.
Those are your ear canals, and based just on the waves going up and down on those little
channels, you brain can figure out where all the boats are, and the ducks and the mobile
phones.
How do we even start?
Well, if we begin with, if a sound starts over on your left, it will be louder in your
left ear than your right and that's because your head is in the way.
There is a sound shadow which means there is a level difference between your two ears,
its much louder if the sound is way over here, then there's a gradient around, and it's even
in the middle, and then louder on the other side.
And that's really useful for high frequency sounds, but at low frequency sounds, the waves
in the air bend more around your head, and that means that the level difference isn't
as big.
So for low frequencies your brain still compares the two ears but this time it uses the fact
say a sound is over there this time reaches just slightly sooner than this one.
It's a timing difference and even for a sound it's all the way around here at 90 degrees,
it's only a teeny tiny 5 or 6 milliseconds.
So your brain has to be really precise with signals.
Now, we've got timing differences for low frequency sounds, and level differences for
high frequency sounds.
But actually nearly everything you hear is a mishmash of high and low so your brain normally
uses both of those cues.
Lets try them out, this bit will work a bit better if you put on some headphones.
Now this is what it sounds like if we make my voice slightly louder in your right ear,
it's as if I'm on your right hand side.
On the other hand if we make the levels normal again, and then make the sound come slightly,
just a few milliseconds sooner in your left ear, then it's as if, not exactly, I'm kindof
on your left side.
Now neither of those is perfect and that's because like I said your brain wants to use
both bits of information to tell where the sound is coming from.
But even if we change both of them at the same time, it still doesn't sound quite right
and that's because there are other factors going on as well.
Including these guys, not just decorations, they filter the shape of the sound, a bit
like those graphic equalisers we all had in the 90's.
That filtering helps us figure out sounds that are up here, or down there or in front
or behind, when comparing the two ears, it doesn't make any difference.
And if you think these are cool, barn owls have special ruffs of sound protecting feathers
that are asymmetrical and actually slow down sound waves on their way into their ears,
so they can make much more of those differences and they can pinpoint dinner squeaking or
rustling at 20 paces in the dark.
I'll give your ears a rest now but for more sound science check out James May on noise
cancelling headphones and for a regular dose of multisensory stimulation subscribe to Head
Squeeze.
Now where's my phone...