How can your brain recognise if a sound is coming from in front of you or from behind you?

Question

I think I understand how the brain knows if a sound is to your left or your right, simply because you have an ear either side of your head, and if one ear is picking up a greater intensity of sound then your brain knows the source of the sound is to that sound. But how does your brain know if a sound is directly in front of you or behind you? I thought of this while listening to a wonderful rendition of Beethoven's Ninth this evening at the Barbican! Thanks in advance.

Answer 1

From a very young age, your brain actually learns by association to interpret sound reflected off your pinnae (the outer folds of your ears) as having a particular directionality.

Left-right directionality is distinguished by the delay and smaller amplitude of sound arriving at the ear further from the sound source.

Front-back and top-bottom directionality are distinguished by the phase-shifted (read: "delayed") reflections off the pinnae, relative to the sound that directly enters your ear.

A sound in front of you creates a characteristic echo pattern which you cannot consciously perceive, but interpret as a "frontwise" sound. A sound behind you creates a different echo pattern, because your pinnae are not front-back symmetrical.

FYI: The Doppler effect has nothing to do with it. The Doppler effect describes the apparent change in frequency when the received is traveling at a different velocity than the transmitter). To use the Doppler effect, one of your ears would have to be traveling at a different velocity than the other. That doesn't happen unless one or more ears is being ripped off.

Answer 2

I asked this exact question myself last year! :)

The answer I got was amazing: In addition to the brain using the lag in processing time to determine relative position to a sound, the shape and curvature of one's ears also acts as a filter for certain frequencies. Based on the way sound waves hit the eardrum, the brain is able to determine the direction from a sound came.

One of my answerers linked me to a fascinating study on the subjet - I'd do the same for you, but I'm on an iPod and can't copy links... :(

You can find the link in my profile, if you'd like. Hope this helps!

Answer 3

The Doppler Effect.
The intensity coming from the left or right would yes be louder in one ear than the other, but not that much. It's more of the Doppler Effect, and the eyes.