[Richard Steinke (OP)]

The latest Q&A show explained how the brain interprets the time delay between sound reaching the left and right ears at different times, to deterimine whether the sound is coming from the left or right.  But what about sound that eminates directly ahead or from behind?  It's easy to sense whether a sound comes from directly ahead or behind, yet such sounds reach both ears at the same time, so how is it possible to instantly know if it's in front or behind you?  Is there some other mechanism? Perhaps the brain uses other clues...

[Pwee]

Great question.

First of all, you have to know that if there is a sound coming directly from in front or from the back (mostly in laboratory situations), people sometimes get confused, so we have a bit harder time determining the direction of sounds like those.

In these cases we sometimes have to stop and turn our heads a little to be sure, so a tiny turn of your head can solve the problem immediately. It usually happens quickly and unconsciously.

One of the things that help us tell the difference in direction is the shape of our ears. They have curves and a parabola shape so the sound coming from the front and the back sounds slightly different.

I think slight differences in the right and left ear can help the brain determin locations too.

In the real world we can use many other cues from our other senses too, so you almost never have to face a situation where you notice this little flaw of our hearing.

[chris]

Excellent question; another ear-adaptation that makes this easier is the shape of the pinna - the bit that sticks out and funnel's sound into the ear. This makes hearing much more unidirectional (forward facing), meaning that when you experience a sound your brain can also compare the initial sound with the intensity of the echo. This will differ according to whether the sound is coming from in front of you or behind.

Chris