There's something funny going on with front-back perception. Normally it shouldn't be doable, but the external part of the ear is like a direction-dependent frequency filter. When the brain hears a familiar sound, but the spectrum is skewed just like this, it goes "aha, this comes from the front side". If the spectrum is skewed the other way, it goes "this comes from behind".
There's a lot more to it than that, and it's not just something that occasionally works. The outer-ear notch filtering is also used in vertical localization.
Also: people who depend on behind-the-ear hearing aids do not get the benefit of these echoes, which means that, for example, it’s harder for them to distinguish the voice of someone talking to them from background noise or from echoes off the walls.
Comments
Yes and no.
There's something funny going on with front-back perception. Normally it shouldn't be doable, but the external part of the ear is like a direction-dependent frequency filter. When the brain hears a familiar sound, but the spectrum is skewed just like this, it goes "aha, this comes from the front side". If the spectrum is skewed the other way, it goes "this comes from behind".
It's not 100% reliable, but it works some times.
http://en.wikipedia.org/wiki/Sound_localization
There's a lot more to it than that, and it's not just something that occasionally works. The outer-ear notch filtering is also used in vertical localization.
Also: people who depend on behind-the-ear hearing aids do not get the benefit of these echoes, which means that, for example, it’s harder for them to distinguish the voice of someone talking to them from background noise or from echoes off the walls.