How can we tell whether a sound is coming from front or behind when we only have two ears?

Answer 1: 

These are two main cues for sound localisation: interaural time differences (a sound from the right will arrive at the right ear first) and inter-aural level differences (a sound from the right will be more intense in the right ear).

A sound from directly in front will produce the same relative arrival time and level in the two ears as a sound from directly behind.

These ambiguities can be resolved in two ways. First, we use head movements. Turning the head to the right, for instance, will increase the intensity and reduce the time of arrival in the left ear if the sound is ahead.

Secondly, we use cues from each ear. The external part of the ear, or pinna, in combination with the rest of the head, modifies the spectrum of the incoming sound in a way that depends on the angle of incidence.

It is believed that the brain learns to identify the characteristic spectral signature associated with each location in space, so a sound from in front will be associated with a different spectral pattern to a sound from behind.

Answer 2: 

For sounds that are equidistant from both ears, each ear receives essentially the same signal, but because the ears point forward there is a significant difference between the frequency responses for sounds that are incident from the front and back.

Sounds from the rear sound different to sounds from the front, because the fleshy part of the ear muffles high-frequency sound from behind the head.

Provided the brain has some absolute reference for the frequency content of the original signal it can work out whether the sound is in front, behind or above.

This means that familiar sounds with predictable, broadband spectra are easier to pinpoint.

The best example is human speech. The hardest sounds to localise it this way are narrow-band signals like electronic telephone ringing tones.

There is, however , a poorly recognised and more mundane mechanism for front/back localisation. One's hearing is influenced by one's sight. If one cannot see any apparent sound source, then the brain assumes it that it must be above or behind. The effect is strong enough to override decisions based on the frequency response. This effect has obvious survival implications for hunter gatherers.

Source : The Hindu