The ear has some natural directional characteristics, mainly due to the presence and shape of the pinna (outer ear). These pinna effects are important for the ability to decide if a sound is coming from in front or behind, especially for frequencies around 2-8 kHz.
One example is the pinna shadow effect. Sounds coming from the front reach the ear canal almost directly, whereas sounds coming from behind are obstructed by the pinna and therefore attenuated before they reach the ear canal.
This pinna shadowing is most pronounced for frequencies around 2 to 5 kHz, where sounds coming from behind are attenuated by 3-4 dB relative to sounds coming from the front. This natural 3-4 dB sound level difference is important for the listener to know whether a sound source is in front or behind.