The head shadow effect is when sound traveling through the air from one side is blocked or reflected by the head. This creates an acoustic shadow on the opposite side of the head, causing the sound to be heard more softly in that ear. In humans, this occurs in the high frequencies, causing sounds above approximately 1300 Hz to be affected.
The head shadow effect is used to localize sounds around us. If a sound is coming from directly in front of us, it will reach both ears at the same time and intensity. If a sound comes from our side, it will reach the ear on that side at a louder intensity than the opposite ear. This gives our brain the information we need to determine which side the sound is coming from. This is known as interaural intensity difference.