Just as our head blocks light (and our parents’ advice), it also blocks sound traveling through the air. This creates an acoustical shadow behind the head; an area with greatly reduced sound energy. Small objects only reflect high-frequency sound waves with short wavelength. Larger objects, on the other hand, reflect low-frequency sound waves with long wavelength. Small objects don’t create a sound shadow for lower frequencies. As a rule of thumb, the sound is reflected if the wavelength is shorter than the diameter of the object. For humans, this means that frequencies above around 1.3kHz are reflected, producing a head shadow; frequencies below 1.3kHz aren’t reflected and no head shadow is created. We use the head shadow to ascertain the position of a sound wave in the horizontal plane. If a sound comes from the front, then it reaches both our ears with the same intensity. However, if it comes from the side, because of the head shadow, it reaches the ear that is turned away with considerably less intensity than the ear turned toward it. Differences in intensity can sometimes be so large that the acoustic pressure on one ear is over 1000 times higher than on the other. This is known as interaural intensity difference, or IID for short.