HRTF is an acronym that stands for Head Related Transfer Function. HRTF is a phenomenon that describes how an ear receives sound from a sound source. When a sound is made, it travels through space in every direction in a sound wave. This wave of sound expands outward from the sound source in every direction, like a rapidly expanding sphere.
Sounds reverberate off of objects near the sound source and the sound waves reach the listener from many different directions and sometimes reach the listener's ear canal directly. When the sound waves reach the listener, they are transformed by the listeners body. The ears, head, shoulders and even the torso contribute to HRTF. Most notably, the size and mass of the head, the shape of the ear, the length and diameter of the ear canal, and the dimensions of the oral and sinus cavities all manipulate the incoming sound waves by boosting some frequencies and attenuating others. These changes in the frequency profile of a sound help create a unique perspective and perception for the listener. These changes also help the listener pinpoint the location of the sound source. HRTF is the change of the sound source's frequency profile by the unique characteristics of the listener.