Pinna, derived from the Latin word for feather, are what a casual observer usually refers to when they are pointing to your ears. The pinna, or auricles, as they are scientifically known, are the visible parts of the ears located on the outside of the head. The biological structure of the outer ear is composed mainly of cartilage, which provides the pinna with a great deal of flexibility and ability to be positioned for optimal listening.
The anatomy of the human pinna is split up into several major components: the helix, the antihelix, the concha, the tragus, and the lobe. The helix is the outer edge of the ear, which usually folds down and inward from the top. The antihelix is the Y-shape that is located just below the helix, and is the second-highest part of the ear. The concha is the hollow part of the ear located right next to the ear canal, and serves as the entrance to the inner ear. It is usually slightly covered by the tragus, the small flap of cartilage that faces backward. When listening to music through earbuds, the tragus is the protrusion that holds the earbuds in place. The final component, the lobe, is located at the bottom of the ear, and is the only part of the ear that is not cartilaginous, being made up primarily of fatty tissues. It serves no known biological function, and is the most common location for ear piercings.
The entire purpose of the outer ear is to collect sonic waves, redirecting them into the aural canal so they can be interpreted and sent to the brain. This is where the pinna's unique shape comes into play, causing them to act as funnels that amplify sonic waves and redirect them straight into the ear canal. In collecting and filtering these sonic waves, the pinna also perform several important secondary functions. The most important of these secondary functions is sound localization, which is the ability to pinpoint the origin or location of a sound after hearing it. The biological architecture of the pinna allows the listener to determine the direction that a sound came from, as well as the sound's distance from the ear.
The concept of sound localization is strongly linked to the idea of head-related transfer function (HRTF) because it allows human beings to locate sounds in three dimensions. Because of HRTF, sounds can be located above, below, in front of, behind, and to either side of the human head. This is due to the fact that the pinna, along with the brain and inner ear, allow us to create a three-dimensional mental map pinpointing the source of a noise. When a sound is heard by both ears, the differences in timing and reception angle for each ear allow the listener to figure out where the sound is at relative to the body and how close the source is. Many species, humans in particular, use this biological mechanism to pick up the slack and supplement the limited range of perception that they receive from their eyes. Since the eyes only allow most species to see a small part of the world around them, the ears serve a crucial function, allowing the listeners to determine if something requires attention.