The purpose of the pinna is to collect sound. It does so by acting as a funnel, amplifying the sound and directing it to the ear canal. While reflecting from the pinna, sound also goes through a filtering process which adds directional information to the sound (see sound localization, head-related transfer function, pinna notch). The filtering effect of the human pinna preferentially selects sounds in the frequency range of human speech.
Amplification of sound by the pinna, tympanic membrane and middle ear causes an increase in level of about 10 to 15 dB in a frequency range of 1.5 kHz to 7 kHz. This amplification is an important factor in inner ear trauma resulting from elevated sound levels.
The pinna works differently for low and high frequency sounds. For low frequencies, it behaves similarly to a reflector dish, directing sounds toward the ear canal. For high frequencies, however, its value is more sophisticatedly reckoned. While some of the sounds that enter the ear travel directly to the canal, others reflect off the contours of the pinna first: these enter the ear canal at a very slight delay. Such a delay translates into phase cancellation, where the frequency component whose wave period is twice the delay period is virtually eliminated. Neighboring frequencies are dropped significantly. This is known as the pinna notch, where the pinna creates a notch filtering effect.
The developing Pinna is first noticeable around the sixth week of gestation in the human foetus, developing from six rounded protuberances (the six hillocks of Hiss) which are derived from the first and second branchial arches. These hillocks develop into the folds of the pinna and gradually shift upwards and backwards to their final position on the head. En-route accessory auricles (also known as preauricular tags - see below) may be left behind. The first three Hillocks are dervied from the 1st branchial arch and form the tragus, crus of the helix and helix respectively. Cutaneous sensation to these areas is via the trigeminal nerve, the attendant nerve of the 1st branchial arch. The final three Hillockes are derived from the 2nd branchial arch and form the antihelix, antitragus and lobule respectively. These portions of the ear are supplied by the cervical plexus and a small portion by the facial nerve. This explains why vesicles are classically seen on the Pinna in Herpes infection of the facial nerve (Ramsay-Hunt Syndrome)