The external ear, which consists of the pinna, concha, and auditory meatus, gathers sound energy and focuses it on the eardrum, or tympanic membrane (Figure 13.3). One consequence of the configuration of the external ear is that it selectively boosts the sound pressure 30- to 100-fold for frequencies around 3 kHz. This amplification makes humans especially sensitive to frequencies in this range—and also explains why they are particularly prone to acoustical injury and hearing loss near this frequency (see Box A). Not surprisingly, most human speech sounds are distributed in the bandwidth around 3 kHz. Most vocal communication occurs in the low-kHz range because transmission of airborne sound is less efficient at higher frequencies, and the detection of lower frequencies is difficult for animals the size of humans.

A second important function of the pinna and concha is to selectively filter different sound frequencies in order to provide cues about the elevation of the sound source. The convolutions of the pinna are shaped so that the external ear transmits more high-frequency components from an elevated source than from the same source at ear level. This effect can be demonstrated by recording sounds from different elevations after they have passed through an artificial external ear; when the recorded sounds are played back via earphones, so that the whole series is at the same elevation relative to the listener, the recordings from higher elevations are perceived as coming from positions higher in space than the recordings from lower elevations.