ear, external

ear, external The ear of a mammal is divided into three regions: the external or outer ear, the middle ear, and the inner ear. The external ear is the only part that is visible from the outside and is what people are usually referring to when they talk about their ears. It consists of a skin-covered flap known as the pinna or auricle, which leads like a funnel into the ear canal (external auditory meatus). Apart from the soft earlobe, the pinna possesses a framework of cartilage moulded to form several ridges and depressions, the most important being a cavity known as the concha, which lies just behind the opening to the ear canal.

The ear canal, which is also lined by skin, is a tubular structure, about 25 mm long, with a cartilaginous outer region and a bony inner region. Fine hairs and glands (which secrete wax) are found in the outer part. The ear canal terminates at the eardrum (tympanic membrane), which forms the boundary between the external ear and the middle ear. The external ear is responsible for collecting airborne sounds and for protecting the delicate eardrum from mechanical damage.

Although experiments carried out in the late nineteenth century provided insights into the role of the external ear in hearing, many textbooks give scant attention to it and even state, quite erroneously, that it is a vestigial structure in human beings, having little function compared with the larger, more erect, and often moveable ears of other mammals. This is largely due to the fact that research on hearing in the twentieth century mainly involved the delivery of sound by headphones, which cover and therefore remove the influence of the pinnae. However, beginning 30 years ago, acoustical measurements using tiny microphones inserted into the ear canals of either real or artificial pinnae have shown how incoming sound waves are modified by the different components of the external ear in ways that aid our ability to detect and localize sounds in the environment.

The external ear has two key functions. Firstly, the resonances of the external ear, particularly of the concha and ear canal, increase the sound pressure at the eardrum for some frequencies of sound by as much as 20 dB. In adult humans, this gain in amplitude is greatest at sound frequencies from 2 to 7 kHz. Consequently, sounds in this frequency range are transmitted by the external ear most efficiently, which, in turn, contributes to an improvement in the listener's hearing sensitivity. (It may be significant that the sound of human speech is largely concentrated in this frequency band.)

Secondly, interaction of sound waves with the external ear provides information that helps in judging the location of sound sources. The primary cues for sound localization result from the fact that we have two ears, one on each side of the head. Sounds that lie to one side of the straight-ahead direction differ in their time of arrival and in the amplitude of the sound at the two ears. These differences can be detected by neurons in the brain, which underlie our ability to determine the direction of a sound source. However, studies in humans and other animals have shown that the external ear, by differentially filtering sounds from different directions in space, provides additional information. Therefore, filling the cavities of the pinna or inserting tubes into the ear canals to bypass the pinnae altogether leads to errors in localization of sounds, particularly in the vertical direction, and to a decreased ability to discriminate sounds in front from those behind. Although two ears are definitely better than one for recognizing the positions of sounds, the filtering of sounds in the external ear can enable us to localize sounds under circumstances where the so-called binaural cues are ambiguous or missing. Such monaural listening conditions occur not only in people who are deaf in one ear, but when a sound on one side is too quiet to reach one of the ears, because of the shadowing effect of the head.

The convolutions of the external ear, particularly the concha, act to increase or decrease the amplitude of different frequency components of a sound as it passes from the free field to the eardrum. These filtering effects are dependent on the location of the sound, giving rise to spectral patterns — characteristic variations in amplitude with frequency — that vary with both the horizontal and vertical angle of the sound source. These spectral cues are most useful when the sound contains energy over a wide range of high frequencies. Eliminating the influence of the pinna by wearing headphones — for example, when listening to music on a personal stereo — gives rise to the impression that the sound is located inside the head. However, if sounds in the free field are first recorded with microphones placed in the ear canal, hence spectrally transformed by the pinna, and then played back over headphones, the listener experiences the vivid illusion of a sound originating from a particular direction outside the head. Generating virtual space sounds in this way not only helps in the scientific study of sound localization: computer-generated virtual environments are also used in training simulators and are found in many amusement arcades.

Because no two ears are exactly the same, we might imagine that each person has to learn to use the spectral cues generated by the particular dimensions of his or her own ears. Thus, listeners usually localize less well when listening to sounds as filtered by another person's external ears.

Some mammals, such as cats and bats, can precisely and independently alter the shape and orientation of their pinnae. Dogs tend to ‘prick up their ears’, i.e. raise their pinnae. Cats readily direct their ears toward the source of environmental sounds, usually in concert with movements of the eyes and head. These movements of the pinna appear to optimize sound reception by placing it at the position where the increase in sound pressure provided by the external ear is maximal, and may also help in sound localization. In bats, pinna movements are additionally thought to play an important role in echolocation.

As well as their auditory function, the external ears can help in the regulation of an animal's body temperature, via control of their extensive blood supply; and they may contribute to the threatening gestures made in encounters with other animals.

Andrew J. King

Bibliography

Carlile S. and and King, A. J. (1993). From outer ear to virtual space. Current Biology, 3, 446–8.
Pickles, J. O. (1988). An introduction to the physiology of hearing, (2nd edn). Academic Press, London.

See also hearing.