position. The amplitudes of the component frequencies have been magnified
(all by the same factor) so that their structures are easily visible.Figure 15.9 illustrates another Fourier analysis of sounds. Consider
several different notes played on the piano: middle C, E, G, and C (one
octave above middle C). These graphs illustrate Fourier spectra of
these four notes, with the horizontal axes representing frequency and
the vertical axes representing a measure of the amplitude of the var-
ious frequency components. The first major peak is the fundamental
frequency characterizing the particular note. The other peaks repre-
sent overtones—frequency components that give the sounds their
unique complexities and associated perceptual timbres. We'll return
to Fourier analysis shortly, because it is central to how the ear makes
sense of sound.
Hearing, or auditory perception, begins with the air pressure varia-
tions entering the outer ear. The most external structure of the ear,
the pinna, is the fleshy flap of skin attached to each side of the head
(Fig. 15.10). It functions as a funnel or antenna, collecting and fo-
cusing the vibrations of air pressure into the ear canal. The ear canal
terminates at the tympanic membrane or eardrum, a small drumskin-
like piece of tissue that is set in vibration when air molecules strike it.
The eardrum forms the boundary between what is called the outer ear
and what is called the middle ear.