the human visual system responds to electromagnetic radiation in the
wavelength range of 400 nanometers to 700 nanometers, so-called
visible light. As discussed in Chapter 11, some animals can sense
electromagnetic radiation of wavelengths shorter than visible light
(ultraviolet) or longer than visible light (infrared). Note that for elec-
tromagnetic radiation, wavelength tends to be the preferred unit of
description, although frequency could also be used. In fact, frequency
is often used to describe microwaves and radio waves. For sound, the
human auditory system is sensitive to air pressure variations in the
range of approximately 20 to 20,000 Hz. Frequency is generally the
preferred unit of description for sound, although wavelength could
also be used.
Higher-frequency (that is, faster) pressure variations are experi-
enced as sounds having higher pitch or tone (Fig. 15.3, left). Lower
frequencies of pressure variation are experienced as sounds having
lower pitch or tone (Fig. 15.3, right). The loudness of a sound is associ-
ated with the amplitude or magnitude of the pressure variation, with
high amplitude variations experienced as louder than low amplitude
variations (Fig. 15.4).
VAVAVAN
Figure 15.4. Sine waves with higher (left) and lower (right) amplitudes.