FoundationalConceptsNeuroscience

Figure 15.1. A bell vibrating after a clapper strikes it generates a rhythmic

pattern of alternating compression and rarefaction that moves out into space

as a series of waves.

A sound wave moves through air at a speed of about 1,100 feet per
second, or 335 meters per second, or 750 miles per hour. For any mov-
ing wave, the velocity of movement is equal to the frequency of varia-
tion multiplied by the wavelength. Thus, for an air pressure variation
oscillating at 200 Hz, the distance of one cycle, or wavelength, is 5.5
feet:

Wave velocity = frequency x wavelength

1,100 feet/second = 200 cycles per second « 5.5 feet per cycle

The higher the frequency, the shorter the wavelength, and the lower
the frequency, the longer the wavelength—a relationship shared by all
propagating waves: sound waves, water waves in the ocean, electro-
magnetic radiation, seismic waves associated with earthquakes, and
soon.
The speed of light is approximately 186,000 miles per second, or
300,000,000 meters per second. This is very fast—certainly much
faster than the speed of sound. Thus, if one is observing an event that
has both visual and auditory features, the visual information will
reach the eye nearly instantaneously, while the sound information
may take noticeably longer. For example, consider lightning and the
associated thunder. If the lightning is 5 kilometers away, then the
flash of light will reach the observer’s eye very quickly, because light
traveling 5 kilometers arrives in about 17 microseconds. However,
the air pressure variations triggered by the lightning will travel at the
speed of sound, 335 meters per second, and thus will require about 3