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Sound

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Sound
98
Chapter 3 Sensation and Perception
Hearing
䉴 How would my voice sound on the moon?
When Neil Armstrong stepped onto the moon in 1969, millions of people back on
earth heard his radio transmission: “That’s one small step for a man, one giant leap
for mankind.” But if Armstrong had taken off his space helmet and shouted, “Whoo-ee!
I can moonwalk!” not even an astronaut three feet away could have heard him. Why?
Because he would have been speaking into airless, empty space. Sound is a repeating
fluctuation, a rising and falling, in the pressure of a substance, such as air. Because the
moon has almost no atmosphere and almost no air pressure, sound cannot exist there.
Sound
Vibrations of an object produce the fluctuations in pressure that make sound. When
you speak, your vocal cords vibrate, causing fluctuations in air pressure that spread as
sound waves. Figure 3.13 shows how these changes in air pressure can be represented
as sound waveforms. The waveforms are drawn in only two dimensions, but remember that sound waves actually move through the air in all directions. This is the reason
that, when people talk to each other during a movie or a lecture, others all around them
are distracted by the conversation.
Just as the amplitude and wavelength of light waves affect our experience of light,
the characteristics of sound waves affect our experience of sound. The psychological
dimension of loudness is determined by the amplitude, or height, of the sound wave.
3.13
FIGURE
Sound Waves and Waveforms
Sound is created when objects, such as a tuning fork, vibrate. The vibration creates alternating regions of greater and lesser compression of air
molecules, which can be represented as a waveform. The point of greatest compression is the peak of the wave. The lowest point of the wave
is where compression is least. In each particular substance, or medium, such as air, a sound’s wavelength (the distance between peaks) is related to its frequency (the number of waves per second). The longer the wavelength, the lower the sound frequency. The shorter the wavelength, the higher the frequency.
More compression
of air molecules
Less
compression
More compression
of air molecules
Less
compression
Amplitude
Amplitude
Baseline
Wavelength
(one cycle)
Long-wavelength
(low-frequency) sound
Wavelength
(one cycle)
Short-wavelength
(high-frequency) sound
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