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Oscillatory Motion and Waves and Physics of Hearing.

128 Sound

Learning Objectives

  • Define sound and hearing.
  • Describe sound as a longitudinal wave.
Photograph of a glass, half of which is shattered into small pieces by a high-intensity sound wave. The tiny glass bits are shattered all over the place.
Figure 128.1: This glass has been shattered by a high-intensity sound wave of the same frequency as the resonant frequency of the glass. While the sound is not visible, the effects of the sound prove its existence. (credit: ||read||, Flickr)

Sound serves as a familiar and powerful example of wave behavior. Because hearing is one of our most essential senses, studying sound offers a natural bridge between physics and biology. Hearing is the perception of sound, just as vision is the perception of light. However, sound waves also have important applications that go beyond hearing. For instance, ultrasound—high-frequency sound waves above the range of human hearing—is widely used in diagnostic imaging and physical therapy.

From a physical perspective, sound is defined as a disturbance in matter that travels outward from a source. Sound waves are compressions and rarefactions in a medium such as air, water, or tissue. At the atomic level, this disturbance is far more organized than the random motion due to thermal energy. In many cases, sound waves are periodic and the atoms move in simple harmonic motion. This text will focus primarily on such periodic waves.

Consider a vibrating string as shown in Figure 128.2, Figure 128.3, and Figure 128.4. As the string oscillates, it compresses and expands the air around it, producing regions of high and low pressure. These pressure variations travel through the medium as longitudinal waves. In gases and liquids, sound waves are always longitudinal due to the lack of shear strength. In solids, both longitudinal and transverse sound waves can propagate. The graph in Figure 128.4 shows pressure variation as a function of position for such a sound wave.

Diagram of a vibrating string held fixed at both ends. The string is shown to move toward the right. The compression and rarefaction of air is shown as bold and dotted line arcs around the string.
Figure 128.2: A vibrating string moving to the right compresses the air in front of it and expands the air behind it.
Diagram of a vibrating string held fixed at both the ends. The string is shown to move toward the left. The compression and rarefaction of air is shown as bold and dotted arcs around the string.
Figure 128.3: As the string moves to the left, it creates another compression and rarefaction as the ones on the right move away from the string.
Part a of the diagram shows a vibrating string held fixed at both the ends. The string is shown to vibrate to and fro toward left and right. The compression and rarefaction of air is shown as bold and dotted arcs around the string. Part b shows a graph of pressure versus distance from the source. The pressure is along the y axis and the distance is along the x axis. The graph is a sine wave along the x axis.
Figure 128.4: After many vibrations, there are a series of compressions and rarefactions moving out from the string as a sound wave. The graph shows gauge pressure versus distance from the source. Pressures vary only slightly from atmospheric for ordinary sounds.

As sound waves propagate, their amplitude decreases with distance. This occurs partly because energy spreads out over a larger area and partly because energy is absorbed by materials. In the body, this is seen when sound waves enter the ear and are detected by the eardrum, as shown in Figure 128.5. Additionally, thermal energy is generated as sound waves cause air molecules to vibrate—some heat is transferred during compressions and less during rarefactions, leading to an overall energy loss. This process connects to the second law of thermodynamics, which governs the conversion of organized energy into disordered thermal motion.

Diagram of an ear is shown with sound wave compressions and rare factions entering the ear as semicircular arcs of bold and dotted lines. The cross section of ear drum marked as A is shown to vibrate to and fro with a force F equals P times A.
Figure 128.5: Sound wave compressions and rarefactions travel up the ear canal and force the eardrum to vibrate. There is a net force on the eardrum, since the sound wave pressures differ from the atmospheric pressure behind the eardrum. Vibrations are converted to nerve impulses and interpreted by the brain as sound.

PhET Explorations: Wave Interference

Make waves with a dripping faucet, audio speaker, or laser! Add a second source or a pair of slits to create an interference pattern.

Section Summary

  • Sound is a disturbance of matter that is transmitted from its source outward.
  • Sound is a type of wave, often longitudinal in nature.
  • Hearing is the biological perception of sound waves by the auditory system.

Glossary

sound
a disturbance of matter that is transmitted from its source outward
hearing
the perception of sound
definition

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