![]() The new wavefront is tangent to the wavelets. The emitted waves are semicircular, and occur at t, time later. ![]() Where s is the distance, v is the propagation speed, and t is time.Įach point on the wavefront emits a wave at speed, v. A complicated mechanism converts the vibrations to nerve impulses, which are perceived by the person.\] There is a net force on the eardrum, since the sound wave pressures differ from the atmospheric pressure found behind the eardrum. Sound wave compressions and rarefactions travel up the ear canal and force the eardrum to vibrate. Wavelength, frequency, amplitude, and speed of propagation are important for sound, as they are for all waves. (These processes can be viewed as a manifestation of the second law of thermodynamics presented in Chapter 15.3 Introduction to the Second Law of Thermodynamics: Heat Engines and Their Efficiency.) Whether the heat transfer from compression to rarefaction is significant depends on how far apart they are-that is, it depends on wavelength. In addition, during each compression a little heat transfers to the air and during each rarefaction even less heat transfers from the air, so that the heat transfer reduces the organized disturbance into random thermal motions. But it is also absorbed by objects, such as the eardrum in Figure 5, and converted to thermal energy by the viscosity of air. The amplitude of a sound wave decreases with distance from its source, because the energy of the wave is spread over a larger and larger area. Pressures vary only slightly from atmospheric for ordinary sounds. ![]() The graph shows gauge pressure versus distance from the source. After many vibrations, there are a series of compressions and rarefactions moving out from the string as a sound wave. ![]() As the string moves to the left, it creates another compression and rarefaction as the ones on the right move away from the string. A vibrating string moving to the right compresses the air in front of it and expands the air behind it. In solids, sound waves can be both transverse and longitudinal.) Figure 4 shows a graph of gauge pressure versus distance from the vibrating string. (Sound waves in air and most fluids are longitudinal, because fluids have almost no shear strength. These compressions (high pressure regions) and rarefactions (low pressure regions) move out as longitudinal pressure waves having the same frequency as the string-they are the disturbance that is a sound wave. But a small part of the string’s energy goes into compressing and expanding the surrounding air, creating slightly higher and lower local pressures. As the string oscillates back and forth, it transfers energy to the air, mostly as thermal energy created by turbulence. In this text, we shall explore such periodic sound waves.Ī vibrating string produces a sound wave as illustrated in Figure 2, Figure 3, and Figure 4. In many instances, sound is a periodic wave, and the atoms undergo simple harmonic motion. On the atomic scale, it is a disturbance of atoms that is far more ordered than their thermal motions. The physical phenomenon of soundis defined to be a disturbance of matter that is transmitted from its source outward. Ultrasound, for example, is not heard but can be employed to form medical images and is also used in treatment. But sound has important applications beyond hearing. Hearingis the perception of sound, just as vision is the perception of visible light. Because hearing is one of our most important senses, it is interesting to see how the physical properties of sound correspond to our perceptions of it. Sound can be used as a familiar illustration of waves. While the sound is not visible, the effects of the sound prove its existence. This glass has been shattered by a high-intensity sound wave of the same frequency as the resonant frequency of the glass.
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