If you are redistributing all or part of this book in a print format, Want to cite, share, or modify this book? This book uses the Rarefactions are formed when the molecules are displaced away from the equilibrium position. Compressions are formed when molecules on either side of the equilibrium molecules are displaced toward the equilibrium position. Notice that the displacement is zero for the molecules in their equilibrium position and are centered at the compressions and rarefactions. The blue graph shows the displacement of the air molecules versus the position from the speaker and is modeled with a cosine function.
(b) Sound waves can also be modeled using the displacement of the air molecules. Note that gauge pressure is modeled with a sine function, where the crests of the function line up with the compressions and the troughs line up with the rarefactions. Pressures vary only slightly from atmospheric pressure for ordinary sounds. The red graph shows the gauge pressure of the air versus the distance from the speaker.
#Book into the waves series
After many vibrations, a series of compressions and rarefactions moves out from the speaker as a sound wave. As the speaker oscillates, it creates another compression and rarefaction as those on the right move away from the speaker. Note that sound waves in air are longitudinal, and in the figure, the wave propagates in the positive x-direction and the molecules oscillate parallel to the direction in which the wave propagates.įigure 17.3 (a) A vibrating cone of a speaker, moving in the positive x-direction, compresses the air in front of it and expands the air behind it. The air molecules oscillate in simple harmonic motion about their equilibrium positions, as shown in part (b). As the speaker moves in the negative x-direction, the air molecules move back toward their equilibrium positions due to a restoring force.
As the speaker moves in the positive x-direction, it pushes air molecules, displacing them from their equilibrium positions. In solids, sound waves can be both transverse and longitudinal.)įigure 17.3(a) shows the compressions and rarefactions, and also shows a graph of gauge pressure versus distance from a speaker. (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 speaker-they are the disturbance that is a sound wave. But a small part of the speaker’s energy goes into compressing and expanding the surrounding air, creating slightly higher and lower local pressures.
As the speaker oscillates back and forth, it transfers energy to the air, mostly as thermal energy. In Figure 17.3, a speaker vibrates at a constant frequency and amplitude, producing vibrations in the surrounding air molecules. When the resonant frequency is reached, the glass shatters.Ī speaker produces a sound wave by oscillating a cone, causing vibrations of air molecules. As the frequency of the sound wave approaches the resonant frequency of the wine glass, the amplitude and frequency of the waves on the wine glass increase. This video shows waves on the surface of a wine glass, being driven by sound waves from a speaker.
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