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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. (Figure)(a) shows the compressions and rarefactions, and also shows a graph of gauge pressure versus distance from a speaker. In solids, sound waves can be both transverse and longitudinal.)

(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), 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.