Sound is not a thing, but a traveling disturbance in matter
In short: We often think of sound as something that moves through the air, but the particles of the air aren't actually traveling with the noise. Instead, sound is a mechanical wave—a rhythmic pulse of pressure and energy moving through an elastic medium.
To understand sound, one must distinguish between the vibration and the sensation. According to the ANSI/ASA S1.1-2013 standard, sound is a dual phenomenon: it is both a physical oscillation in pressure or particle displacement within a medium, and the auditory sensation that results from that oscillation. In physics and engineering, the focus is often on the mechanical disturbance itself—the way a source, such as a vibrating loudspeaker diaphragm, creates compressions and rarefactions that propagate outward. This mechanical process involves variations in particle velocity and stress as the wave passes.
This propagation requires a transmission medium, which can be a solid, liquid, gas, or even plasma. Because sound relies on the internal forces of an elastic or viscous medium to move, it cannot travel through a vacuum, where no matter exists to support the disturbance. The speed at which these waves move is not constant; it is determined by a complex relationship between the medium's density, pressure, and temperature. Whether the medium is a crystal or water, the energy travels while the matter remains relatively stationary.
The study of these waves, known as acoustics, spans many disciplines, from the study of ultrasound to the engineering of architectural spaces. While the particles of the medium do not travel along with the wave, the mechanical energy does. This allows for a wide range of human perception, with the human ear typically sensitive to frequencies ranging from 20 Hz to 20 kHz, though the field of acoustics also explores the much lower infrasound and the much higher ultrasound. Specialists like acousticians study these vibrations, while audio engineers focus on the technical manipulation and reproduction of the resulting signals.
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Sources
- Source: CrashCourse — Sound: Crash Course Physics #18 · YouTube (embedded player only)
- Also: Sound
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