Sounds are vibrating waves of energy that travel through a material medium. There are many kinds of sound waves with different shapes and patterns, including normal traveling waves in air, standing waves, surface waves, internal waves, and shock waves. Sound waves travel at speeds ranging from subsonic velocities of several meters per second, to hypersonic velocities that approach the speed of light. Sound waves vibrate at frequencies that range from billions of cycles per second, to a single cycle within a period of several days, months, or years. Sounds are created wherever there are free-moving objects in close proximity, such as molecules that make up air, liquids, and solids. Normally, a room full of air is moving around randomly with molecules constantly colliding with one another because they are so close together. When a sound source, such as a crying baby, disturbs these motions, it creates a coherent pattern of molecules that we call an acoustic wave, or sound. When the baby cries, the sound wave it produces spreads out in all directions, forming an expanding sphere in the air. This is the result of molecules that collide with one another, pushing their neighbors forward then rebounding, repeatedly, causing the wavefront to move forward at the speed of sound. The full range of sound not only occurs in gas, liquids, and solids, but extends from tiny, microacoustic waves to giant, macroacoustic wave disturbances including weather patterns, ocean waves, seismic waves, global waves, solar waves, and galactic waves. Acoustic waves travel through the Earth’s atmosphere, hydrosphere, biosphere, and lithosphere, as well as on or near other planetary bodies and satellites, in the stellar wind, on the surface of stars, in interstellar dust clouds, in spiral galaxies, and in giant molecular clouds that occur in the regions between galaxies and clusters of galaxies.
A Collection of Writings on Nature, Science, and Art by John Holland