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Sound is different from what your computer normally deals with. The electrical signals inside and at its ports, even the light from your monitor, are electro-magnetic phenomena. Sound is purely physical. It is simply the rapid change in air pressure.

When a physical object vibrates, it forces air to move along its undulations. After all, air and the object cannot take up the same place at the same time. Air is pushed away from the place the object bows out and rushes into the empty place where it bends in. The movements are tiny to your eye, but on the molecular level, they are huge.

Air, being made from physical molecules, has its own inertia. It can't immediately move out of the way of a vibrating object but instead gets squeezed, which raises the pressure between molecules. Where the air gets sucked toward the vibrating object, the pressure goes down. Consequently, the air around a vibrating object is repeatedly squeezed and expanded as the object moves. The pressure changes affect the adjoining air, so the changes in pressure move away from the vibrating object as a series of high-and-low pressure waves. Your ear detects the changes in air pressure as sound.

Unlike light, which happily traverses the vacuum of space, sound requires a medium (usually air) for transmission. The speed of sound depends not on the moving object but on the density of the medium. The higher the density, the faster the sound moves—for example, sound propagates faster in water than air.

Because the waves of sound spread out as they move away from a vibrating object, the intensity of the sound pressure declines with distance. Unconstrained, this decline would follow the famous inverse-square law, because as the sound travels in one dimension, it must spread over two. By confining or directing the air channel, however, you can alter the rate of this decay.

Fitting its digital signals into this world of pressure waves is a challenge for the computer in several ways. The computer needs a convenient form for manipulating sound. Fortunately, sound has an analog in the electronic world called analog audio, which uses electrical signals to represent the strengths of the pressure waves. Computers turn these electrical signals into digital audio that's compatible with microprocessors, other digital circuits, and sound systems. Of course, the computer is not limited to sounds supplied by others—it can create the digital signals itself, a process termed synthesis. To turn those digital signals into something that approaches sound—back to audio again—your computer uses its own audio circuitry or a soundboard that includes both a digital-to-analog converter and an amplifier. Finally, your computer plugs into loudspeakers, which convert the audio into pressure waves once again.

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