HOW DIGITAL AUDIO WORKS
HUMANS PERCEIVE SOUND WHEN THINGS VIBRATE at a frequency and amplitude that is detectable to them. Frequency is the rate at which things vibrate and corresponds to pitch. It is expressed in hertz, or cycles per second. Pitched (or harmonic) sounds vibrate at a regular, periodic rate. Nonpitched sounds, such as noise, have more random vibrations. The frequency of the A above middle C on the piano is approximately 440 hertz, for instance. If anything—whether it is a guitar string or a rubber band—vibrates consistently at that rate, you will sense you are hearing the A above middle C. Amplitude is the strength of the sound and corresponds to volume. Sometimes the term decibel is used to describe amplitude. A zero-decibel sound is undetectable; a 100-decibel sound—like that you would hear standing near a jet engine—causes pain to one’s ears!
The physical vibrations of an object (such as a guitar string or a singer’s vocal chords) cause quick fluctuations in the air molecules around them, producing waves of compressions (molecules bunching up) and rarefactions (molecules expanding) that move through the air. The quicker the vibration, the higher the frequency (or pitch). The stronger the vibration, the greater the swing or fluctuation, and therefore the greater the amplitude or volume.
We often hear the term “analog” with reference to older recording technology, yet it is very relevant to digital recording. Analog is, in one sense, the opposite of digital and refers to continuously changing values (as opposed to the discretely changing ones—ones that change in steps—in the digital realm). For instance, imagine you are turning up a volume dial on a sound producing device. Let’s say this dial is labeled 1 (softest setting), 2, 3, 4, 5, 6, 7, 8, 9, and 10 (loudest setting), but you can sweep the dial to any position on or in between these numbers. Theoretically, there are an infinite number of settings for the dial in between 1 and 2, or 2 and 3, and so forth. Now imagine a dial that “clicks” or locks into place only at the numbers 1, 2, 3, and so on, instead of sweeping continuously—you can set the volume to 1, 2, 3, and so forth, but nowhere in between. On this dial there are only 10 discrete settings. This is digital, and from this example it does not seem as good as analog, does it? But let’s get back to our vibrating object.