A three-way audio speaker produces sound from three separate devices called the mid-range, woofer and tweeter drivers, each of which has its greatest efficiency in a specific range of frequencies. Because the drivers are optimized to work in a given range, the speaker delivers clearer, more accurate sound than it would with a single, general-purpose driver for all audio frequencies.
The woofer is the largest driver in the speaker, typically measuring from 5 to 12 inches in diameter. It generates low-pitched sounds in the bass range, such as drums, explosions and thunder. Most woofers consist of a cone made of paper or plastic, a strong permanent magnet and a wire coil. The audio signal in the coil creates a magnetic field that pushes and pulls against the permanent magnet's field, causing the cone to vibrate and make sound. The woofer produces the strongest sound vibrations of the three drivers and consumes relatively large amounts of power.
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The tweeter is the smallest driver in the speaker, usually measuring no more than a few inches across. As the name suggests, it works in the treble end of the audio spectrum, producing high-pitched sounds such as flutes and the crash of cymbals. Without a tweeter, music from a speaker loses much of its brightness and clarity, taking on a dull, stifled sound.
Although a mid-range speaker driver resembles a woofer, it is smaller, seldom exceeding 8 inches in diameter. Because of the size, the mid-range produces sound frequencies midway between the low notes of the woofer and the highs of the tweeter; speech, vocals and most instrumental music falls in this range. A mid-range driver can produce reasonable sound on its own, although it will lack bass punch and treble brightness. Some speakers have more than one mid-range driver, letting the speaker handle more power and producing louder, fuller sound.
To optimize the speaker's sound, a set of electronic circuits called crossovers feed each driver with a filtered version of the audio signal from the amplifier or receiver. One crossover circuit feeds the woofer with only low-frequency signals, another crossover sends high frequencies to the tweeter and a third provides middle-range frequencies to the midrange driver. These circuits are typically tucked away inside the speaker cabinet; in most cases, they never need adjustment or maintenance.
All of the speaker's parts, including the cabinet, drivers and crossover, influence the sound in complex ways. Although a speaker may have excellent specifications on paper, it might not sound great to your ears. Because good and bad sound are highly subjective, it pays to hear speakers first in a good listening environment and with the type of music you enjoy before buying them.