Woofers are loudspeakers that are designed to reproduce the lowest frequencies, or bass end of the audible sound spectrum. They may be anywhere from 4" in diameter all the way up to huge 36" giants meant for highly specialized scientific purposes. For most consumer purposes however, sizes are pretty much standardized at 6, 6.5, 8, 10, 12, 15, and 18 inches with the 6.5 to 12 inch sizes being the most popular. For a general discussion of speaker operating principles see, How Speakers Work.

Because woofers are specialized reproducers, their design maximizes their potential for reproducing the lowest frequency. Thus, they will have cones that are suspended in such a way as to promote the maximum XMAX or back and forth motion. Middle frequencies by contrast, have smaller cones and less excursion, or cone travel. Tweeters have cones, domes, or emitters that may seem practically motionless. Even so, they can produce sound in their frequency range that is just as powerful (and piercing) as their larger siblings. The higher the frequency, the smaller the waveform at similar amplitude (loudness).
But woofer cones must move in and out with the longest possible extension because sound waves get longer as they get lower in frequency. Current technology provides us with stiff and rigid cone materials such as Polypropylene, Carbon and Polymer Laminates that maintain there shape under stress, resist moisture, and are very light. The cone works as a piston to compress and alternatively evacuate large masses of air. To do this most effectively, it needs to be connected to an amplifier that produces the high amplitude waveforms that can produce this motion. Because such signals require considerable electrical power behind them to control the cone movement, the highest power available is needed strictly for bass reproduction. Similar levels in the midrange and high frequency areas require comparatively little power for the same apparent loudness, all other things being equal.

Because of the powerful dynamic motions of the woofer cone, the frame or basket may be cast in a heavy metal to reduce the tendency for the speaker to "ring" at certain resonant frequencies. This arrangement also allows for the most precise alignment of the voice coil within the magnetic field and is particularly useful in speakers at, or greater than 15" in diameter. In the larger drivers, after years spent resisting the tug of gravity, there is a tendency for the heavy magnet to be pulled out of alignment on the stamped frames of the larger sizes. This is of little practical concern with 10" and smaller woofers.
Magnetic structure design is fairly standardized with pole pieces constructed to put a concentrated magnetic field in and around the voice coil. Over the past 30 years, the magnetic materials used have changed considerably to permit ever-larger concentrations of magnetic energy rated in "Gauss" to be created from smaller quantities of lighter compounds. Coupled with the use of Kapton multiwound coils and more powerful amplifiers, speakers when designed with even a modicum of commitment to high fidelity, can much more easily achieve that end.

For speakers to truly work well and achieve their maximum audio potential, they must be housed in an appropriate enclosure. Indeed without some elementary means of preventing the front radiated wave from interacting with the back wave, no bass would be heard at all. This is simply because the two waves are precisely 180 degrees out of phase with each other and cancel out when they meet. For more on this important subject, see our article on Enclosures.