The range of human hearing extends from approximately 20Hz to approximately 20kHz. Actually, 20Hz is so low that we don’t so much hear it as feel it, like a rumble. Very few instruments can play this low (organs, synthesizers, certain very big drums). On the other end of the spectrum, a 20kHz tone/squeal/whistle is so high that many of us can’t quite hear it anymore. This depends on age and other factors like hearing abuse (high-level rock concerts or work-related noise pollution). "20 to 20" is the frequency spectrum a loudspeaker is supposed to reproduce. Note the word supposed. Very few speakers actually manage to reproduce this entire range. While that is a subject for future investigation, we can already mention that no single loudspeaker driver can reproduce this whole range except for headphones -- because they need to produce sound only inside the very tiny cavity of your ear.

Since a single loudspeaker drive unit can’t reproduce the whole frequency spectrum of sound, the full spectrum is usually divided into at least two parts: high and low. Specialized high-frequency and low-frequency drivers are each assigned specified portions of the speaker's response range. High-frequency drivers are called tweeters (in reference to the noises birds make), and low-frequency drivers are woofers (for the low barks and growls of a dog). The device that separates the frequencies is called a crossover.

So we now have a loudspeaker box with two round drivers attached to its front: one small tweeter and one bigger woofer. This basic type of loudspeaker is called a two-way design because the whole frequency spectrum has been divided into two parts. That’s the meaning of "-way" -- how many ways has "20 to 20" been divided. It’s also true that the speaker only has two drivers. However, it could have more drivers and still be a two-way. A designer might decide to use multiple drivers for each portion of the frequency spectrum. That’s hardly ever done with the tweeter, but it is done quite frequently with woofers. A designer might double up on the woofers, using two (or even three) of them. In a two-way speaker with multiple woofers, these woofers are said to operate in parallel. This simply means they’re doing exactly the same thing, like dual tires on the back of semi trucks.

Some speakers that use three drivers divide the frequency spectrum into three sections -- then they are called three-way speakers. And, not surprisingly, the names for the three drivers, corresponding to their low/mid/high responsibilities for the different frequency portions, are woofer, midrange (or mid) and tweeter. A speaker with more than three drivers could either be a three-way with multiple woofers, or a four-way or five-way. But most speakers are two-way or three-way designs.

We’ve talked about drivers quite a bit. In a conventional box speaker, the moving parts that produce sound are called drivers or transducers. They literally push the air back and forth like miniature pistons, to create the sound waves we hear as music. Remember what we learned about frequencies -- higher-frequency sound waves have more individual ripples, or up and down cycles, per second than lower ones. A 20kHz note has 20,000 peaks and valleys per second. To reproduce it, a driver has to push out and back 20,000 times per second. That’s much faster than a human eye can perceive. Watch a two-way speaker’s tweeter while it’s playing really loud -- it won't seem to move at all. But watch the woofer. Play the music loudly enough, especially with bass notes, and you will easily see the woofer move.

So far, we've dealt with the different drivers as though they were the same, but of course they aren't. Tweeters are usually small and dome-shaped, whether they are made from treated silk or ultra-thin metal, while woofers are much larger and cone-shaped or flat. Since tweeters deal with the high frequencies, they need to move back and forth very rapidly. A small, lightweight dome is perfect for this. But the lower frequencies of the midrange and bottom-end of the spectrum require both greater in-and-out motion (also called excursion) and greater surface area on the part of the driver. While the cones (or sometimes flat diaphragms) used for woofers look simple, they are very carefully designed from their shape to the type and thickness of rubber surrounds used to anchor them to their frames. Woofers can also be made from a wide range of materials -- from paper to metal to high-technology ceramic.