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This is a good question. (Note that what I have written here is a very gross oversimplification and in no way is intended to define how all systems work, but to give you an example to start from.)

First, understand that the majority of music happens within the range of a standard piano, which is 27.5 Hz (A0) to 4186 Hz (C8). The human ear can theoretically hear any sound wave between about 20Hz and 20KHz, but most humans over the age of 3 cannot actually hear much lower than about 25Hz or much higher than 17KHz. Most sounds higher than about 6000Hz are sibilance, harmonics and other non-musical tones.

Sound is divided into octaves (and other intervals, but for now let's stick with octaves.) An octave is a doubling of frequency. So, if 20Hz is the lower limit of hearing, the first octave would be 20-40Hz. Second is 40-80Hz, and so on. Most music exists mainly in the fourth through eigth octaves (160Hz to 5120Hz.) The area our ears are most sensitive is between about 400Hz to 4Khz, which is called the mid-range, and is actually further divided into lower, mid and upper midranges. Our ears are MOST sensitive to 3-4KHz, and too much loudness in this area is interpreted as "harsh" or "shrill" or some other negative concept.

SO, now to answer your question. In general, bass is anything lower than 400Hz and treble is anything higher than 4Khz. To translate that into speaker terms, here's an idea of how it would work. Woofers generally handle bass and lower midrange frequencies. So-called subwoofers are actually just woofers that have been optimized for the first two (or so) octaves. Since sound above the second octave is directional and in the first two octaves (or lower, which we cannot hear but can certainly feel) non-directional, subwoofers are generally crossoed over at 80Hz. Woofers handle 80Hz up to somewhere in the lower-midrange, say 6-700Hz. Midrange drivers handle 6-700Hz through the rest of the musical spectrum to about 4Khz. Tweeters handle the upper registers and sibilance areas above 4Khz.

There are of course tweeters that can handle the midrange, and midranges that can handle the upper registers. There are woofers that operate fine through the entire piano spectrum of music. There are speakers designed for extemely narrow bands and are highly specialized (like subwoofers or "super tweeters.") And also, just because a speaker can reproduce a frequency does not mean it sounds good doing so. There are many so-called full-range drivers out there that can indeed reproduce the full range from 20-20Khz. But they cannot necessarily do so efficiently nor with very much musicality.

So-called "flat" frequency response means that all frequencies are reproduced with equal intensity. Many people think this sounds artificial or even bad, as it tends to emphasize those areas where our ears are already more sensitive and to lack authority in those areas like bass or upper registers where our ears have trouble. That's why most people add woofers to create more bass, or place tweeters near ear level, or use EQ devices to compensate for our human shortcomings.

Anyway I hope this gross overview helped you. There are lots of web pages out there with additional and much deeper information. Try doing a Google search for something like "frequency ranges loudspeaker."

Cheers.

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