But there doesn't seem to be much information out there targeted at a basic understanding of crossover points for the guy who just wants to know what speaker to buy and whether to override the Audessy-chosen settings.

I assume that most of us understand what a crossover does. Without getting into the details (involving low and high pass filters and the like), the basic function of a crossover, what makes it a crossover, is to divide sound. One signal comes in, two signals go out, one containing high frequency sound, the other containing low frequency sound.

Some things I don't think are as well understood.

Crossovers are Gradual

It appears a common misconception that a crossover acts like a hard barrier... that a crossover point of (for example) 80Hz means that 100% of the 79Hz sound comes out one side and 100% of the 81Hz sound comes out the other. This misconception seems to cause a lot of confusion.

This is less than ideal, but will hopefully give an idea:

The red line is the "sum" line... that is to say, it represents the entire signal coming out of both speakers.

The dotted line is the low frequency channel. The dashed line is the high-frequency channel. The chart itself is arbitrary. The crossover point (1) can represent any frequency.

As you can see, at the crossover point, sound is coming out of both speakers equally. As we move away from the crossover point, one speaker gets more an more quiet while the other gets louder until the sound is entirely to one speaker.

How rapidly this happens is called the "slope"... which is measured in "decibels per octave".

Many people have not really considered how an octave ties in to a frequency. An octave can be measured as a doubling of frequency. To the ear, an octave is linear (every 8 keys on a piano is an octave), but to the frequency it is logarithmic.

That means that 20Hz-40Hz is one octave. 5Khz-10khz is also one octave.

The higher the decibel-per-octave, the steeper the curve.

Now as I mentioned before: the crossover is the point at which both speakers are receiving equal power. If the sum is flat, that means that the crossover point is -6db or so.

For setting subwoofers in an AVR: the most common crossover slope seems to be a very gentle 12db-octave (also called "2nd order"). Let's do the math for an 80Hz crossover point.

At 80Hz, the sub will be at -3db and falling at a rate of -12db per octave. At 160Hz, the sub will be a -15db, and so essentially silent. Working backwards, we find that the sub actually started getting less than 100% signal at a half-octave earlier (about 50hz).

Looking at the other side: the speaker starts loosing about a half-octave higher (100 Hz or so), crosses -3db at 80Hz, and at 40Hz is -15db.

This is why it is important that a subwoofer perform above the crossover point, and a speaker perform below the crossover point... because they are asked to. Ideally, both systems would perform 1 octave from the crossover (to both double and half the crossover frequency), to ensure that any distortion was inaudible. But this is not always possible, meaning there are trade-offs to consider.

What to give up

If we set the crossover too high for the subwoofer, then we get high-frequency (likely >160HZ) breakup.

If we set the crossover too low, we get low-frequency drop off.

Generally low-frequency drop-off is preferred (and easier for calibration to correct for) than high-frequency breakup. So when we err on crossover, we generally want to err "too low".

Another problem with letting the crossover point rise is one of localization. Our ability to tell the direction from which a sound comes (left vs right for example) is a function of our distance from the sound and the frequency of the sound. The lower the frequency, the less we can tell direction.

This is why a single sub, sitting on the floor, can handle all the low-frequency: but we want 2-9 speakers in our stereo to handle high frequencies.

If we choose to set the crossover point high, we risk that the subwoofer starts putting out localizable sound. Noise starts sounding like it's coming "from the sub" (which it is).

It is for this reason that, when in doubt, we try to set lower (80Hz) rather than higher (120Hz)... but as with anything: play with the settings to see which works best for you.