What is a Crossover?
****Basics****
A crossover is probably the most important part of a speaker. A bad crossover design can cause a speaker that uses the best drivers in the world to sound like crap. An extremely well executed crossover network however can enable some very inexpensive drivers to sound excellent. The intracacies of crossover design are way over my head (and certainly not all that helpful in a newbie document), but a basic understanding of what a crossover is and what it does can help immensely with understanding home theater.
In a loudspeaker (like your home stereo speakers) you almost always have more than one driver (components making the sound).
In a "
two way" speaker system you have a tweeter (usually 0.75" or 1" in diameter) and a woofer (usually 4"-8" in diameter). The tweeter is used to cover the higher frequencies sounds while the mid/woof is used to cover the lower frequencies.
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The crossover is what makes the tweeter only get high frequencies and the mid/woof only get low frequencies.
A
crossover consists of two filters. A high pass (HP) filter and a low pass (LP) filter. When you combine the two you have a crossover.
A HP filter allows higher frequencies to pass through it while attenuating lower frequencies and is therefore connected to the tweeter (
makes sense right: a High Pass filter only allows HIGH freq to PASS though).
A LP filter allows lower frequencies to pass through it while filtering away higher frequencies and is therefore connected to the mid/woof (
again: a Low Pass filter only allows LOW freq to PASS though).
What is defined as lower frequencies and higher frequencies is defined by the guy who designed the filters. The designer will choose a frequency based on the tweeter and woofer components being used in the speaker. Above this point will be the higher frequencies (that get sent to the tweeter) and below this point will be the lower frequencies (sent to the woofer).
****Advanced****
To understand the finer points of a crossover it also helps to know what an
octave is. An octave, most basically, is a doubling of audio frequency. So when you see someone complain about subwoofers not being able to play the first octave, they usually mean 16hz-32hz. The next octave is 32hz-64hz, the next is 64hz-128hz etc. Although depending on what scale definition you are using, the particular frequency ranges that make up an octave differ.
It is very important to understand that the choosen crossover point frequency mentiond above is not a brick wall divider.
Lets say the choosen crossover point was 2000hz. This does not mean that the tweeter plays the frequencies from 2001hz and up and the mid/woof plays the frequencies below 2000hz.
It means that below 2000hz the HP filter will start to filter off the lower freq at a specific rate. The farther below 2000hz you go the more the signal will be filtered away.
The LP filter is the reverse. Above 2000hz the LP filter will start to attenuate the high freq signal at a specific rate. The farther above 2000hz you go the more the signal will be attenuated.
The rate and shape of this filtering will be determined by the "type" and "order" of the filter.
There are a bunch of different names for the types, but they aren't overly important to this discussion. If you get into speaker construction and design, you'll want to check out the DIY/ADVANCED area of this forum to get tips on crossover types.
The
order of the crossover is how steep the slope is (how quickly it filters away audio). A first order filters the signal gradually as you move away from the crossover point, while a 4th order filters much more drastically.
Anyone interested in the specifics: A first order filter will attenuate the input signal 6 dB/octave. Second order at 12 dB/octave. Third at 18 dB/octave. Fourth at 24 dB/octave and so on.
The neat part is, that given how the dB scale works, when you have a tweeter with a HP filter and a mid/woof with a comparable LP filter the frequencies where the filters overlap (freq that both drivers are playing) will be filtered in such a way that you get an even level across the entire frequency range the two drivers are capable of!
Now this is a little bit of an over simplification, the filters won't both use 2000hz. The HP will start to attenuate a little above 2000hz and the LP will start to attenuate a little below 2000hz so that the two filters will sum correctly. The frequency quoted as the crossover point, is usually the point at which both sides are down by 3dB.
No speaker will play a perfectly flat 85dB from 20hz-20khz. It will uaually be +/-3dB over some frequency range (not necessarily 20hz-20khz, could be from 48hz-22khz). There is also a lot of other things the filter design takes into account. It accounts for the size of the speakers baffel and the layout of the drivers on the baffel. It accounts for oddities in a drivers frequency response. It will also account for differences in sensitivity between the drivers. And likely a few other things I'm not even aware of.
So that's a two way speaker. What about a three way. Well a three way just has two crossovers. It has a HP attached to a tweeter. A HP and a LP attached to the midrange driver and a LP attached to the woofer. There are also 4way speakers, and I'm sure some fool somewhere has designed higher way speakers.
You may have also seen speakers listed as 2 1/2 way. What this means is that you have a 2way speaker with a HP attached to a tweeter and a LP attached to a mid/woof. But you also have a second mid/woof or woofer with another LP filter. This second LP filter will start to attenuate higher frequencies at much lower frequency than the first LP filter.