I'm starting this thread because I see so much lack of understanding and misinformation on how crossovers, subs, mains and surrounds actually interact. I don't pretend to know all of it, but I know there are a few others here who can accurately fill in some of the holes. Beyond that, there is a lot of grey area, because manufacturers don't always provide all the specs on how their crossovers work.
I will provide what I can and ask some others to fill in some of the blanks. I am hoping members will refrain from adding speculation or things they think are correct, because to paraphrase Mark Twain, "
so much of what we know simply ain't true."
My point for this thread is really just HT crossovers set in the receiver or processor between the speakers and sub, because this is such an enormous and intricate subject, taking on more will just make if too big a topic.
Crossovers are not a brick wall...
It is common to think the crossover setting is a sharp change from the speaker to the sub. As though if you set it to 80Hz, then anything 80Hz and above is sent only to the mains and anything 79Hz and below only to the sub. The truth is, there is a rolloff. Typically, the crossover frequency is where both the speaker and sub are at -3dB, because, in theory, if you have two sources running at -3dB, it is the same volume as 1 at 0dB.
You have a low pass filter, which filters out the higher frequencies and passes the low ones to the sub. The rate at which this rolls off is usually pretty steep, 24dB/octave for example, because sub drivers are typically not intended to reproduce frequencies into even low midrange. You also have a high pass filter, which filters out the low frequencies and passes the higher ones to the speakers. This one has a more gradual rolloff, 6dB per octave for example. Now, listen to something at a given volume, then reduce it by 6dB and you'll see that isn't much of a change. To know what one octave is, either half or double the Hz.
So, it you set your crossover to 80Hz, in this example, it only drops 6dB at 40 Hz. People are constantly confused why I keep telling them not to set the crossover at the minimum usable frequency of their mains, and this is why. If you do that, the crossover not only doesn't have the chance to work right, you put more of a strain on your speakers than you need to. Plus, if you have invested substantial money in a good sub, you are not taking full advantage of it. More on that later though.
There are more reasons to not drop the crossover to the minimum. By relieving the mains the strain of going to their low extreme, you not only reileve your amp of the significant burdon low frequencies cause, but the speakers as well. This can give noticeable improvements to the entire system. So, you say you have large full-range floorstanding speakers and tons of power and they shouldn't be limited that way? Read on.
My "Large" horror story...
My mains are very capable, solidly built Thiel
CS 3.6. You can see they are rated flat, within 1.5 dB down to 29 Hz. Depending on what rating you accept, (I have seen them rated from 4 ohm to 3 ohm) my Aragon 4004II amp delivers between 400 and 600 wpc to them. So, power is not really an issue. When I first bought them, there was no home 5.1. At first, I had a Velodyne sub with its own external amp and crossover. The main channels were run through the amp, which split off the sub signal, then on to the Aragon for the Thiels. This is how I first ran my 5.1 system, which wasn't true 5.1, since I had to set the sub to "Off" and the mains to "Large" the get the LFE and other bass to the sub.
Then I got a sub designed for 5.1 use. I set the sub to "On" and without even thinking about it, left the mains to "Large". After all, with those speakers and all that power, I'd have to be an idiot to set them to "Small". The very idea was absurd.
I got it all tuned in and listened to music, which sounded great. Then some movies. Also great. Then one night, while I was cranking out an action flick at reference level I discovered what happens when you apply that much clean power and infrasonic signal to any speaker. It was the sound of the woofer voice coils slamming into the end of their travel with a blood curdling "Chunk!!". I sat in horror for a minute while I thought about what had happened. It was obvious. Presented with those kinds of demands, the mains had to be set to "Small" if they were going to live and I was going to use the system to its full potential. The chunk never happened again and the system has never lacked for bass. At first I though the 80Hz minimum setting on my processor was too high, but I've never thought about it again, after just letting the system do its thing and not dwelling on it.
Of course, the slope rates I give are not the same for every system. Unfortunately, receiver and processor manufacturers don't seem to like to give up what the slopes actually are. Plus, some allow different settings for each speaker, which is good. Just don't set it too low.
This is just scratching the surface and I hope some of the truly knowlegable members will add to it.
