I purchased a tweeter pair for some speakers of mine. I read the installation instructions and it said that if I don't have the tweeter connected to a crossover, I need to use the capacitor. But if I was using a crossover, I didn't need it. My question is simple. WHY?

I'll let someone get more technical for you, but the simple answer is this. The Capacitor acts as a cross-over, it is in a sense a passive cross-over, higher frequency impulses "essentially" pass right through it, while lower frequency impulses get trapped < give me a brake guys I'm trying to not be too technical> since your tweeter can't handle or is incapable of reproducing the lower freq's anyway this keeps you from damaging your costly speaker.

Well... how technical do you want... I essentially just spent a term learning about this and all the implications of filters (low pass/high pass/band pass) at a college level! Essentially, you can think of it like this... when you have a capacitor in the line, it can 'fill up' and 'let out' electrons, but it does this slowly (think low frequencies). By filling up and letting out electrons, it essentially absorbs the low frequencies in your signal. High frequencies however, act quicker then the capacitor can absorb them so they pass right through. To see it from abit more mathematical stance... in the frequency domain, the impedance of a capacitor is given by 1/(wC) where w is the frequency and C is the capaticance. If you calculate this number for various frequencies you'll notice the the higher the frequency, the smaller the impedance, so it impedes higher frequencies less then it does low frequencies. This removes low frequencies from the signal before they are sent to the tweeter! I'm sure someone else can explain all this much more eloquently then I can, and I can try to explain it in more detail if you'd like.

Peter is correct, a capacitor is a filter, many times used in a design to smooth out the variations. In a cross over design you have a capacitor AND a coil, and by selecting these componets you have far more control over the frequency and the slope of the crossover. An inline Capacitor is basically a quick and dirty way to filter out the low frequencies and protect the tweeter. Not very accurate and certainly not the best but is more a protection than performance. Here is a whole write up on crossovers http://sound.westhost.com/lr-passive.htm Sharyn

Adding to what the others have said, the capacitor also blocks the lowest frequency signal you can get, DC. DC kills drivers as a general rule...

Not to be overly picky but it is technically not the capacitance that changes but the impedance that changes. IE it is not how much the capacitor stores but how it affects the flow of the electricity, so as frequency approaches 0 or dc, the capacitor becomes an open circuit, and as the frequency increases the capacitor becomes a closed circuit. This is how it looks, a capacitor unless it fails does not actually form a direct path for the electricity, but as the plates keep switching faster and faster as the frequency increases, the effect is as though the capacitor is passing the electricity, in fact it becomes low impedance, as in low prevention of the flow. Sharyn

Thank you all. I had taken electricity class, and knew what a capacitor was, but is didn't quite make sense. It does now, though. Thanks.