frequency and ohms

I cannot understand what the correlation between adding high and low pass filters and the frequency that the sound is and whether ohms go up or down. In example if I use two high pass filters on the same series does that affect the ohms besides for the fact that you need to do the basic ohm math for series speakers? Thanks

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well, first off i will say that i'm not expert on acoustics, and with that said im not exactly sure what you mean. its pretty clear that you are talking about passive crossovers (the ones that wire directly to the speaker) so why on earth would you wire more than one crossover to a set of speakers? here is how a crossover works: the audio signal passes through a series of copper coils and capacitors. depending on the value of these parts and the resistance of the speaker, it will filter the audio above or below a certain frequency. the frequency is determined by the values and ratings and wiring of the parts of the crossover. each crossover is specifically designed to work with the set of speakers it came with. there is no one size fits all crossover, every situation is different because every speaker is different. the crossover and speaker work together as a system, if you put a different speaker or multiple speakers on the same crossover then it will change the way the crossover works. thus using any crossover for anything other than its intended purposes could result in something bad happening to your speakers. if you add more speakers to a single crossover it might cause a shift of the crossover point above or below the physical limits that the speakers can handle and they could blow. crossovers serve two functions, to combine two speakers and make them sound like a single full range speaker and also to protect each individual speaker from frequencies that they are not designed to play. any questions?

A filter does not change the impedance of a speaker. If you have an improperly designed crossover will allow the amp to see multiple drivers at certain frequencies. If the amp sees (4) 4 ohm drivers from 250Hz to 1KHz, the amp will see a 1 ohm load at that range, you will have more output at that range of frequencies. That is how frequency is related to impedance in my initial post to your other question. A properly designed crossover will allow the amp to see only one driver, or a combination of the 2 drivers at a diminished impedance. At the crossover point, the output of each driver will be 3dB down. At the 3dB down point the amp will see half of the impedance of each driver. These 2 combined will be near the nominal impedance of the set. A 4 ohm driver is not always 4 ohms. At certain frequencies it can be MUCH greater than 4 ohms. Frequency has a lot to do with impedance.

building on the "frequency has a lot to do with it" statement, every speaker has what is called an impedance curve. this is basically a graph of the speakers impedance versus the frequency it is trying to play. each speaker has its own impedance curve but in general as the frequency goes up so does the impedance and when it is playing music the impedance is constantly changing. since you seem to be wanting to have tweeters on each channel of a four channel amp playing at 2 ohms i guess its necessary to tell you that even if you get the tweeters to a resting impedance of 2 ohms to hook up to the amp, as soon as you play music you will lose that 2 ohm load in favor of an ohm load that is much higher. this is why it was said that an amp doesnt really "see" tweeters, because when they play music their ohm load goes so high that the amp wont produce more or less power. so even though your tweeters say they are rated for 75W, you will not honestly ever see 75W while playing music unless you go below their lowest recommended frequency range, either way it will be certain death for your tweeters. just because a tweeter is "rated" for a certain wattage doesnt mean that it needs that amount of power to get maximum sound. tweeters are high sensitivity speakers which means that most tweeters will do good off of 15 watts or less. they only give you this rating to make it easier for you to match it up with an amp. you are stressing over nothing and even if you do all this work you will end up with very very little to no noticeable gain in sound.