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When a woofer, midrange, or tweeter fails, it is time to replace (or repair) it. Exact replacements are a necessity in this case. If the left side tweeter is not working and you can get an exact brand and model number replacement then do it! If the replacement is of the same brand (but not model number) do not waste your time or money. Non-exact replacement mids and tweeters will "fill the hole" but not give you the sound of the original. If a non-exact replacement part is used, you will not only have to replace the damaged part but both drivers to keep a balanced stereo image. As previously stated, crossovers were designed for the original drivers and any compensation that was added might not sound good on the new drivers.
All crossovers (passive or electronic) are not rigid frequency dividing lines. In a simple two way design (one woofer and one tweeter) the designer might set a crossover point of 2000 hertz (also written as 2 Khz or 2000 cycles per second (cps)). This does not mean that all of the energy above 2000 hz goes to the tweeter and below it goes to the woofer. In fact, in some low budget designs, there is no upper frequency roll off for the woofer. This means that for at least one octave of sound above and below this frequency (1 Khz to 4 Khz) the sound from the two drivers will intermingle and cause interference. This can be either constructive or destructive interference and the designer takes this into account by knowing not just the response curves of each driver but how they work together. If the woofer and tweeter both have a peak of 3 dB at the same frequency, they could combine at this one band of frequencies to make the sound "harsh" (like the sound through a megaphone). Conversely, if the speakers cancelled each other, you would lose many of the important midrange frequencies making the sound a lot quieter, thinner and less present. This is why I would not recommend clone replacement mids and tweeters in a quality speaker system.
The woofer, should it fail, is the most critical component to replace as it works in conjunction with the cabinet. If you haven't already, I urge you to read my importance of box tuning page.
Last updated 12/17/2012