In all speaker systems, regardless of how many drivers are operating, it is desirable to have a smooth, linear response, with as few peaks and valleys as possible. Since woofers have an extremely attenuated response in the high range and tweeters can be destroyed by powerful low frequencies, a crossover circuit is used to isolate, optimize, and integrate the drivers. A crossover is an electronic or mechanical means of separating frequencies in the audio range so that they can be rendered most efficiently in a multi driver system. There is normally, one passive crossover in each cabinet. In less expensive speakers, this means that the tweeter may be efficiently isolated from the woofer through a High Pass Filter, usually a single capacitor that limits the low frequencies and passes on the high frequencies delivered to the tweeters and midranges, from the amplifier.
Another popular alternative is the Active Electronic Crossover in which the crossover action takes place in a unit positioned before the amplifier in the circuit. This requires the use of two or more amplifiers, each connected to the particular drivers ( midranges and or tweeters ) whose frequencies are delivered by the electronic crossover. This is a much more expensive, but very efficient, way to achieve the desired result. Individual speakers that have separate inputs for the various drivers are said to be Biamplified.
The term 'Crossover' indicates that point on the normal frequency spectrum between 20 to 20 thousand Hertz, (or cycles per second) where certain frequencies are muted, or rolled off to one driver and permitted passage to another. Thus, in a typical 3-way crossover, frequencies from 5, up to 1800 Hertz (HZ.) or so, are filtered and diverted to the woofer. Frequencies above that are sent to the Midrange up to 3500 Hz or so, and muted above that. Frequencies above the midrange band are filtered out and the remaining high frequencies are passed to the tweeter.
A mechanical crossover is typically a small dome or cone affixed to the center of a woofer that extends it's high frequency response, so that it becomes a so-called full range speaker. Electronic crossovers connected to independent tweeter and midrange drivers usually are much more effective in isolating the frequency bands within a spectrum, and at the same time, reduces intermodulation and other forms of distortion.
Above is a schematic of a typical 2-way passive electronic crossover. Such crossovers can be 2, 3, 4-way or more. They are circuits that for the most part, use passive components such as coils (MH), capacitors(MFD), and resistors(Ohm).
The woofer is connected directly to the amplifier and all the crossover action is applied to the tweeter, in our example. Be aware, however, that while this may work fine for certain types of 2-way speakers, the exact value, number, and configuration of the electronic components are contingent upon the particular drivers used and the cabinet characteristics of the speaker as a whole.
No crossover (passive or electronic) should be seen as providing rigid frequency dividing lines. In a simple two way design (one woofer and one tweeter) the designer might set a crossover point of 1800 hertz (also written as 1.8Khz or 1800 cycles per second (cps). This does not mean that all of the energy above 1800 Hz are sharply routed to the tweeter while only frequencies below, go to the woofer. There is actually a gradient, or slope up or down from the attenuation point. In fact, in some low budget designs, there is no upper frequency roll-off limitation for the woofer, beyond it's own natural response roll-off.. This means that for at least one octave of sound above and below the 1.8 kHz example frequency, (1 kHz to 4 kHz) the sound from the two drivers will intermingle and may cause interference. This can be either constructive or destructive interference; the knowledgeable designer takes this into account by utilizing not just the response curves of each driver, but their roll-off characteristics as well.
If the woofer and tweeter both have a peak of 3 dB at the same frequency, they could combine at this one point in the frequency band to make the sound "harsh". Conversely, if the speakers canceled each other,(i.e. one peaked while the other dipped, thus nullifying that particular frequency ) some of the important midrange frequencies would be lost, making the sound a lot quieter, thinner and less present. Thus, it is important to understand the characteristics of any replacement parts and how well they will integrate with your existing system, before they are put into your speaker.
However, and this is important, while this information could be very useful in helping you optimize your system, it is also very difficult to come by. Most car audio manufacturers do not provide the kind of detailed technical analysis, that makes it possible for the average audiophile to achieve optimal results. Thus, most of us make do with replacing defective units with drivers that are most similar to the original parts. Even though this method is obviously not as precise, the overall results are usually quite acceptable. This is one reason why most drivers should be replaced in pairs, (or four's, with surround systems) thus avoiding obvious imbalances on the sound stage.0998