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We all know that tweeters reproduce high frequencies and woofers reproduce bass notes. Occasionally there’s even a midrange driver (sometimes called a “squawker”) to reproduce what’s in between.
But this just leads to questions. How do those flute tones get to the tweeter in the first place? And how do those bass guitar riffs get to the woofer? The crossover tells ‘em, that’s how.
In one sense, a crossover is a traffic cop for electrons. It directs some one way and some another way. Sure, an engineer would scoff at this description but for the rest of us, it gets the concept across and that’s the important thing.
So a crossover is first a set of filters that divide a wide range signal into segments. It then routes each segment to the driver or drivers best able to reproduce it.
A crossover has a second function, too. And it’s just as important as the first. It compensates for the idiosyncrasies or anomalies in every driver to insure that all the drivers in a loudspeaker system work together to bring you the sound your deserve.
Crossover Filters: What They Are and Why We Need Them
Briefly, a filter is an electric circuit that selectively passes one frequency or frequency band while blocking others. Most speakers need crossovers because tweeters can’t reproduce bass (they’re too small and can’t move enough air) and woofers can’t reproduce high frequencies (they’re too big and can’t move quickly enough).
Filter Characteristics
You’ll often see two specifications for a crossover:
Let’s imagine a two-way speaker with one woofer and one tweeter. The crossover frequency is simply the point at which the crossover begins to separate the flutes and bass guitars. The crossover frequency in most two-way speakers is around 1 to 2 kHz. Some are lower, some are higher.
The slope (or “rate of attenuation”) is expressed in dB/octave. Let’s say our two-way crossover acts at 1.5 kHz. This does not mean that all the frequencies higher than 1.5 kHz go to the tweeter and everything lower goes to the woofer. (If only things were that simple . . . ) Let’s look at the following graph.
If the high frequencies are reduced very gently, the crossover probably has what we call a slope of 6 dB/octave. This simply means that a frequency of 3 kHz is reduced by 6 dB compared to a signal at 1.5 kHz.
You’ll sometimes see a reference to a crossover’s “order”. This isn’t mysterious at all once you know that a 1st order crossover’s slope is 6 dB/octave, a 2nd order crossover has slopes of 12 dB/octave, a 3rd order’s slopes are 18 dB/octave, etc. We rarely see slopes steeper than 24 dB/octave in loudspeaker crossovers.
To make this clearer, here’s a low pass 3rd order (18 dB/octave) slope. Compare it to the first slope to see how much more quickly this filter reduces or attenuates high frequencies.
Caption: Notice that the high frequencies are reduced much more quickly
Now remember that we defined a crossover as a set of filters. What we’ve seen so far is just half the story.
Just as our low pass filter (the filter that passes only low frequencies) keeps high frequencies away from the woofer, we need a high pass filter to keep low frequencies away from the tweeter.
When we add this high pass filter to our crossover, the result looks like this:
Symmetrical vs. Asymmetrical
If the slopes of both the low pass and high pass filters are identical, we usually refer to the crossover as “symmetrical”. But if, for example, a crossover’s low pass filter has a slope of 18 dB/octave and the high pass slope is 6 dB/octave, we call it an “asymmetrical” crossover.
Let’s Count The Ways
You’ve undoubtedly seen crossovers referred to as a “two-way” crossover, a “three-way” crossover, etc. Although you might be tempted to think this is identical to a “two-way” or “three way” speaker, it isn’t.
A two-way crossover splits the full range signal into two parts – lower and higher frequencies. A three way crossover simply splits the same full range signal into three parts – low, middle, and high frequencies. You might even see four- of five-way crossovers but they’re increasingly rare today.
Filter Types
We’re not quite done yet. In addition of frequency and slope, filters have other characteristics. Rather than numbers, these characteristics are usually describes by the names of the engineers who developed particular filters. If you see them at all, the three most important are:
Active and Passive Crossovers
To add another bit of complexity to our discussion, there are two broad classifications – active and passive – based on where the crossover is placed in the signal chain.
Passive crossovers are the most common. Generally located within a full range loudspeaker’s enclosure, they handle an amplified signal and, as do all crossovers, filter it into segments that are directed to the appropriate drivers.
Here’s a diagram to make things clear.
Active crossovers, often called electronic crossovers, divide the full range signal before it gets to an amplifier. Of course, this means that the low and high frequency bands produced by an electronic crossover must be amplified before they are sent to the drivers responsible for reproducing a particular frequency range.
Caption: This is called “bi-amping” because you need two amplifiers.
Today, many powered subwoofers use active crossovers even if the exact configuration varies.
Odds, Ends, and a Conclusion
Even though we’ve given you a lot to digest here, you should know that a good crossover may involve many other considerations, too. There are relative driver efficiencies, maybe some aberrations the designer wants to compensate for, etc. In fact, the crossover is often responsible for the loudspeaker system’s “voice,” the tonal color the often distinguishes one speaker from another.
The good news is that the only thing you really have to remember is that crossovers are important. Good listening!