|The Crossover Design Cookbook|
Chapter 1: Simple Crossovers
by Mark Lawrence
What are Crossovers?
1st order Crossover
2nd order Crossover
How Crossovers Work
Final Watt-V Crossover
What We've Learned
I recommend FireFox
Your ears are really quite remarkable instruments. They work over a rather incredible range of both frequency and power. Unfortunately, it's pretty much impossible to make a single speaker driver that can cover the full frequency and power range of your ears.
For low frequencies (tubas, bass drums, tympanis, organs with large pipes) we need something that can handle a lot of power and move a lot of air. So, we make big, relatively heavy and slow woofers. Roughly speaking, the larger the woofer, the more air it can move and the more power it can handle, but the more distortion it will have when trying to work at higher frequencies. Very roughly speaking, woofers have performance like this:
|Diameter||Power||Low Freq.||High Freq.||Box|
Of course, there are many available speakers and by no means do they all fit these parameters, but the table above does roughly show the general trend. Larger woofers require larger boxes with an associated low WAF (Wife Acceptance Factor) value. Also, larger woofers have progressively more distortion at high frequencies.
Tweeters reproduce high frequencies - flutes, piccolos, sopranos - but cannot move much air or handle much power. Typical tweeters will melt if you try to put more than a couple of watts of power into them.
So, we need something which will distribute the music to appropriate drivers - low frequency, high power music to the woofer, high frequency, low power music to the tweeter. We must block high frequency music from getting to the woofer, because the woofer will just produce a lot of noise and distortion. We also need something to block the low frequency music from getting to the tweeter, because although the tweeter will pretty much make no sound while trying to reproduce the low frequency music, it will likely melt.
The circuitry which distributes the music to the appropriate drivers is called the cross over. It's what we're going to learn to make.
Tweeters typically cannot reproduce music below about 1,500hz, so if you want to make a two-way system (one woofer, one tweeter, nothing else), you pretty much have to use a 6" or 8" woofer. If you want to make big boxes that can handle lots of power, you're going to have to make a three way system and use a mid-range driver to cover the range from about 300Hz to about 2,000Hz. An alternative is to make a simple small two way system, then use a sub-woofer to fill in the lowest bass.
In the picture below, we see a piano keyboard laid out next to a frequency chart. Pianos produce tones from about 27Hz to about 4,200Hz. Men can typically sing in the range of about 70Hz to 300Hz; and women can typically sing in the range of about 150Hz to about 600Hz. People can typically hear from about 20Hz to about 14,000Hz - a bit more if you're young, a bit less if you've spent a lot of time firing guns or flying small aircraft.
It is very difficult to reproduce sounds below about 35Hz. Very few speakers can actually do this, in spite of the myriad claims to the contrary. It is relatively easy to reproduce sounds up to about 25,000Hz; most decent speakers can do this.
The range of about 100Hz to about 2,000Hz is by far the most critical. This is where most music happens, and it's where your ears are most sensitive to sound and distortion. Frequencies below 100Hz and above 2,000Hz are icing on the cake - it's very nice to have them, but if you don't have a good, low distortion midrange, you've got nothing.
Two way system typically have cross overs at about 2,000Hz, so the cross over is mostly not active in the critical mid range frequencies. Three way systems typically have an additional cross over at about 300Hz - right in the middle of the midrange, in the most critical region. Careful cross over design is very important in three way systems. To get around this problem, lately satellite - sub woofer systems have become popular. The satellite speakers are small two way speakers which can go down to about 100Hz, with a single cross over at about 2,000Hz. The sub-woofer handles frequencies below 100Hz. This moves the low cross over out of the critical mid range band.
The speed of sound is about 1,000 feet per second, so sound at 1,000Hz (1,000 vibrations per second) has a wavelength of about 1 foot. Sound at 10,000Hz has a wavelength of about 1 inch. Sound at 100Hz has a wavelength of about 10 feet. You can't localize a sound source to less than 1 wavelength, so in a typical room in a house there is no stereo image available below about 100Hz. You can get by just fine with a single subwoofer hidden in the corner of the room.
Copyright © 2002-2019 Mark Lawrence. All rights reserved. Reproduction is strictly prohibited.
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Revised Thursday, 15-Aug-2019 09:30:53 CDT