In preparation for the construction of the final speaker units we have been researching various approaches to designing a crossover. A crossover is essentially a high/lowpass filter circuit that limits one speaeker driver to high frequencies and another to lower frequencies. They are categorized and arranged in various sequences and order as are frequently used filters. The progression is parallel to greater attenuation at the cutoff per octave. Each step in the order increases the attenuation by 6db. “A 1st order crossover filter 6 db/octave, a 2nd order 12 db/octave, a 3rd order 18 db/octave, and so on’.
https://www.diyaudioandvideo.com/Guide/WireSpeakerCrossover/
Components involved include capacitors and inductors, which are similar but operate in different ways. A capacitor stores voltage in a circuit and limits the total output, while an inductor also limits the output but stores the current in a magnetic field. We have the means to wind our own copper coil inductors if we wish to save money.
Phase changes introduced by a crossover:
“Each order of crossover introduces a 90 degree phase shift. A 180 degree shift is an inverse of the wave. If 2 speakers are 180 degrees out of phase then they will cancel each other wherever they produce the same frequencies. Even with crossovers, both speakers will produce sound for several octaves beyond the crossover point. If this problem occurs, there will be a noticeable dip in the frequency response at the crossover point. To solve this problem, wire one, but not both, of the speakers backwards (+ to -). Usually, phase shift problems only occur with 2nd order (or 6th order) crossovers, but can also occur when using multiple 2-way crossovers in a 3-way (or more) speaker system. The only way to really find and fix a phase shift problem is trying all possibilities in reversing the speaker leads.”
Phase changes introduced by Inductors:
“When using more than one inductor in a crossover, the electro-magnetic fields of the inductors can interfere with each other causing an unpleasant result. That is why it is best to keep the inductors as far apart as possible. Also, keep the fields out of phase with each other by rotating the inductors 90 degrees. It is possible to have 3 inductors out of phase, as shown below.”
It is important to note that when building a speaker it is important to use components that have the least resistance possible so that resistence in the overall speaekr system doesn’t build up too much for the amplifier. Saia has been experimenting with how to arrange the amplifier for the passive speaker we are building with the other components and we have landed on using an external amplifier.
There are three different types of capacitors, Electrolytic, Mylar, and Polypropylene. They are categorized by performence, Electrolytic to Polypropylene. Metalized polypropylene being the best, but most expensive.
Back to the crossover, it is important to focus the crossover onto the middle of the overlapping frequency response of the two drivers used. This is in order to have the maximum amount of flat octave response on both speakers.
“mid/woofer crossover there are 4 octaves between 200-3.2k Hz, 200-400-800-1600-3200. 800 Hz is the middle frequency, with 2 octaves flat in either direction. For the tweeter/mid crossover, there are only 1 octaves, 2000-4000. 3k Hz is the crossover point with 1/2 octave stable in either direction. These two drivers have little overlap, and normally would not be used together.”