Home Speaker Asylum

General speaker questions for audio and home theater.

Some thoughts on this

The proposed high mid/tweeter crosspoint might not be too bad IF you listen on-axis and IF your room is large/dead enough that the reflected off-axis response is significantly lower in level than the on-axis. I've built speakers intended for nearfield listening with quite a high crosspoint (6kHz) and they were OK in this regard. Do try to keep the center-to-center distance between tweeter and mid under one wavelength at crossover to reduce vertical lobing. (An MTM will have comb-filtering issues, due to the separation between the mids.

You WILL need to highpass the tweeter, even if you run the mid full range. Look at where the mid is down -6dB from its "average" sensitivity and make this your crossover point. Devise a second-order highpass filter for the tweeter that is also down -6dB at this frequency. You will definitely need a resistive L-pad on the tweeter to set its output level.

Unless the speakers are to be placed right against a wall, you have to consider the baffle diffraction step (look it up if you're not familiar with the concept). Since you are considering using two mids in an MTM or TMM, you can do a 2.5 way arrangement. Run one of them fullrange up to where it meets the tweeter, and lowpass the other to fill in the low frequencies. The low fill driver wants to be down -3dB where frequency = approximately three baffle widths. Or just divide your baffle width in inches into 4560 to get the approximate frequency. A series inductor plus impedance compensation (Zobel network) on the .5 way driver should suffice. OR you can build the cabinet as a bipole, with both mids wired in parallel and in phase, and one of them firing to the rear. Both could then be run full range, with no crossover components on either, and still take care of the baffle step. Bipoles need a lot of air around them (no near-wall placement) but can throw a huge soundstage.

There's more to it than this, of course, but this can get you started. Post (or PM me) your intended baffle dimensions and a link to the response curves of the Focal drivers, and I can crunch some numbers for you.

Recommended reading: Ray Alden, Speaker Building 201. This book has the clearest explanations of the basic concepts. Welcome to the addiction -- er, "hobby."


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