I just finished building a pair of MTMs, using Zaph Audio's "Bargain Aluminum MTM" design.
This system uses Dayton DA175-8 woofers and the SEAS 27TBFC/G tweeter.
Crossover is at 1450Hz.
Testing with REW and a calibrated Dayton UMM-6 mic revealed high THD between 100 and 500Hz, tapering off above 500. Both speakers are similar. See plots (I included a shot of a measurement on another speaker I have, a Klipsch RF7-II, for comparison. It shows very low THD).
Notice that above the crossover point THD drops steeply. The tweeters, at least, seem to be on spec. Notice also that most of the distortion is odd-order. 2nd order is actually quite low.
I also tested the left speaker drivers separately, bypassing the crossover, the woofers together w/o the crossover, and the two crossover filters separately. The woofers show very low distortion when connected directly to the amp (< -50 db), and also when the tweeter is in the circuit via the high-pass filter, but the low-pass section is bypassed. It is clearly the low-pass circuit in the crossover causing the problem.
Anyone have any idea how that can happen?
Another speaker, using same test procedure, just for comparison:
Left speaker with tweeter connected via high-pass, but low-pass bypassed:
Left speaker woofers only, via low-pass: