(Prior thread on the subject here
. Note Dennis Murphy's remarks on what appear to be two types of MTM interference.)
Originally Posted by ggunnell
The number of well reviewed horizontal MTM centers from highly regarded manufacturers such as B&W, Epos, Dali etc. is large, as is the number of speaker design posts where you could learn more about the effects of crossover design on off-axis frequency response
For the skeptical view of MTM designs this is an important point and one that can get insufficient attention. The trick to MTM's with very good acoustic symmetry on the long
axis involves tight driver spacing, aligning their acoustic centers, and a specific type of crossover.
Originally Posted by Bill Fitzmaurice
The MTM was invented by Joe D'Appolito to address the specific issues of horizontal dispersion, vertical pattern control and baffle step compensation...
In the 80s, D'Appolito wrote up his theory in Speaker Builder magazine. He also presented "A Geometric Approach to Eliminating Lobing Error in Multiway Loudspeakers"
D'Appolito included a lot of variables in his original and quite elegant 3-driver, 2-way solution to solving uneven directivity, although the difficult combination of spacing, first order responses, and acoustically-aligned sources is hard to include in all MTMs, which is where MTM skepticism can and probably should occur. When done correctly, however, the response was said to deviate a theoretical +/- 1.5dB on the long axis.
More conventional MTMs, as opposed to the true D'Appolito array, cause/allow lobing dependent on the combination of the array's deviation from those elements, although this can be seen as a pro or a con, depending on design goals. Lynn Olson's nearly legendary Arial project
used the vertical MTM w/o the D'Appolito requirements, thereby reducing long axis response instead of actually improving on it.
It's worth noting that the third order implementation also sums to quite flat off-axis, although as with all MTMs, only within the constraints of the driver's raw responses and their relationships to one another.
PS: As sivadselim just said:
Originally Posted by sivadselim
...most (if not perhaps all) horizontal MTMs designed for center channel use are not truly D'Appolito array speakers. D'Appolito array speakers are designed with very specific crossover characteristics and correspondingly very specific driver spacing (among other things).