this is very enlightening. thank you so much! zonkers

Some manufacturers prefer to go with a TMM layout, as this way there's less beaming in the vertical plane.

In my opinion Tyler Acoustics is doing the MTM right in their Pro Dynamics series - using a horn tweeter whose pattern is wider than it is tall, roughly corresponding to the pattern of the woofers.

MTM places the two cones farther apart than TMM does, so MTM beams more in the vertical plane (below the crossover frequency) than a TMM does. On the other hand, a TMM puts the effective center for the midrange frequencies physically fairly far below the center of the high frequencies. So, in choosing between the two, I'd look at where the crossover frequency is. If the crossover frequency is around or below 1 kHz, I'd go with a TMM as the ear is not very good at detecting the height of a sound source down that low. But if the crossover is much above 1 kHz, I'd probably go with an MTM.

Now with either one of these formats, assuming a dome T, you will have an even greater radiation pattern discrepancy in the crossover region than if you were only using a single woofer. So, many designers go with a "2.5 way" TMM format - that is, the lower woofer is only active in the bass region, and then it is rolled off well below the main crossover point, while the upper woofer remains active all the way up to the crossover.

Hope this helps.

Duke

An alternative way to get directivity down at low frequencies is to use a dipole or cardioid type enclosure (the former is pretty simple, and the latter is pretty complicated). Dipoles inherently have a figure-8 radiation pattern at low frequencies. The drawback is that they need a lot of equalization to do bass well, unless they are very large.

According to researcher Earl Geddes, radiation pattern control below 500 Hz isn't really necessary in a normal home listening room. Often even getting down to 500 Hz requires tradeoffs.

A loudspeaker designed with a great deal of attention to radiation pattern control is the Gradient Revolution. It's a dipole below 200 Hz, then it's a cardioid (lilly-pad shaped pattern) from 200 Hz to about 2.5 kHz, then it's 120 degrees wide above that point (uses a coaxial tweeter, and the angle of the midrange cone is about 120 degrees). But, it's only about 85 dB efficient - that's one of the tradeoffs.

Duke

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Your "squishing" analogy is quite close, and provides a good mental picture. Actually the pattern doesn't get any wider in the horizontal plane for an MTM, but if we could see it, it would indeed be fat-looking, as if a squishing had occurred.

Duke