MTM behaviour questions

[AdamZuf]

Hello!
I've read in Speaker Building 201 and on the net about these d'Appolito configurations, and have a few questions.

1. The vertical dispertion is lesser then a TM, but more uniform lobing? Because the schematic shows that the "lobeless" portion of the response has enough wide vertical angle not to be heard if you sit vs. stand. I don't follow

2.If the listening distance is about 1m but tweeter is not exactly height level, I should get better results then TM, right?

3. I didn't understand the part about using mixed slopes for the tweeter/woofers, doesn't that screw up the total response?

Thanks!
Adam

[AdamZuf]

Also, how does offsetting the tweeter affect lobbing, assuming that I follow's a'Appolito's configuration?

[augerpro]

http://www.birotechnology.com/articles/VSTWLA.html

[HiFiNutNut]

Augerpro,

Thanks for the link. A nice article.

[Samuli^]

I was about to make a new thread out of this, but it seems I can't so I'll just post it here. Hope you don't mind the little thread-jacking Adamzuf.


This is somewhat a follow up to Augerpro's link to a study [http://www.birotechnology.com/articles/VSTWLA.html] sent in the "MTM behaviour questions" thread. The study in so many words described a phenomena - in MTM the two midwoofers will interact in destructive and constructive phases at different angles and/or frequencies forming lobing off-axis. This should be quite obvious, anyways.

Of course two or more drivers reproducing the same frequency range, not tall enough to form a line-array or acoustic centers too far apart to couple, will lobe when the angle is chosen so that there is different path-lengths to the transducers. Am I in the dark here, thinking it's obvious when perhaps it shouldn't be? Maybe I haven't fully understood the MTM-configuration. Here is my input on the topic anyways, feel free to correct me if there's something where the logical leap doesn't quite follow:

One less talked about problem with MTM-configuration is the forming of a line-array at some frequencies which will limit the dispersion vertically, although the near-field formed by such line-array will be small. To me the idea of MTM has always been in getting the extra output or less distortion without the need to have more complicated crossover, as would be the case with, for example 2.5 way (and 2.5 configuration wouldn't be of use reducing excursion at higher frequencies, anyways). If going for 2.5, it's the same trouble to go full-bore with a 3-way, imo. D'appolito definately has it's benefits, those being reduced midbass driver excursions and simpler crossover designs with surprisingly little ill-effects (or positive?) on the off-axis performance.

Summing up, and all imo or to my knowledge, D'appolito arrangament has three possible main modes of operation:

1. Low frequencies, where the path-length to wave-length ratio and the resulting phase difference is insignificant. Aka. The two drivers will work much like a perfect point source.

2. Where the two mid-drivers (and possibly the tweeter) couple to form a miniature line-array limiting the vertical dispersion as if one tall driver was used:

The near-field of such wouldn't extend far, but if standing on a reflecting surface as table, might the effective lenght of the array might grow in size enough for the line-arrays near-field to extend especially at higher frequencies enough to be a problem in dispersion AND in volume. This is because in the near-field line-arrays output diminishes 3 dB per doubling the distance but monopole will decrease at 6 dB. This is because of the cylindrical waveform of line-arrays versus 4pi radiation of monopoles.

3. Higher up where the drivers don't couple, but form lobing of destructive and constructive interferences depending on angle and frequency.

IMO, there is only three full-proof ways around the problem of vertical lobing in usual speaker designs: Place the drivers as close to each other as possible, use low XO frequencies and use steep XO slopes. D'appolito doesn't strike me as very efficient way of _reducing_ lobing problems, more like it introduces them less than you'd initially expect.

For what it's worth here is two links of stereophiles measurements of vertical off-axis of what I would call a good MTM and a good TM. The MTM is with 5.25", as closely as possible spaced drivers and steep crossover slope @ 2 kHz designed by Joe himself. The TM is 6" midbass and a tweeter with about equally steep crossover @ 1.5 kHz cut-off.

MTM, Snell LCR7 XL: http://www.stereophile.com/images/ar...606Snefig5.jpg

Paradigm Active/20:

http://www.stereophile.com/images/ar...t/P20fig05.jpg

[AdamZuf]

Thanks!
I read the article and it approves what I was thinking about the subject.

So, it's a trade off. Either you get a cancelation on lower frequencies with the standart d'Appolito's configuration and smooth dispertion on the crossover frequency, or you go by minimzing the space between the woofer's centerpoints and shift the nulls up, closer to the midrage. But the lower the frequency, the harder it will be it absorve/diffuse it in a room, so cancelation is ok.
A bold statement for somebody who never designed a speaker, but considering the need for baffle step many times, wouldn't it be logical for it to left the sum of it and the nulls to be calculated in the same circuit, when you fine tune after experimentation?
If the nulls shift up to the crossover frequencies, then if there's a need to compensate for that, so will it be harder to apply to a parallel crossover?

Plus, correct me if I'm wrong here, but seems logical that the more the nulls comes up to the ear's sensitive area and the crossover point, the worse.

Comments?

[AdamZuf]

Another small issue is that the further away the woofers are from each other, the less cancelation will result in the null frequency, because of SPL getting weaker by distance.

Samuli,
I don't think that even a tight MTM can form a line array effect to a significant measure, and I don't think the 3dB drop per meter will even get past the first meter. I can't remember the calculation, but Jim Griffin's article will explain it easily.

From the MTM / TM graphs, it seems that the MTM also introduces irregularities to upper frequencies, and it makes sense, they are probably harmonicaly related? That's bad.

This thread makes me think more then twice about the subject..

[bjorno]

Hi,

AdamZuf,

I can't remember the calculation, but Jim Griffin's article will explain it easily.

I don’t know if you read other treads dealing with this subject like this one:

high cross point d'Apolitto

Or the one containing short array considerations that compliments Jim Griffins Nearfield Line Arrays:

Line Array question

If you look at the picture:

http://www.diyaudio.com/forums/attac...amp=1179784790

You can see the directivity for short arrays is plotted for n= 10 to 4 and that Lambda criteria must be lowered to be of any use.

Now an array with n=2 as for a M-T-M can be estimated from the picture = Lambda (p) = 0.75 x Lambda

I believe that this number should be used for estimating performance for an M-T-M, as the very upper limit to crossover at.

Consequences when applying to LR M-T-M: s, see picture:

b

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