You asked!
A D'Appolito Array--or MTM if you like has a set of rules that Joe published but is generally not followed by manufacturers typically because of (wait for it) cost and complexity.
Generally speaking that design was only meant to be done vertically and follows some rules as far as center-to-center distance between the drivers at the crossover point. It works very well when the rules are followed and not so well when they are ignored. Ever see one laid on it's side? If so, those things are not following the rules and when done wrong, make a mess of things.
The "rules" if you will demand that the midranges "match" very closely and if any way possible, the midrange to tweeter crossover point center-to-center distance should be within 1/3rd of the crossover wavelength....or as close as possible.
For example, say you have a pair of 4 inch midranges and a 1" dome tweeter to cobble together. Stack up the pair of fours and the dome then measure the center-to-center distance to get the proper crossover frequency. Say, for example you can have a center-to-center distance of 7 inches. Multiply that by 3 and whatever a 21 inch wavelength is...done! Roughly, that is 635Hz or so.
Uhhh.... there are no 1" dome tweeters that crossover that low so how far can you push it until lobing becomes obvious? Well, about one full octave works decently enough or around 2KHz which can be done with rather expensive and stout dome tweeters. Thanks to some recent dome tweeters, you can cross them over at 1,500 Hz so a pair of 5" mids can be used and crossed over at less than an octave center-to-center distance without too much lobing becoming noticeable.
Cost? Well, those tweeters that crossover that low are not cheap, they can struggle with those low frequencies at higher SPL since two mids are naturally 3dB more efficient and considering two mids can handle twice the power--the tweeter should be able to output 6dB more output to "keep up" with the dual mids. Not only does the dome have to be more efficient to "keep up", it has to operate at the low frequency limits at a higher peak SPL...that costs money.
Money? Yeah, I built a D'Appolito array with 5" mid-woofers, selected a 1850Hz crossover point and used a steeper crossover filter (18dB/Oct) to protect the 30mm stout dome tweeter. I paid three times the cost for that low frequency output of the tweeter (compared to 3,000Hz) and the passive crossover parts were twice as much being 18dB/Oct 1,850Hz VS 12dB/Oct 3,000 Hz. Of course, you are spending twice as much on the mids because they are doubled up so. Tweeter cost tripled, crossover component cost doubled and mid-woofer cost doubled so quite expensive.
Could you get better results, about the same efficiency/power handling with a 6.5" mid-woofer and crossing the tweeter higher at 2.5 KHz with a much lower cost passive crossover? Could you get the same or better efficiency, peak SPL, power handling with an 8 inch mid-woofer crossing over at 2 KHz and use a waveguide to get the dispersion to match at 2KHz at around the same price?
In audio, there are certain styles and designs manufacturers push because they sell! They don't work for free, want to stay in business so will produce things that the public wants. The D'Appolito MTM array was originally done back in the 1980's as a way to mimic a point source full range speaker--like a coax but without the problem of IMD mucking up the treble response as the soundwaves of the tweeter bounce off the moving midrange cone. The two mids would couple at a specific frequency which cut the vertical dispersion to prevent floor/ceiling bounce roughly how a multi-driver vertical line array works. It works well when done correctly but for the most part, is the price worth the benefit?
The D'Appolito Array worked well when done right but Joseph D'Appolito himself stated it was time to move forward as technology, driver improvements and measuring accuracy have really taken off since the early 80's. Basically, it was a good design for the time but you can do better at the same or lower cost. Since Joe designed it, him stating it is time to abandon his own design is rather interesting...I get it.
I've owned a D'Appolito Array speaker for 17 years, purchased it in the late 90's. Worked very well for the most part and I built one of them 10 years ago. Five years ago, I completely redid my multi-channel system and did not go with MTMs--nothing personal, used waveguides to control dispersion and more efficient, higher power handling drivers to get the SPL I required without requiring arrays.
I'd say the reason you are seeing a decline in the amount of D'Appolito arrays is driver and crossover filter costs, more build complexity at a performance level that allows better options. As always, manufacturers have to be different from each other so the styles change as with most things. Don't forget, Klippel released a robot that will take thousands of measurements to determine speaker accuracy so manufacturers are responding by improving the off-axis and lobing/beaming performance of their speakers in response. The days of talking heads and a simple measurement are going away as speaker designs are now put under the robotic machine overlords producing massive amounts of accurate data and the audio companies have to focus on performance more these days. The markets are shifting east and coupled with actual accurate measurements, the days of using your ears is eroding away. An MTM done wrong is obvious with measurements so why risk it?
Just various ponderings, a great design if done right and a poorly measuring design if done wrong. The cost VS benefit ratio for all speaker designs depends on available tech and other factors. Be aware that in audio, the "right" designs are not always the best selling ones. Although we all wish that every company has pushing the audio frontier forward with ground breaking designs--wish in one hand and take a dump in the other--see which one fills up first.
Hope that helps, keep your stick on the ice.
A D'Appolito Array--or MTM if you like has a set of rules that Joe published but is generally not followed by manufacturers typically because of (wait for it) cost and complexity.
Generally speaking that design was only meant to be done vertically and follows some rules as far as center-to-center distance between the drivers at the crossover point. It works very well when the rules are followed and not so well when they are ignored. Ever see one laid on it's side? If so, those things are not following the rules and when done wrong, make a mess of things.
The "rules" if you will demand that the midranges "match" very closely and if any way possible, the midrange to tweeter crossover point center-to-center distance should be within 1/3rd of the crossover wavelength....or as close as possible.
For example, say you have a pair of 4 inch midranges and a 1" dome tweeter to cobble together. Stack up the pair of fours and the dome then measure the center-to-center distance to get the proper crossover frequency. Say, for example you can have a center-to-center distance of 7 inches. Multiply that by 3 and whatever a 21 inch wavelength is...done! Roughly, that is 635Hz or so.
Uhhh.... there are no 1" dome tweeters that crossover that low so how far can you push it until lobing becomes obvious? Well, about one full octave works decently enough or around 2KHz which can be done with rather expensive and stout dome tweeters. Thanks to some recent dome tweeters, you can cross them over at 1,500 Hz so a pair of 5" mids can be used and crossed over at less than an octave center-to-center distance without too much lobing becoming noticeable.
Cost? Well, those tweeters that crossover that low are not cheap, they can struggle with those low frequencies at higher SPL since two mids are naturally 3dB more efficient and considering two mids can handle twice the power--the tweeter should be able to output 6dB more output to "keep up" with the dual mids. Not only does the dome have to be more efficient to "keep up", it has to operate at the low frequency limits at a higher peak SPL...that costs money.
Money? Yeah, I built a D'Appolito array with 5" mid-woofers, selected a 1850Hz crossover point and used a steeper crossover filter (18dB/Oct) to protect the 30mm stout dome tweeter. I paid three times the cost for that low frequency output of the tweeter (compared to 3,000Hz) and the passive crossover parts were twice as much being 18dB/Oct 1,850Hz VS 12dB/Oct 3,000 Hz. Of course, you are spending twice as much on the mids because they are doubled up so. Tweeter cost tripled, crossover component cost doubled and mid-woofer cost doubled so quite expensive.
Could you get better results, about the same efficiency/power handling with a 6.5" mid-woofer and crossing the tweeter higher at 2.5 KHz with a much lower cost passive crossover? Could you get the same or better efficiency, peak SPL, power handling with an 8 inch mid-woofer crossing over at 2 KHz and use a waveguide to get the dispersion to match at 2KHz at around the same price?
In audio, there are certain styles and designs manufacturers push because they sell! They don't work for free, want to stay in business so will produce things that the public wants. The D'Appolito MTM array was originally done back in the 1980's as a way to mimic a point source full range speaker--like a coax but without the problem of IMD mucking up the treble response as the soundwaves of the tweeter bounce off the moving midrange cone. The two mids would couple at a specific frequency which cut the vertical dispersion to prevent floor/ceiling bounce roughly how a multi-driver vertical line array works. It works well when done correctly but for the most part, is the price worth the benefit?
The D'Appolito Array worked well when done right but Joseph D'Appolito himself stated it was time to move forward as technology, driver improvements and measuring accuracy have really taken off since the early 80's. Basically, it was a good design for the time but you can do better at the same or lower cost. Since Joe designed it, him stating it is time to abandon his own design is rather interesting...I get it.
I've owned a D'Appolito Array speaker for 17 years, purchased it in the late 90's. Worked very well for the most part and I built one of them 10 years ago. Five years ago, I completely redid my multi-channel system and did not go with MTMs--nothing personal, used waveguides to control dispersion and more efficient, higher power handling drivers to get the SPL I required without requiring arrays.
I'd say the reason you are seeing a decline in the amount of D'Appolito arrays is driver and crossover filter costs, more build complexity at a performance level that allows better options. As always, manufacturers have to be different from each other so the styles change as with most things. Don't forget, Klippel released a robot that will take thousands of measurements to determine speaker accuracy so manufacturers are responding by improving the off-axis and lobing/beaming performance of their speakers in response. The days of talking heads and a simple measurement are going away as speaker designs are now put under the robotic machine overlords producing massive amounts of accurate data and the audio companies have to focus on performance more these days. The markets are shifting east and coupled with actual accurate measurements, the days of using your ears is eroding away. An MTM done wrong is obvious with measurements so why risk it?
Just various ponderings, a great design if done right and a poorly measuring design if done wrong. The cost VS benefit ratio for all speaker designs depends on available tech and other factors. Be aware that in audio, the "right" designs are not always the best selling ones. Although we all wish that every company has pushing the audio frontier forward with ground breaking designs--wish in one hand and take a dump in the other--see which one fills up first.
Hope that helps, keep your stick on the ice.
