These measurements are not wide enough. 60 degrees off axis may look like a lot, but it is not with regard to sound power. Sound power is a 360 degrees measurement. Your link inspired me to a attempt a brief explanation of the sound power problem with an MTM setup.
Look at the picture (in the link Adason posted) of the interference paths from the top and bottom of the cone. Now imagine these two points are the two mid drivers in an MTM.
On axis, both drivers are coincident sound sources, which means that their SPL adds up 6dB when both fed with the same signal. Not the 3dB you would expect from the electrical power injected. There is a 3dB efficiency gain, but only as long as the two mids are coincident. At higher frequencies this breaks down, as per the picture, because destructive interference comes into play.
Now, what happens if you fix the crossover so that the frequency response is straight on axis? On axis, both drivers are coincident, so that they combine to deliver + 6dB. However, off axis, they are no longer coincident, to the point where the outputs null out. This creates a dip in the power response, with audible consequences.
This really is an issue that has only been highlighted for a couple of decades now, after Harman's spinorama and Tool's work came out. I did not invent this stuff.
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Well.... Betsy has a variation of min. 5dB beginning around 2100hz - when going off-axis. That's less than I would expect... but still way too much if we're talking even and smooth directivity. I get that it might not be that important for some.... but saying that beaming does not occur at all - is kinda questionable 
Adason, this is about academically derived knowledge directly applicable to the improvement of sound reproduction. But science is being frowned upon these days, I sense that.
Tannoy is a fine example of of loudspeaker where horn and midbass driver are exactly the same size. And always coincident. A waveguide avant la lettre, prescient engineering. I played a set of K3838's for years. Best flush wall mounted.
Tannoy is a fine example of of loudspeaker where horn and midbass driver are exactly the same size. And always coincident. A waveguide avant la lettre, prescient engineering. I played a set of K3838's for years. Best flush wall mounted.
Thank you all very much for your answers! This is a little beyond my understanding of acoustics, I'm afraid.
That is, you are talking about the fact that the MTM will have dips at high frequencies of the mid-frequency when deviating from the axis? Is this the phenomenon of interference?
To summarize, MTM will reduce the sweet spot, but what about the positives?
That is, you are talking about the fact that the MTM will have dips at high frequencies of the mid-frequency when deviating from the axis? Is this the phenomenon of interference?
To summarize, MTM will reduce the sweet spot, but what about the positives?
I agree with you that for 4.5 "it shouldn't be a problem for 2.5 kHz looking at its frequency response and distortion graph.
according to your math, 15" woofer would have to be crossed way below 400Hz
as far as I know, not tannoy, not altec or anyone else does that
everything else you write about being coincident, and rest is nonsense...
flush wall mounted including
I am outa here
All sound interferes once you move into 3 dimensions. The word 'interfere' sounds bad, but it really depends what is happening.
Yes, they are saying that the two mids are far apart, at frequencies just below the crossover where the tweeter isn't helping you might see little energy toward the ceiling and floor. Or maybe not, it depends on your design.
If you want to be sure you need to do careful measurements and/or sims. Like I said earlier, either you learn the method or you just do it anyway
The good thing is you don't need to build it again if you want to try the other way, you just rearrange the crossover.No, it can make the reflection from the ceiling sound different.
I agree with AllenB.Oh, how did you know about the concept of my project?
You might not hear the clear differences before you try it out and get used to the sound. When I changed to a waveguided tweeter rather than a normal dome, I had to simply play with it for some time and change back, to realize that I actually preferred the waveguide more - simply because I relaxed more and forgot about the speaker more. So my comfort chose for me, rather than the logical and technical part of me. That the two sides of me ended up agreeing, is just proof that we are all maybe more the same and not so different after all - even though that modern society tries to make us so focused on individualism
Pro's for MTM is for me:
1. Higher sensitivity to match most tweeters
2. Narrow sound pattern in the vertical plane - may reduce some reflections from the floor and ceiling. Toole and Geddes disagree slightly on this. Toole believes that we can "listen through" rooms and that reflections add spaciousness - whereas Geddes use damping at the floor and diffusion in the ceiling. Geddes makes a great point and maybe it's best if you hear him yourself, at the 57:21 mark:
Chat with Dr. Earl Geddes of GedLee Audio - YouTube
Point is, that MTM might help a bit with vertical reflections, but this will also narrow you vertical listening window - not you horizontal.
Here's an example of BMTMB, where you can see that it's dispersion is actually very nice and to me, it looks close to what you get with one midrange:
3 ways d'appolito floor standing speaker | Fully active with DSP
When building an MTM, you simply have to be careful that you do not cross to high and use a tweeter that easily meet the lower crossover point, which the midranges need - to not have to long a CTC distance. If the distance is to large compared to the wavelength of the frequency you chose for your cross-over. Then you'll have problems in one of the most ear-sensitive areas of the human frequency range - and this IS audible
MTM reduces sound to ceiling and floor over relatively small bandwidth, just about below the crossover frequency. It is not constant for the whole bandwidth. Much better idea would be to use waveguide which can restrict sound to ceiling and floor over much wider bandwidth, in my opinion. Of course one can use waveguide with the MTM as well but this pushes the c2c spacing even further apart.
Basically yes MTM benefit over waveguided TM loudspeaker is just the looks in my opinion, maybe size is a constrain as well. Waveguide provides better performance for the vertical polars than MTM solution. If two woofers are required for sensitivity or other reasons, maybe then. There are many ways to design speakers all being compromises in some way. Sometimes it is audio that is compromised over looks or cost.
MTM benefit over waveguided TM loudspeaker is just the looks in my opinion, maybe size is a constrain as well. Waveguide provides better performance for the vertical polars than MTM solution. If two woofers are required for sensitivity or other reasons, maybe then. There are many ways to design speakers all being compromises in some way. Sometimes it is audio that is compromised over looks or cost.
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