How compromised is this dipole tweeter?

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I’ve just built the frames for a 3 way active dipole, with “nude” mids and tweeters. Now a tweeter “dilemma”:

I want a dipole tweeter that is high efficiency to allow high peaks with a moderate power tube amp, by which I mean drivers > 95 dB. I’m prepared to sacrifice some dipole “true behaviour”. How compromised is this:

I have a pair of 97 dB tweeters (Supravox, that I got 2nd hand); and two pairs of Scan Speak D2608 Discovery tweeters. The D2608 is c 92 dB.
My idea is to have the Supravox front firing, in the centre; and rear firing: one D2608 a side, wired in parallel (and attenuated to match the Supravox). The crossover will be c 2000 Hz. Mounted “nude” in a frame.

By one D2608 a side, I mean the 3 drivers form a row; with the two D2608s from above probably set a little forward, so the acoustic enters are in a line. The left – right distance between the centres of the two D2608s would be about 220 mm.

To the rear there would be comb filtering, above frequency ...?
But how much, and how would that rear comb filtering affect the sound?
 
Thinking further, the comb filtering could be reduced by mounting the rear firing tweeters further apart.
The way I've done the frame, they could by about twice that width apart, c 450 mm.
And they do not need to fire directly back, eg they could be angled at 10 - 30 degrees to either side.
Both these mountings having less comb filtering, but being less dipole in overall system response.
 
I'm having trouble visualizing what your talking about. A drawing would help.

I've tried nude, back-to-back tweeters, and found them to sound off. The comb filtering/dipole peaking is obvious.

If your set on using two tweeters per side, you might have better luck if you separate them more - like Linkwitz does. His baffle is about 33cm wide. The tweeters probably will have a much smoother response this way.
 
How would it be, if the Left – Right distance between the centers of the two rear firing tweeters were about 120 mm?

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Top row is the horizontal pattern, bottom is the vertical pattern. Left is to the left side, right to the right side. "vorn" is 0°, "hinten" is 180°

If you mount the drivers vertically instead of horizontally, you have to switch top with bottom.
 

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Much appreciated Rudolf.

Did you use Boxsim? By “the vertical pattern”, you mean with the rear tweeter above and below the front tweeter – yes, much better in that direction.

In the other direction, somewhere between 30 and 60 degrees, the behavior goes bad. Can I ask please, how does it sim at 45 degrees?

I recall a thread about cutting down the faceplates on conventional tweeters like these (I think cheap Daytons were used), as small as possible, to push the good polar behavior higher in frequency. http://www.diyaudio.com/forums/multi-way/161299-directivity-dipole-tweeters-4.html#post2125177

I’m a bit reluctant to cut into the expansive Supravoxes. And even if I do, as the magnets are larger (also on the SS tweeters), I will still only be able to get good dipole behavior only so far.

So maybe I could cross these tweeters, to a Neo3 used as an active supplementary tweeter. I know the Neo3 is an ideal dipole tweeter, but rejected it because of SPL capability is just a little low for me.
However its power handling maximum of “20 watts RMS/ 50 watts max”, limits SPL ~ when it’s crossed around the “normal level” of 2 kHz. 91 dB + 20 watts gives a maximum of about 103 dB at full excursion.

I recall if frequency is doubled, that excursion reduced by factor of 4.
So if the Neo3 were crossed around say 5 kHz, 4th order, maybe I could safely put 60 watts into it (without quite full excursion, I’d guess) and achieve c 107 dB. IIRC the Nao Note is crossed (passively) around 5 kHz.

I would still need to trim the Supravoxes and Scan Speaks. But in conjunction with the Neo3, could get a much higher range of good dipole behavior.

Cuibono, I appreciate it could be difficult to visualize, I will try to arrange a drawing, but have no CAD experience, nor scanner

It’s actually 3 tweeters per side: one (97 dB) front firing; two (91 dB) rear firing, probably side by side.

What do you think of combining
  • a wide Orion style baffle (330 mm), with
  • no trimming of drivers; and possibly also
  • the Neo3 crossed around 5 kHz
 
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To reduce comb filtering from the two rear firing tweeters, perhaps i should, instead of having these two fire directly back in parallel, have their aimed at say 20-30 degrees apart. Then there would be much less interaction between them.

Possibly mount them each on rotating gimbals, to fine tune their angle of radiation.
 
Did you use Boxsim?
Yes, it automatically generates 30°, 60° and all other angles spaced 30° apart. But sorry no 45°!
By “the vertical pattern”, you mean with the rear tweeter above and below the front tweeter – yes, much better in that direction.
No, all diagrams shown are - as requested by you ("Left – Right distance between the centers of the two rear firing tweeters about 120 mm") - for the horizontal arrangement of the drivers - rear tweeter left and right from the front tweeter. "vertical pattern" means vertical "measurement" - up and down from 0°.
In the other direction, somewhere between 30 and 60 degrees, the behavior goes bad. Can I ask please, how does it sim at 45 degrees?
You can ask, but you can't get it ;). You will have to interpolate 45° from 30° and 60°. The behavior doesn't go bad above some angle, but above some frequency, which depends on the spacing of the drivers. For more info you probably need to read my "Open baffle 1.pdf". You can download it from here.
I recall a thread about cutting down the faceplates on conventional tweeters ... I’m a bit reluctant to cut into the expansive Supravoxes. And even if I do, as the magnets are larger (also on the SS tweeters), I will still only be able to get good dipole behavior only so far.
I would not cut into those Supravoxes either. Just mount the tweeters in the vertical arrangement and don't think about their vertical behavior any longer. Stereo is a horizontal effect.
So maybe I could cross these tweeters, to a Neo3 used as an active supplementary tweeter. I know the Neo3 is an ideal dipole tweeter, but rejected it because of SPL capability is just a little low for me.
However its power handling maximum of “20 watts RMS/ 50 watts max”, limits SPL ~ when it’s crossed around the “normal level” of 2 kHz. 91 dB + 20 watts gives a maximum of about 103 dB at full excursion.
You are trying a bad compromise with good tweeters. A bad decision IMHO. If you want to have the Neo3, then stack as many Neo3 as you will need to achieve your SPL needs at 2 kHz and forget about those dome tweeters.

Rudolf
 
(Been off camping for over a week, away from civilisation and the net)

Thanks again Rudolf, much appreciated. You've got the heart of the issue.

Two Neo3s in parallel would reach the SPL target. As they are 4 ohm tweeters, I'll need either two amplifiers or a very load tolerant one, but I have a few.

Would you expect much comb filtering between the two vertical Neo3s?
 
Hello Otto88,

you can look into that yourself with EDGE. The program allows for rectangular drivers.

You can not expect to be totally free from interference in the vertical plain, but you should get a 10° wide angle where the difference between 0° and +/-5° is never more than 2 dB. By tilting the Neos up or down you can aim them at your listening height.
I don't think that you expect equally good listening conditions for standing up and laying on the floor. ;)

Would you expect much comb filtering between the two vertical Neo3s?
 
Even the master can learn . .
First rule if you want to "master" anything: Be prepared to learn something new every day and to correct yourself every other day. :D
Looks $ wise somewhat like a "giant killer"
Not really. Those Neo tweeters can take you only so far volume-wise. My system can't play VERY loud.
Undistorted SPL level is what separates the boys from the men. And that's were you need REAL money to buy decent drivers. Listening at low volume levels can be had for cheap, yes.
 
Expolinear have just released an AMT with 95dB sensitivity and a very flat enclosure for just 75€. As far as I know it can't be used open, but two of them are still quite cheap. FR not nice, but AMT-typical dynamical qualities.
 
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