The Endeavour, a 3.5 way active speaker concept
After a few stabs at dipoles in the last two years, I've recently worked on an active speaker concept I call Endeavour – and it really is, since I'm still very much a beginner (although I've learned a lot in theory from you all). By now, it's concept only, and before going any further, I'd like to hear your comments on it!
It’s a ported 3.5 way construction that I tried to optimize for the following design goals:
I tried to achieve these goals by the following design choices:
Outer dimensions (W*H*D): 21cm * 99.5cm * 30cm
Acoustical crossover points (ca.): 270 Hz, 370 Hz, 1.8 kHz
Crossovers are LR2 for the enforcement woofer and LR4 for W-M and M-T.
I simulated the concept in Boxsim as good as I could, using published data from Wavecor, Scan Speak and http://http://www.lautsprecherbau.de/. Tilting had to be emulated by placing the drivers "behind" the front baffle, and digital notch filters had to be simulated by analog rejection circuits. The stuffing was configured to match the ~15% volume extension I found in online resources for the kind of absorption I'm planning to use. The results (individual and combined frequency responses on axis, horizontal and vertical off axis responses) are attached.
I didn't optimize the bass for maximum linear extension because the speakers wouldn't be too far from the back walls and I tried to limit excursion below 30 Hz; but that could easily be changed by different digital filters, of course.
How does the concept look to you? Any obvious faults or mistakes? Any details I haven't thought of or trade-offs I should rethink?
I'm especially uncertain about phase tracking between the drivers, especially because the driver arrangement is a bit unorthodox.
Looking forward to your feedback!
It looks like an awesome and well thought-out project. I can only think of 2 small things that I would do differently.
3.5 designs usually roll off the helper woofer with a 1st order slope. That will ensure the woofers combine with a coherent phase. The roll off on the helper woofer can be strategically placed to compensate for the baffle step loss.
I see no reason to tilt the speaker. The sloping baffle will not help time-align and since you have the tweeter under the woofer can make matters worse. The DXT tweeter sits further back than regular tweeters so it probably doesn't need to be time-aligned with the mid.
With careful fine-tuning of the crossover frequencies and driver spacing you should be able to achieve exemplary directivity. I assume that you have measurement equipment and the miniDSP will make that process easy.
Following Boris's suggestions and some feedback on HTGuide, I changed two aspects of the design:
a) I removed the tilt, swapped the MT arrangement, and re-simmed. And voilà: Vertical dipersion still meets my goal of being reasonably smooth up to 30° upwards, and downward dispersion got a bit better in the first 30°. Plus construction got a whole lot simpler!
b) I changed the LP slope of the helper woofer to first order, but added an additional LR2 filter higher up to bring the woofer down in the higher midrange where it exhibits a rather nasty breakup peak. The resulting on axis response is a lot smoother than before.
The changed arrangement brings up the mid's SPL at its floor bounce frequency only 2-3 dB (and it's still 12 dB down at that point), so all in all I'd say: Improvements on all fronts!
Next up: Trying a 2nd order MT configuration to see if I can get better power response.
Thanks for the feedback so far – I'll keep you posted!
So far your simulations look very good. You mention phase tracking and I assume your simulator will allow plotting of phase of the individual sections as would reveal this. At the same time we can infer that it is very good since the summed response is so smooth and also because the tweeter crossover has 6dB sum gain (even improved on the second version after good suggestions from Boris).
I think there is a missed opportunity to totally remove floor bounce with this design. You have calculated the floor bounce frequencies (first notch frequency?) but you seem to have placed the crossover frequency right at the bounce frequency of the mids. If your simulater allows, you could sim a second system below the reflecting floor and see the reflection effects. Pushing the woofers down in the cabinet would raise their floor bounce freq. You might then be able to cross between woofer and mid bounce frequency and remove all effects.
At least worth a sim.
Can you translate some of the German? Hinter is rearward response?
Thanks for the detailed feedback, David!
Yes, I began playing around with the phase plots after posting my initial version, and at the moment I'm trying out different baffle slopes to get the required time delay for the tweeter physically instead of electronically. I think I'm slowly getting the hang of the aligment/delay/phase/FR interdependencies involved.
Concerning the floor bounce: Yes, the frequencies shown are for the first notches. Since my simulator (Boxsim) doesn't allow me to sim more than one system, I used the Boxycad spreadsheet (http://audio.claub.net/boxycad2.xls) and the lobing calculator at mh-audio.nl - Home to arrive at the ideal XO frequency – but I haven't redone the calculations since changing the driver arrangement. Will do that to see if I can realize your suggestion. Thanks!
As for the German: The graphs (clockwise from top left) are for off-axis response to the left, right, downward, and upward; and "hinten" indeed means rearward response.
As suggested on the HTGuide forum, I redid my math to see how a sloped baffle might help with time alignment of the drivers:
I arrived at a 6° slope that ensures good phase tracking and would thus in principle allow the Endeavour to be driven passively and without electrical time alignment. (Plus, the more complicated construction is back! Yay!)
Furthermore, I followed David's suggestions concerning floor bounce cancellation avoidance: I moved the woofers quite a bit lower and recalculated the target W-M crossover frequency, the main constraint being keeping W-M distance below 1/2 wavelength at XO frequency, while at the same time crossing over at the geometric mean of the W and M floor bounce frequencies (first notch). For the resulting driver responses, see the FR graph:
One drawback of the current iteration is increased vertical lobing above listening height (at least in the M-T crossover area.)
This leads directly to the questions I hope you can help me with:
Thanks in advance I hope I'm nearing actual build planning, so keep the feedback coming!
P.S. I attached a more "realistic" rendering of the Endeavours' current incarnation I think I like the looks, do you?
Any comments or feedback, especially on the crossover and enclosure questions?
nice driver choice for M and T. If I were to build a box speaker I would certainly think about the two as well.
1. Would using a 3rd order Butterworth instead of a 4th order LR for M-T XO help with upwards vertical dispersion? If so, what would be the trade-offs involved?
=> A BW3 tilts the combined beam 15° up or down depending on the polarity of the drivers. I am not sure about the height of your couch but I guess you will be better off with a straight beam and steeper slopes of the LR4.
2. It looks as if the facetting could be sufficient but only measurements will tell the truth.
Some more comments, maybe you have simulated the following already and put aside as non-working. If not, maybe worth a thought:
a. I would try to flank the Seas with two drivers. With the Discovery 15 you will certainly not achieve a clean D'Appolito but a floor bounce in the mid could be minimized. If the 15 is too big, there is a 12cm discovery as well.
b. Then I would cross over to the woofers a bit lower and turn the helper woofer into a 2nd real one. Should be no issue at around 200..250 Hz
c. I would move those woofers to the side and make the front as slim as possible to preserve the horizontal dispersion as far up as possible. Of course the directionality will need to be matched between M and T.
d. I would not tilt the speaker either and use a cross over with a "straight" beam and sufficient steepness like LR4.
These are my suggestions. Take what you like and toss the rest :D
Thanks for the detailed feedback, Oliver!
Maybe I'm off the mark with some of those points any thoughts?
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