The Endeavour, a 3.5 way active speaker concept

[wowo101]

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:

• even and wide lateral dispersion (Toole-style)
• reduced baffle diffraction
• minimal floor bounce cancellation
• good vertical polar response above the speaker at least for angles up to 30°
• minimized internal reflections, resonances, and enclosure vibrations
• small speaker size while preserving full bandwith for music reproduction
• listening distance between 2 and 2.5m
• adaptability to room and placement constraints

I tried to achieve these goals by the following design choices:

• combine a 5" midrange with a waveguided tweeter or more precisely a tweeter with a diffraction lense
• facet the upper half of the front baffle
• place the floor bounce frequencies as far in the speakers' stop bands as possible, while still maintaining driver distances below 1/2 wavelength at XO frequencies
• reverse the usual MT arrangement to TM, thus reducing vertical lobing above the speaker
• choose 6.5" bass drivers suited for small enclosures
• use a second bass driver as a 0.5 way for enforcement
• use an 18mm plywood/4mm bitumen/3mm HDF sandwich enclosure, but only average bracing
• use absorption on the back- and sidewalls plus around 5" of porous absorption to eliminate the longitudinal standing wave resonance of the bass chamber
• tilt the box slightly upward
• cram 4 150W switching amplifiers and an SMPS into each box
• use an external MiniDSP 4x10 Hd as a digital preamp and crossover box

Outer dimensions (W*H*D): 21cm * 99.5cm * 30cm

Drivers used:

• 2 * Wavecor SW 182 BD 02 with dual 2" ports
• Scan Speak 15W/8424G00
• Seas 27TBCD/GB-DXT

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!

[gainphile]

Very nice looking design. I also have DXT and liked them.

What do you use to draw this design ?

[wowo101]

Thanks! For the drawings I used Adobe Illustrator as I'm familiar with it. Will have to switch to at least Google Sketchup, though, before construction gets real.

Glad to hear you liked the DXT; do you have any measurements yet of how it performs on an open baffle?

[madmac]

It all sounds great to me!!. I find speaker building fascinating !!

[cjd]

The MT + baffle angle *adds* to acoustic offset between these two, which is usually not what folks strive for. I can't tell the height for sure (did I just gloss over that?) but this is why you sometimes see the upper section kick forward (avalon) - spec out distance to your ear to see about time alignment.

Of course, this is solvable in the digital domain, but worth mentioning anyhow; it always seems to me that things work better when you use physics instead of trying to overcome it.

Can't comment specifically on box tuning without pulling up driver data and all that, but I know of only one woofer of this size that comes close to "full spectrum" for my kind of music. That said, don't undersize the box - it's worth giving drivers the room they really want.

Box construction sounds complex but good. Me, I just go for lots of braces and dead simple construction.

Remember, when it comes to managing tweeter diffraction, thick wool felt can be your friend.

C

[wowo101]

Thanks cjd for your detailed answer!

You're absolutely right about the M-T arrangement – the added offset might be the reason I see bad phase tracking between the drivers in the simulation. My reasoning for this arrangement was

a) tilting the vertical lobes upwards and
b) lowering the floor bounce frequency of the mid so it's in the driver's stopband.

I'll see how vertical dispersion looks with a reversed arrangement in the sims. Considering b): If I move the mid down, I'll move up the cancellation frequency to 300 Hz for my intended listening arrangement. Do you think that will be noticeable in the in-room FR?

The Wavecors for the bass require a volume of around 15l each in a ported box, so the common enclosure with a gross volume of 29.3l (minus bracing, drivers etc., plus ~15% via absorption materials) should be fine. I lowered the tuning ab bit to get ~3dB down around 40Hz.

I also calculated how the RS180s would perform in the concept, but they would in fact requirequite a bit more volume.

Thanks again for your feedback!

[wowo101]

Next iteration

Following cjd's suggestions, 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!

Additionally, I changed the LP slope of the helper woofer to first order following advice I got on diyaudio.com, but I 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!

[meb46]

wowo101/CJD - I'm a bit confused on the need to remove the tilt? I thought the drive for doing this was essentially to improve the time alignment of the drivers and essentially put the acoustic center at equi-distance to the listeners ear. On my MIC build I used the following model to generate my vertical dimenstions and speaker positioning. For the dual Bass Drivers I used the Acoustic center of these two as the mid point between their respective Acoustic centers... this was based on the fact that both Mass Drivers utilize the same internal volume cavity.



Super work thus far on The Endeavours, love the look of them... looks like the faceted "Avalon Look" is the flavour of 2012

I'm all for complex builds, the assembly complexity is the bit I love the most.

Any thoughts on the build method? For complex designs like these I fully recommend the CNC Routing approach. It is naturally more expensive but you will reap the rewards when it comes to assembly ease and speed. Further to that, when you are tackling complex cabinets like The Endeavour, CNC accuracy will make the process more simple... and reduce the possibility of "interest loss". I worried in the early days of the MIC design on the possibility of me loosing interest and wasting both time and a great deal of $$$ by simply getting lazy. CNC Routing took all of the dusty time consuming part away so I could save my attention span for the assembly and finishing. I guess some people love the dusty MDF cutting process, but it could take its toll on some people (Myself for one).

Just my 2 cents...

Looking forward to seeing the build!!!!

[cjd]

I didn't suggest that a straight baffle should be used - just that the time alignment should be considered because the MT put the mid behind the tweeter by quite a bit, when usually you want to have the mid's baffle forward of the tweeter.

If you sit down and do the math, it's pretty tough to be off. Well, unless you mess up the math but...

[meb46]

CJD - Roger, understand

[wowo101]

You’re both right, of course: By removing the tilt *and* swapping the MT arrangement at the same time I've thrown out the baby with the water – that dawned on me right after having posted... ops:

Since the arrangement with the tweeter at the top seems to work well dispersion-wise, I'll next sit down and do the math for an optimal z-axis alignment relative to the listening position, and I'm almost certain the tilt will come back. Thanks for the heads-up!

Concerning the build: Yes, I'll definitely go down the CNC road, especially since I’ve had very good experiences with http://www.speaker-space.de for CNCed custom enclosures.

[wowo101]

Update!

As promised, I redid my math to see how a sloped baffle might help with time alignment of the drivers (see attachment). 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 speaker dave's suggestions on DIYAudio 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 attached FR graphs.

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:

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?
2. What would be the minimum depth for the front baffle faceting to be useful? I sketched and simmed a 25mm baffle – would that be enough in your opinion?
3. When using a constrained layer/bitumen sandwich approach to enclosure damping, how thick should each layer (plywood, bitumen, HDF) be in your opinion?

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?

[David G]

Why are you worried about vertical dispersion above the tweeter? Do you listen to your system standing up? Design for the area in which you'll be seated.

[wowo101]

Two reasons: One, I'd generally like to have as even dispersion as possible to avoid spectral imbalances in the diffuse/reflected sound; two, I'd like to avoid that tonal balance will change when listening standing up, which will sometimes be the case, the speakers being placed in a combined living/dining room.

I know there are limits to what's achievable in these respects, but I'd like to pushn them a little. And if you have a look at the graphs, you'll see that the speakers are designed to be maximally flat on axis (i.e. at the listening position when sitting) anyway.

[wowo101]

A fresh start

Hey all,

after a hiatus of a few months, I've reconsidered the design decisions of my Endeavour project and entered a second design iteration which I'd love to get your feedback on.

I've reconsidered midrange driver, baffle width and woofer section, starting with the following assumptions:

• A smaller mid could be used to reduce CTC distance between tweeter and midrange since max. SPL is limited by tweeter and woofer.
• Front wall reflections should be reduced down to at least 500 Hz, i.e. in the range where our hearing is most acute. Cardioid solutions were ruled out because of implementation complexity and uncertainty about behaviour near walls.
• Boundary effects/room gain simulations showed that a CB configuration would yield the flattest bass response in room; at the same time, impulse response could be optimized by switching to CB.
• Going 3-way instead of 3.5-way would reduce cost and complexity.

Following this, the design I arrived at differs from the first approach in the following aspects:

• Scan Speak Discovery 12w midrange driver instead of 15w, which also matches the DXT's directivity at XO frequency better.
• Baffle width expanded to 26cm, resulting in a baffle step center frequency of 442 Hz; accompanied by larger chamfers to avoid increased edge diffraction effects.
• Bass is now handled by a single 8" long-throw woofer (TIW 200 XS - 8 Ohm) in a 35 l closed box per side; possible bass extension by adding a Linkwitz transform to a suitable Fc.
• W-M arranged to achieve a good compromise between floor bounce reduction and driver integration.
• W-M XO moved to 500 Hz (geometric mean of first woofer and midrange floor bounce nulls).

The preliminary results are these:


(Construction sketch)


(Room gain simulation)


(Driver and summed FR)


(horizontal and vertical response plots)

Please ignore tweeter behaviour at angles > 0° since the DXT waveguide can't be modeled correctly in BoxSim.

My main concern now is power response at W-M XO which isn't as smooth as I hoped it would be – any hints on that?

And apart from that: Any other feedback?

[JonMarsh]

First thoughts are that the mid is covering a fairly narrow bandwidth- though you must have your reasons. I suppose you may not be able to push the Discovery mid lower or higher- but the DXT works best with around a 2K or higher crossover, from other's experience. Any way to stretch your buget to the SS 12MU/4731. In general, the time you've taken to mull this over appears to have paid off- which is usually the case! And it's cheaper to think things through than to build several versions!

Strictly from a gut feeling, I like the design, but I'd be tempted to use the more expensive SS midrange, and go for 350-400 Hz low end crossover, and 1800-2 kHz top end crossover. But that's just my 0.02 without any serious analysis, other than knowing where the DXT waveguide quits functioning.



The midrange is so critical to overall presentation and enjoyment, I tend to be agreeable to more budget woofers and tweeters within reason, but sticking to a top flight midrange when possible. (like the lower end Ardent MkII configuration I'm working on-)

[wowo101]

Thanks for the feedback, Jon!

The reasoning behind the – admittedly counter-intuitive – crossover points is this:

The tweeter height is determined by listening distance and height. The midrange is placed as close to the tweeter as possible, and the M-T crossover is set to the highest possible frequency that still guarantees CTC distance <= crossover wavelength / 2 to minimize vertical lobing. (As a reference for how low the DXT could be crossed using LR4 slopes, I took the Grimm LS1 which crosses it at 1.550 Hz: http://www.grimmaudio.com/hifi_loudspeakers_ls1.htm).

With the midrange height fixed, its floor bounce null for the intended LP is around 400 Hz. The woofer is placed as far below as the simulations suggested would be ok for driver integration, which resulted in a floor bounce null at around 630 Hz. The crossover was then set to the geometric mean of these frequencies, i.e. around 500 Hz. A check against other designs using the same woofer showed that it can be used up there without problems.

Does this make sense, or am I getting my priorities wrong?

(And of course you're right – the Illuminator midrange would be a worthwile upgrade, but it's beyond my budget for this project.)

[Face]

Just because it "can" be crossed that low, doesn't mean it'll sound it's best there. There's nothing worse than the sound of a strained tweeter.

[wowo101]

I think Mark K had distortion measurements of the DXT on his blog, but that's long gone – in his measurement summary, though, he concludes that "based on these distortion measurements the usable lower limit of this tweeter is 1.5k" (http://www.audioheuristics.org/measu...n/Seas_DXT.htm). In the ER18DXT he uses it with a 1st order electrical crossover around 1.8 kHz. I don't have any personal experience with the DXT, but Grimm Audio and Mark seemed to be good advisers to me…

[wowo101]

Jon, Face,

today I ran my simulation again with crossover points probably more to your liking, and the results on- and off-axis were equally good (except for vertical above the listening axis). Incidentally, the crossover points shown would avoid the first floor bounce nulls for a shorter listening distance (2.5 m instead of 3.1 m).




Since I'll be handling all the crossover and equalizing stuff digitally, I guess I should be fine with the construction as proposed and find out by experimenting, measuring and listening whether avoiding the floor bounce nulls or having a more boradband midrange coverage will be more rewarding.

Thanks again for the feedback!