[PaulFolbrecht]

Short background: Got into OBs recently with a ready-made product featuring the Supravox 8" field coil paired with a super tweeter (which I never even connected) and a 17" bass driver (also Supravox).  I've traded this baffle for another with short wings that will have a bit better bass reach; it uses a 15" bass driver. 

I am expecting this speaker to be very, very good (the SV on the baffle all by itself is amazing except for what's missing under 80 Hz), but even so I believe I am going to want to build or have built another baffle, for functional and aesthetic reasons.  I want to use two 15" bass drivers and I want to have a lacquered piano black finish (!).  I will, most certainly, be using the superb Supravox as the main (wideband) driver.  I will cross a super tweeter (probably the same Aurum Cantus) probably around 14K with a cap.

I would like to share some thoughts and solicit some input.  Here's a more-or-less brain dump:

- Goals: Coherence is a big one, and the best mids possible - thus the wideband field-coil driver.  Getting the bass to integrate perfectly is also a major concern.  As for the bass itself, flat to 40 Hz is the requirement with lower being a bonus.  My room is very large.  I am not a "bass fiend"; I listen mostly to jazz but require tight, well-defined bass to 40 Hz at least (thus my love of OB and horn bass).

- Baffle shape: I lean towards a bottom-to-top layout of the two bass drivers, then mid, tweet.  I have more vertical than horizontal space.  Concern: this might put the mid driver a bit higher than desired, maybe 38".  I could also do an irregularly shaped baffle with bass drivers side-by-side on the bottom, for maximum floor bounce, tapering up, but that would probably be a bit fugly.  And I'm not sure if the increased floor bounced would be necessary or desired.

- Bass drivers: Probably the ubiquitous Augie.  I plan to bi-amp so do not care too much about sensitivity (no need to match the Supravox).  I favor the lower Fs and higher Q of this driver over, say, the Eminence Alpha 15A.  But I could be talked out of that.  Would the higher sensitivity Eminence perhaps be faster?  It's resonance of 41 Hz is probably low enough.

- Crossover: The SV will run full-range and the bass drives low-passed around 70 Hz with a PLLXO or possible an active XO.  I favor steep slope - I would prefer to do 24 dB/octave which may be better done actively.  Thoughts on that?  I am open to high-passing the SV driver at 70-100 Hz as well if I can better achieve phase coherency with the bass that way - see below!

- More about xovers and phase.  I would like to have phase coherency between the bass & widebander but with the added complications of the driver and baffle rolloff I don't know how to do that.  Meaning, the SV driver is rolling off on its own (at 12 dB/octave) at about 70 Hz, and the baffle will be rolling it off another 6 dB/octave a bit lower.  What does that do to phase WRT the bass if the bass drivers have a 12, 18, or 24 dB crossover?  (Without the baffle, I could cross them both at 24 dB/octave for 360d of shift each and phase coherence...)  I really do not understand this part of OB design at all - how the baffle rolloff and driver rolloff affect phase - and would really appreciate some help there. 

- Are short wings on the bottom half, sloped and possibly non-parallel, to increase the effective baffle width advisable?  I envision the baffle being about 20" x 60" and with floor reflection, I'm pretty sure that's large enough to support 40 Hz bass.  I do have to do some playing with The Edge to finalize baffle size.  (And I just might outsource the whole thing.)

- Another area I don't understand too well, and I'm not sure how important it is, is the effect of driver placement/spacing.  Is it going to make much different how close the 8"er is to the tweeter or the bass drivers, other than how far you need to be from the speaker for them to 'integrate'?  Obviously, the spacing will affect the lobing & interference, but as to what matters, and how you determine what the spacing should be, I have absolutely no clue about.  CAN SOMEBODY RECOMMEND A GOOD BOOK about this topic and speaker design in general?

- I would like to mount the SV driver indirectly on some type of suspension for vibration isolation, unless this is generally deemed to be overkill.  Specific ideas on that would be welcome.

Thanks in advance for thoughts and advice!

[JLM]

By reputation, the 8 inch Supervox field coil should be an excellent choice.  You must have young ears to crossover at 14k!  Is that a misprint?  An 8 inch driver (without whizzer) will start beaming high frequencies around 4k. 

I also believe in coherency and foundational bass.  I've heard twin Augies in a ceiling/attic mounted infinite baffle arrangement driven by an HK Cititation 16 that supplemented a Hawthorne Audio Silver Iris in very small OB.  The Augies contributed very little, except with extremely bass heavy material.  So I doubt baffle size/shape will matter much compared to the crossover design you come up with.  I'd recommend a highly stiff/heavy baffle and directly mount the drivers.  Dead/inert is the way to go with cabinets as you don't want to hear driver wobble and/or baffle vibration.  You're designing a reproducer of music, not a violin that you're trying to hear.

Look to the Nomad speaker design here at AC.  Woofer is at the floor, mid/tweeter are coaxial.  That's the design ideal.

Where have you heard good horn bass?  IMO (anything much under 60 Hz) that requires a garage sized concrete structure to develop the size and stiffness needed.

[PaulFolbrecht]

By reputation, the 8 inch Supervox field coil should be an excellent choice.  You must have young ears to crossover at 14k!  Is that a misprint?  An 8 inch driver (without whizzer) will start beaming high frequencies around 4k.

Yes, it beams; that is the price you pay (small, IMO) for its other qualities.  The driver is about 3 dB down at 14K which is about the place you want to roll in a super tweeter.

Look to the Nomad speaker design here at AC.  Woofer is at the floor, mid/tweeter are coaxial.  That's the design ideal.

Perhaps - not all speaker designers like coaxials by any means - they have their own set of trade-offs.  In any case, since the fullrange driver is decided upon, that's moot.

Where have you heard good horn bass?  IMO (anything much under 60 Hz) that requires a garage sized concrete structure to develop the size and stiffness needed.

I hear it from my Lamhorns all the time.  I was referring to back-loaded as well as front horns bass.  And yes I've heard some 50-60 Hz front horns that are great as well.  I love the very low distortion characteristics (fundamental only) of both horn and OB bass.

[Anglo]

Great thread.

My buddy distributes Supravox drivers so I do have some decent exposure to these drivers.  I do also own a pair of 4" in a Sipral Cornu horn and do have 3 way OBs myself with 15`drivers for bass and midrange.  I have been playing around with OBs for 4 years now and have realised a few things.

Coherence was the very first thing I had been trying to achieve since about 2 years, that is, once the WOW factor of OBs passes and you have a synergistic system i.e electronics that sing well together.

So, here`s my base theory.  I believe that coherence is ultimately revealed by a fullrange driver that is properly implimented.  That means that the bass isn`t late and lingering that to me, is not a cohesif message hence lacking coherence.  Also, a reference in coherence is Acoustats.  Limited frequency response but coherent.

If we agree up to here we will be able to say that achieving the coherence of a fullrange driver will never happen.  Just the phase shifts(of multiple drivers) become apparent once listening to a fullrange driver.

This said, I have done what I call a seamless integration of drivers through vraious ways.

I have a ribbon tweeter that is amped by a GAinclone.
I have an EV SP15A that is amped by 845 monoblocks
I ahve an Altec 515B that is amped by A Bryston 8B

This means homogenity is very hard to achieve compared to someone such as yourself who has drivers of the same company with same properties.
The tweeter is time aligned with the voice coil of the EV wideband, so it is pretty back(You don't have this to do because you don't have a tweeter).  Plus the phase shift of the 1st order crossover that adavnces the tweeter, I just have them toed in compensating for this.  So the EV's beam is  beside me and the tweeter is right at me.

The wideband should run free IMO

It should be time aligned with the 15" and IMO isolated because the vibration caused by the bass response driver is very bad on the wideband.

I have a 3rd order active xover wher I reversed the phase on the drivers from the the wideband driver (EV) and then adjusted the active over to 90 degrees.  It also cuts off at 100hz so I can get the lower mid harmonics that OBs lack IMO.

My tweeter is beside the wideband driver and is cut off with a mundord cap 3.3 MFD, which would be 6db per ocatve at 5.6 KHZ.

I have absorbed my 3 first reflections helping listener`s fatigue immensely.
Corner bass traps.
 RT60 in the room is at .5


You have a really nice project ahead of you!

[PaulFolbrecht]

Anglo,

Have two questions for you:

1) How can toe-in compensate for the 90d phase-shift of the cap on your tweeter?  That makes no sense to me.  Frankly, since people using super tweeters almost always seem to filter them with a cap, resulting in a 90d shift, I've figured it just really doesn't matter since the xover is so high when we're talking super tweeters.  I mean, is a 90d phase shift relative to the main driver audible with a 10 or 12Khz or higher xover?

2) "I have a 3rd order active xover where I reversed the phase on the drivers from the the wideband driver (EV) and then adjusted the active over to 90 degrees.  It also cuts off at 100hz so I can get the lower mid harmonics that OBs lack IMO."  You're talking about the bass driver's xover here?  Can you go into more detail on how your achieving phase alignment with the fullrange driver here?  I see you're using a 3rd order filter and connecting the wideband driver out of phase - but I don't follow "and then adjusted the active over to 90 degrees".

Is your buddy the U.S. importer, Levinson?

[Anglo]

Hi

The tweeters are placed beside the wideband driver.  They are in phase with the voice coil of the wideband driver as if there wasn't any xover at all.  Now then, we add the 1st order to a tweeter that is "in phase" with the EV but adavnced because of the 1st order cap.  I toe them in till they (being the tweeter and the wideband) hit me at the same time.  As I toe in the tweeter goes further than the wideband.

Hope I am explaining this well

My buddy sells Supravox in Canada, so no he isn't the US guy.

For the 2nd question

Yes it is the bass driver.  The voice coils of the Altec line up pretty much with EV wideband driver.  So the phase shift is 270 degrees + 90 degrees because I can change the phase on my active xover.  It remains at 18db per octave.

[PaulFolbrecht]

Thanks, I've got it now.

As for toeing in to achieve phase alignment - I used worry a lot more about the subject in general until really thinking about the fact that it's only correct in one spot (or one plane) anyway.

Ah, so you know Jean-Marc.  One of my drivers was damaged in shipping and he repaired it for cheap.

[Anglo]

...actually his name is Cyr Marc but that's close enough!

Keep us posted on your project, I am very curious.

[PaulFolbrecht]

Doh!

Cyr-Marc, of course,

[PaulFolbrecht]

FYI - Martin King has been helping me out off-line with some of these questions.  This guy is very knowledgeable about this stuff and hugely generous with his time.

[PaulFolbrecht]

As a short detour, I decided to throw together a pair of JE Labs style short, wide baffles.  Wow, what a revelation - they're great!



The baffles are 36" wide by 32" high, with wings 10" at the base and 5" at the top, with the single Supravox 8" driver mounted 13" from the bottom (center).  The baffles tilt back 10d and at 8-10' the image height is perfect.

They are flat to 50 Hz or so and probably have a bit of a midbass baffle hump, but they sound *great*!

[Anglo]

...glad you are enjoying the world of open baffles!

Very nice.