[jeffreybehr]

This is the 2nd prototype of what I called the SC658.



The slightly wider front plate (14" at rear) accommodates 4 bass drivers.  Even tho I'll be actively filtering and biamping these, Sonic Craft's Jeff Glowacki tells me even 8 of these won't move enough air in the bass to be useful, but I have to try  . Before I abandon OB bass, I might add 2 or 4 more drivers per channel in the corners of the wings if these 1st 4 show some potential but don't quite get there.  If this OB bass system simply isn't good enough, I can pull the 4 and build, attached to the rear of this front plate, a sealed box for a real bass driver.

Have determined that the Foster/Kindel 1" dome tweeter is certainly adequate (= maybe 2-1/2 stars on my 5-star grading scale)--its output is never offensive in quality (nor inadequate in quantity, either; I drive its filter cap with a 10-Ohm resistor), but it definitely lacks 'air'.  Will try probably the $32 Dayton PT2B-8 http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=275-085 mag.-planar tweeter next.

Am wiring this with better stuff, as the very-high quality of the SC65 driver deserves the best I can find.  More later.

[JoshK]

Interesting! I might have to agree with Jeff, at least on paper, that the 4 drivers aren't going to be adequate for bass, but it doesn't hurt to try either. 

Are those SC drivers polycones?  How do you like their sound?  They certainly seem like a viable option for line arrays.  I like that you can stake them so close together.

[hurdy_gurdyman]

The slightly wider front plate (14" at rear) accommodates 4 bass drivers.  Even tho I'll be actively filtering and biamping these, Sonic Craft's Jeff Glowacki tells me even 8 of these won't move enough air in the bass to be useful, but I have to try  . Before I abandon OB bass, I might add 2 or 4 more drivers per channel in the corners of the wings if these 1st 4 show some potential but don't quite get there.  If this OB bass system simply isn't good enough, I can pull the 4 and build, attached to the rear of this front plate, a sealed box for a real bass driver.

What sized speakers are those? OB bass is very practical, In my experience. I have used a 15 inch Augie on each baffle and had solid and powerful bass down to 30 Hz. If you use enough small drivers to move the same amount of air, you should be able to get good bass. I haven't had enough coffee to think clearly this morning, but I think it's something like four 8 inch drivers to equal a single 15 inch in surface displacement. It would probably take six 6 inch drivers to do the same. I could be off a bit, but I think that's close. They need to have a low fs, of course, and enough X-max for the job. Four 6 inch drivers should have about the same displacement as a single 12 inch driver, which can still make satisfying bass, although not quite as much of it.
A line array tends to be very efficient, so the bass drivers will have to play louder than for a conventional single driver OB, so that needs to be considered as well.

Hmmm, the more I think on this, the more I suspect you'd probably want about twice as many bass drivers to really get good bass impact and weight to match the main array.

Just my caffine starved opinion this morning. 

Dave

[JoshK]

P.S. I know this is a shameless plug, but you may consider my 4 10" bass drivers I am trying to get rid of.  Dirt cheap so it will give you some more ammo to work with in your project. 

[JohninCR]

Jeffrey,

With Fs, Sd, and Xmax it's simple to compute your bass capacity
for a given front/rear distance differential using Linkwitz's SPLmax
excel spreadsheet.

Ideally you'll want array behavior for bass also.  That is line length
and frequency dependant.  Driver spacing can be much greater in
the low frequencies, so a 4 driver manifold on the top and bottom
may work depending how high you want to take them.  With a manifold
you also get lowered Fs due to air mass loading, which can be quite
significant with narrow pathways like used with the Ripole approach.

How many of those drivers do you have?

[jeffreybehr]

Josh, 1--those are plastisized paper pulp, if I remember what JG said.  The driver is highly transparent; see my thread on the SC658 OB project.  Two experienced GEAs said within the system's frequency limitations, they were about the best-sounding  speaker systems they'd EVER heard.  I'll forward all their remarks, unedited, if you'll send me an e-mail.

Hurdy, they're nominal-6-1/2"ers.  They measure 6-7/16" across the flats and something like 7-1/4" OD.

John, I don't have those details on this driver, perhaps because I've not asked.  I own 24, one carton, but I'm trying to buy 8 from a friend who has ordered a carton of 24. 

The 1st (8-driver) system that I called the SC658 has excellent-quality bass, just not enough output in the middle-bass and bottom octaves.  I'm hoping that shallowing the wingtops will remove some lower-midrange thickness; then I can attack the mid- and bottom-bass octaves.

One channel is now breaking in with BB PN, and those bass drivers are thundering away.  We'll soon hear what it sounds like.

[JohninCR]

Sd is easy, just compute the cone area (including 1/3rd of the surround).  Guestimating Xmax is easy too.  Just use a very low frequency tone and back off the volume a bit once the driver makes any audible noise of operation.  Then look at the back of the driver where the leads connect to the cone to see their point-to-point excursion.  With a ruler in hand you can get a pretty good idea of their useful excursion.  Divide by 2 and that's their useable Xmax.

That thickness you were hearing was most likely panel resonance in the wings and/or cavity resonance.  Wings need more bracing than a box.  Damping material can eliminate cavity resonance, as can changing the depth or taper angle one wing at a time.  The first OB array I built using 12 4"ers had equal depth tapered wings (10" at the top & 18" at the bottom with about a 60 degree open angle).  They had a terrible resonance at about 120hz, which went away totally when I trimmed 4" off the bottom tapering to a 2" trim at the top.  Unequal taper angle combined with unequal wing depth causes real problems for a resonance trying to develop itself.

[jeffreybehr]

John, could you be more explicit on your wing dimensions, before and after?  And yes indeed, I can feel the wings resonating.

[JohninCR]

John, could you be more explicit on your wing dimensions, before and after?  And yes indeed, I can feel the wings resonating.

My little 4"s had an Fs of over 90hz.  Plus the much smaller cone area, means yours are a different league, so bracing and damping are required for yours.  Just because it's open doesn't mean the same forces aren't acting on the panels.  Splayed open helps, but otherwise all you avoid is reflections inside a box, so without the structural integrity of a corner like a box has, wings flap in the wind, which can be quite audible.  Mine were sold long ago, but going from memory the wings were 6ft tall, 10" and the top and 18" at the bottom.  I cut the wing toward the center on each speaker down to 8" at the top and 14" at the bottom.  That gave me unequal wing depth and unequal tapering, so if there was still resonance, it was spread over a wide range of frequencies.

[jeffreybehr]

Here's what they look like this morn. 


 
Screwed some 2x3s to the outer wings yesterday and will do more; a bass drum is NOT what the panels are supposed to sound like.  Have ordered the pair of Dayton mag.-planar tweeters... http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=275-085 ...and if I love the sound of them, I'll order 3 more pairs for a short line-array about 25" tall.  Without tweeters, the main drivers (the top-8 in LA) are running full range.  They have a surprisingly satisfying amount of treble thru about 8KHz, but as Sonic Craft's Jeff Glowacki warned, it's a little harsh sometimes.  The outside wings are fixed at 60 degrees back while the inner wings are hinged--and I'm not very satisfied with the latter.  (Dig the very special mass-loading devices.)
 
The bass drivers are the bottom 4--I just HAD to try dipole bass using these little things even tho JG advised against it, saying they wouldn't produce satisfying levels of bottom- and mid-bass.  Well, for me and in my room, they do indeed produce highly satisfying levels of high-quality bass, well into the bottom octave.  The 4 are wired series-parallel for a 4-Ohm load (as are the 8 for an 8-Ohm load), and I'm driving them thru a Dahlquist DQLP-1 active crossover and 2 channels of the Outlaw 770.  With the crossover point at 45Hz and the bottom-octave boost (all of five deciBels) full up, the bass is tuneful, appropriately strong, and extended into the bottom octave.  The big bassdrums in the EMI/Boult Planets and the Telarc/Goldsmith discs sound GREAT, and bassfiddles sound like the same instrument throughout their range.  The sound of the dipole bass is different than that of direct-radiator bass such as from the ET8s--it's bloomier and with slightly less initial-stroke impact, but I think it sounds more like the rest of the system, which is what I was after.  Large-panel resonances are still thickening male voices a bit, but I'll fix that.

I wired these with better stuff than last time.  I had one chunk of old Audioquest Forest+, and I used the large and small conductors from that for the B/MR drivers.  The system's hard-wired with AQ Type 8 on the fullrange and 14g.-times-4 solid cable on the bass.
 
I LOVE them.  $275 for the drivers and screws delivered to me, a $10 inductor, and $100 of MDF and drywall screws.  Add $100 for a pair of tweeters and filter caps and I'll have maybe $500--or about $700 with 4 LA tweeters in each channel--invested in these VERY-fine-sounding (and somewhat ugly) speakers.  Am I ever glad I mentioned my speaker quest to JG!  BTW they're very sensitive, too, as in the high-90s.

[jeffreybehr]

Finally (I think) have finished stiffening the wings; they did indeed sound like bassdrums originally.  The bass-side wing, the far one in this pic, is fixed, while the tweeter-side wing is hinged and is quite open in this pic.  This thing must weigh 300 pounds by now; I added a strap on the rear to assist while trying to move it.



Tweeter arrives Friday.

[jeffreybehr]

Have taped the Dayton mag.-planar tweeter on to evaluate.



Even clip-leaded and driven thru a humble Rel. AudioCap (3µF), it sounds VERY good.  Seems to have very good horizontal dispersion but quite narrow vertical dispersion.  I'll install, initially, 4 per channel. 

Retuned the B/MR stack's lo-pass filter point to about 3K (from about 1.5K) to increase the 'life' and transparency of the system and am now driving the tweeter with a 10-Ohm resistor before the cap; this reduces output 3dB at 10KHz driven with 1-octave-wide pinknoise.

[Bob in St. Louis]

  Seems to have very good horizontal dispersion but quite narrow vertical dispersion.  I'll install, initially, 4 per channel.

Jeff, since your doing multiple per channel, make a "T" out of them, with two on top horizontally, and two below vertically.
That might spread out your sweet spot.

Bob  (No jokes about 'sweet spots' being spread out, either  )   

[jeffreybehr]

I got prototype-3 running over the weekend.


The 8-tweeter array sounds VERY good.  I still have some..lots of?...work to do damping the wings.  Probably I'll use 1/4" wool felt on the outsides, and I already have a bunch of 1/8" SoundCoat* to apply to the insides.  I'll add more left-right braces, too.  That's a Sonic Craft SCC300 woofer, dipole, of course. 




* which is MUCH less expensive from Sonic Craft than from Parts Connexion, and SC has 1/8"-thick stuff.

[Bob in St. Louis]

Damn your cool! 

Bob