[James Romeyn]

Would greatly appreciate driver recommendations with which members have had experience, for open-baffle single or multiple drivers, for the range of about 70 Hz to 300 Hz, approximate sensitivity 89 dB, minimum power handling 400 WRMS, prefer 4-Ohm impedance. 

Crossover at both ends will most likely be active, 1st order high & low pass.  Maximum baflle height 18". 

For horizontally sited multiple drivers, what are the generally accepted upper frequency limits?  (I have appreciated the greater focus, narrowed polar response, & less room interaction resulting from side-by-side mouting of drivers crossed as high as 4kHz.  300 Hz would appear to be fine...though I'd prefer a narrower enclosure rather than wider.

For Isobarik (front-back) loading, what are the generally accepted upper frequency limits?  My understanding is that beyond a certain frequency, Isobarik loading presents definite audible drawbacks. 

Would appreciate a refresher of the pros/cons Isobarik vs. horizontal siting of multiple open-baflle drivers.             

Thanks all.   

[Bob in St. Louis]

Iso-loaded open baffle? I'm having a hard time picturing that....
Is this some kind of hybrid baffle your thinking about?
I have an excellent idea for an OB driver. It's an amazing driver, but I'm not quite sure what your goal is. This driver wouldn't 'fit the bill' for some of your specs though.

Bob

[James Romeyn]

Bob
Sorry for confusing readers w/ what may be incorrect terminology. 

In the 80s or 90s (19, not 18 you wise guys ) a British company called Celestion built & sold an OB "sub" w/ two-12s, one directly IFO the other.  It included an active xo/eq & was sold as an add-on to their then-highly-rated mini-monitors (or as a complete system).  Chassis was steel if memory serves; design was modern, grill was fabric stretched over plastic hemispheric-shaped frame.

Also just saw pics from RMAF '06 including a new OB speaker w/ a sub having similar architecture as above...am pretty sure Linkwitz' current kits & his earlier failed commercial line also included a similar sub. 

Linn's "Isobarik" terminology was previously applied to front-back loading, but possibly the term only applies to standard enclosures & not to OB systems.  Sorry if I confused readers.

Anyway, would love to read about any OB design/architecture only for the range 70-300 Hz, 89 dB sensitivity or better, 400 WRMS power handling or better. 

The previously mentioned maximum height specification of 18" could be increased to 7' to accomodate a tall linesource. 

[Bob in St. Louis]

Well Jim, That 70-300 is killin' it for me. Sorry, I've got no suggestions.
May I ask what your building. Why such a small HZ range?

Bob

[hurdy_gurdyman]

Jim,

The Celestion bass system you are refering to is nothing more than two open-backed drivers firing into a standard manifold. Any decent bass driver that is good for OB use should work.

Dave

[James Romeyn]

I'd appreciate very much any specific drivers suggestions.

Is it corrrect that a Q of between .7 to .8 is ideal?

For a low-frequency cutoff of 70 Hz what is the target maximum fs?

[scorpion]

So, it is about room modes ? Well OB Bass might be a good choice.

Going up to 300 Hz I would put the drivers on a flat baffle.

Just to test if this may be a good solution: Buy a pair of Goldwood 12" or 10" OEM  woofers 8 ohm impedance from PartsExpress.
Place them parallelwired on a testbaffle (1" MDF) 18" wide and put that on the floor and start playing, testing and experimenting.
If the test is satisfying you might consider going over to OB Bass altogether and then we could start talking about elements more precisely.


/Erling

[James Romeyn]

Erling
Thanks. 

My main interest in OB is not currently room modes, but rather eliminating the rear wave reflections bouncing off the enclosure walls & hitting the back side of the cone.  The current system is sealed from 300 Hz to about 8kHz, but those rear wave reflections will soon be eliminated.  So that leaves the 70 Hz to 300 Hz question.       

My current understanding is that an OB system to the mid-low-20 Hz range would require a too-large baffle surface area.  So for that range I'm planning on sealed equalized corner-loading. 

Corner loading maximizes boundary reflections & minimizes direct output.  This virtually eliminates the difference between the two.  Those differences normally cause modes & distortion.  Maximizing boundary reinforcement also has the benefit of lower THD & increased output.  The only known down side is that it may require more eq. 

But I'm always open to new viewpoints unless my presonal experience dictates otherwise.   

[Rudolf]

If you already intend to use the room corners, J. Dunhams design could be a suggestion:
http://www.bd-design.nl/forum/forum_entry.php?id=11270
You will find a link to some pictures in his postings.
Originally it is a design of Larry Selmer:
http://www.diyaudio.com/forums/showthread.php?s=2585c437b22d511e1c71fe5ad11cd144&threadid=57265&goto=nextoldest

Rudolf
www.dipolplus.de

[scorpion]

Jim,

Some flesh on the bones. I think combining Rudolf's and mine suggestions would be a good idea for a start.
With cornerplacement push-pull arrangement of drivers might be useful.

/Erling

[James Romeyn]

Going up to 300 Hz I would put the drivers on a flat baffle.

Vs. what other baffle shape? 

...a pair of Goldwood 12" or 10" OEM  woofers 8 ohm impedance from PartsExpress

Dual voil coil models?

...and put that on the floor...

Vs. what other location?

Thanks.

[scorpion]

1. Popular 'enclosures' for OB-woofers include H-shape, U-shape, N-shape and W-shape. Depending on size they will all suffer at least one serious resonance in the frequency range you are planning for, 70 - 300 Hz. Thats why flat. If you go to Siegfried Linkwitz' web: http://www.linkwitzlab.com/ and take a look at the Phoenix design (both the woofer he uses and the prototype woofer) you have examples of the W- and H-shape.

2. These were only ment as elements to test with without ruining you. Take GOLDWOOD GW-212/8 12" OEM WOOFER 8 OHM for instance. It is a simple woofer, PE's price is $ 18.95, but it has TSPs suitable for OB use, but Xmax is limited. Two of these on a baffle say 24" x 18" should be able to produce over 100 dBs at 70 Hz irrespectable of baffle placement. The 10"ers about 2 dBs less. 8 ohms impedance will be a 4-ohm load to the amp when paralleled.

3. Bassoutput from these woofers will be helped by their parameters. Floorplacement will give you something like 8-10 dBs more output in the bass compared to the baffle beeing lifted from the floor. If you follow Rudolf's suggestion and place them on the floor in a corner then you will gain even more. I found his links very interesting. Your problem will be the directivity content in music above 100 Hz and eventually choosing a good quality element that is up to your standards. With my suggested elements you will for the try get away with the need to equalize woofer response, I think.

I hope you will find the Linkwitz' web useful for your purposes even if it is very technical. 

/Erling

[Russell Dawkins]

Scorpion - have you seen any of John Kreskovsky's writings on U frame woofers?

http://www.musicanddesign.com/u_frame.html

he appears to know his stuff and the comparison at the bottom of this page between the H and U frame performance is surprising.

Has anyone heard his NaO speakers??

http://www.musicanddesign.com/

On the surface of it they seem to have anticipated a lot of the current design and configuration buzz - MTM OB dipole with rear tweeters (for dipole HF) - even dipole (if not strictly OB) bass.



Russell

[scorpion]

Russel,

Obviously John Kreskovsky knows what he is doing. His parting a bit with OB but his damped U-baffle is a very interesting approach for a subwoofer design although I suspect that the dampening procedure is not so easy as perhaps described. Being on the other side of the Atlantic I have no experience of the Nao but element choice is impressive.

/Erling