design help needed: monopole 15" / 8" MTM open baffle with Manger

[tinitus]

Nothing like a big woofer

 

[AJinFLA]

Re: 15" or 4 x 8"

AJ and others,

Firstly, I really like the idea of Neo motors for speakers. For dipole, it also makes more sense due to reduced rear masking... achieving a more proper dipole pattern versus a cardioid resulting from a large ferrite magnet.

For midrange (dipoles) this is true. Neo's/minimal masking are desirable. For sub/woofers, its mainly a weight advantage. A 275hz wavelength is over 4' long. It won't care much about a 6" dia obstruction like a ferrite motor.

So the question is: is a 15" preferable to 4 Dayton RS225 8" speakers?

Yes, if is of comparable quality/greater Vd vs the Daytons. Like a http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=295-468
Used below 300hz, it would be a superb dipole woofer.
The Eminence would be more if you are looking for lightness and very high sensitivity. Thats why I recommended it here http://www.diyaudio.com/forums/showthread.php?postid=1042207#post1042207
If you interested in purchasing some, thats probably the best non-used price you will find in the link.
Vd is the most important parameter for dipole bass
If you are building dipoles, you should have read this site 20x by now. Enter your data here http://www.linkwitzlab.com/spl_max1.xls
You can see the free space SPL of your 4 Daytons and decide if it is sufficient (/ch) for you desired target, especially if you intend to use monopole bass at very low frequencies.

cheers,

AJ

 

[taloyd]

45 degree wing angle critical?

Hello,

So I'm liking the simplicity of a single 15", and it leaves me the 4 Daytons for another project... however, is it absolutely crucial that I have a 45 degree angle from the baffle? If the baffle is wide enough to fit a 15", then adding two 12" wings at a 45 degree angle from the baffle adds 17" to an already wide baffle... this makes me want to go with the 4 x 8 configuration.

In the frequency range of 70 - 190, do you think a resonance can/will develop with a rectangular (from above) 18" x 12" shape?

thanks again,

-tal

ps: are all neodymium speakers inherently shielded, or just the Aurasound marvels?

Tal,
No experience with the drivers you are using, but I will comment on the baffle.

A few years ago, I built a test baffle with hinged wings to listen for any sonic signature with various wing angles. With any included angle of less than 90 degrees between the wings (each wing at 45 degrees) an audible resonance developed to the rear of the speaker. The resonance can be handled, but it is worthy of consideration.

For test baffles, hinges can be an easy and worthwhile addition. As well as changing angle, hinges make it very easy to change wing size by simply mounting different boards.
Paul

 

[Paul W]

Tal,
Yes, you will likely have a resonance to deal with...probably around 280 Hz and possibly quite strong. This is outside the stated operating range but close enough to xo that, at minimum, it should be considered part of the xo slope.

You could EQ it electrically but I think acoustic solutions are better. I've had good success damping a 15" U-baffle with a 1/4" layer of real wool felt stretched across the U. John K uses fiberglass stuffing.

Any higher in frequency, and some serious experiments would be appropriate. Really try to avoid straight U's in the midrange.
Paul

 

[taloyd]

Paul W: rotate 90 degrees

what if I rotate the cabinet so that the wings are extending vertically, not horizontally? So the width is still 18", but now the height is 18" + 8.5" for each 12" length wing? Ideally, I'd also like to only have only one angled wing, so as to keep the cabinet height decent. (As the full-range sits above it at ear-level).

The cabinet, from the side, would look like the crude attached jpg. The cabinet would be sufficiently braced and damped, of course...

Thoughts?

 

[Paul W]

If the sides are closed at 90 degrees, resonance will probably still exist, but perhaps lower Q (somewhat lower amplitude and broader).

If the sides are open, like a simple flat baffle, resonance shouldn't be much of an issue. Of course an 18" baffle will require much more EQ boost and driver excursion for the same LF SPL.

From the extremes, a reasonable compromise should be possible. Shorter sides will move the pipe resonance up in frequency. Move it far enough, and the resonance will be insignificantly excited because the xo will nearly eliminate any driving signal. (12"=1/4 wavelength at 280Hz, 6"=1/4 wavelength at 560Hz). Angling the sides/wings should also help, as will damping. No free lunch though...shortening the sides increases LF excursion requirements.

 

[taloyd]

compromise

Life is full of them.

I think what I'll do is make the wings 8", which will raise resonance out of the passband, as well as increase volume displacement requirements, HOWEVER, given that I have nearly a liter per 15" driver, that should be fine. That will also bring the pattern a little more dipole and a little less cardioid, as Jon Marsh, Linkwitz, and a few others are earnest believers in dipole (=symetrical radiation) being superior to cardioid...

So: I'm going to go with the Eminence Kappalite 3015LF, one per side, just as soon as I confirm that Aurasound is going to take a while to release a 15" version of the NS##-513 series. As I'm completely enthralled by those drivers, and would vastly prefer an underhung aluminum neodymium driver over anything else (and since they make pretty much the only one on the market, and VERY reasonably priced) then I want to make sure that they're not going to be releasing one soon after I purchase the Eminence drivers.

So as soon as I get word back from Aurasound, I'll go ahead with ordering the Eminence drivers, and building a very simple 18" wide box for the Eminence, on top of which will sit the Manger driver in either a sealed or dipole configuration. Going to play with dipole first, see how it sounds...

What I can do is make the box for the Eminence tall enough to accomodate a 2nd driver if SPL requirements are an issue. Also, I can try to run them clear down the range, without a 70 Hz high-pass. That might work out splendidly, we'll see.

But if Aurasound ARE going to soon release a 15" driver... than I will wait for that, as they're too lusty to pass up. And two of those per side should un-necessitate a subwoofer... if I ever decide to go that route (and sell 4 AV15 drivers)

So that's that for now...

cheers,

-Tal

attached is a jpg of the lustiest of Aurasound - the NS18