NADS - what you get for your free time

[454Casull]

Goals:

• Smooth response and minimal distortion (of course)
• Moderate cost
• Relatively high sensitivity
• Minimum 110 dB @ 1 m combined peak output
• Semi-constant power output

Drivers, per side:

(2x) Vifa DQ25SC16-04
Eminence Alpha-6C
Eminence Beta-12A-2
Dayton RSS390HO-4

I ended up with a hybrid design to satisfy the constant power requirement without too much hassle (the alternative is to mix mono and dipole...) The tweeters operate in bipole mode, and I think the rear tweeter should be around -3 dB, maybe a little higher in level. The mid and the midbass are operating in dipole, while the bass is enclosed in a sealed volume between 2 cubes and 3 cubes (before stuffing). I've gone for a pretty narrow effective baffle width because I'm not exactly sure how far I can push it before the dipole peaks intrude into the passbands. The baffle width as shown is 17.75"/451 mm.

The crossover points are 200 Hz, 600 Hz, and 1800 Hz. In the beginning I was considering using a combination of passive and active filtering, but in the end I decided on fully active.

I believe it would be significantly advantageous to remove the dust caps and slice the cones on the Eminence drivers. The stamped basket arms may need constrained-layer damping (applied on the back) and perhaps some wool on the inside surfaces to attenuate the reflection.

In the next post I'm going to attach a Word document with a bunch of calculations.

Comments?

[454Casull]

Word doc

[sreten]

Hi,

just some observations :

Your sealed bass will have 6dB of baffle step loss countered
to a degree by 3dB gain from the close floor coupling.

The impedance wiggles in the Beta 12 300Hz and 400Hz
indicate some treatment may be beneficial (or not).

For open baffle baffle step is replaced by baffle loss,
this is related to half space sensitivity not full space.
Your sensitivity numbers for the two Eminences
I find hard to fathom where they have come from.

Are you planning to drive the tweeters in series ?
AFAIK the tweeters should be out of phase to give
the 90 degree null of the open drivers, so as such
they should also have "baffle loss" low down.

/sreten.

[454Casull]

Originally posted by sreten
Hi,

just some observations :

Your sealed bass will have 6dB of baffle step loss countered
to a degree by 3dB gain from the close floor coupling.

There's baffle step even when something is right on the floor? Shouldn't it be radiating into half-space?

The impedance wiggles in the Beta 12 300Hz and 400Hz indicate some treatment may be beneficial (or not).

Yes, I noticed that the slicing mod on a Peerless CSX (it's somewhere on this board) fixed its impedance irregularities.

For open baffle baffle step is replaced by baffle loss, this is related to half space sensitivity not full space. Your sensitivity numbers for the two Eminences
I find hard to fathom where they have come from.

The sensitivity is into full space. Should I be using the sensitivity into half space?

Are you planning to drive the tweeters in series? AFAIK the tweeters should be out of phase to give the 90 degree null of the open drivers, so as such
they should also have "baffle loss" low down.

/sreten.

From what I recall of the baffle simulations for the tweeter, it radiates into half space down until the crossover point, so I don't think it would have much dipole loss. The tweeters will be driven separately; I'm thinking of using an AMP9 to do all 4. An AMP6 unfortunately wouldn't cut it for the peaks, but an AMP9 is actually cheaper than two AMP6 units.

[454Casull]

I ended up with a hybrid design to satisfy the constant power requirement without too much hassle (the alternative is to mix mono and dipole...)

My apologies if anybody was confused by this sentence. I can't edit the post anymore, so here I'll state that I meant to say "the alternative is to mix mono and dipole for the bass section alone".

[454Casull]

Another revision. I think I win the "worst-looking functionality" award. Rear braces not shown.

[454Casull]

Rear isometric view

[jamikl]

Not the worst at all 454Casull, unless I have no design skills myself as I have just been working out a very similar shape with two beta 15s and a mid and tweeter above.
jamikl

[Pano]

Pretty cool design!

I've done box bass at the bottom of OB before, but could never get it sounding right. I'm not too good at BR boxes..

Your design looks much better, tho.

Gary Pimm did some very nice work with the Alpha-6, but he hasn't published it yet. It should apply to the Beta-12, too.
You should be able to contact Gary thru the forum.

[454Casull]

redone calcs