Altec 828 horn flare without a rear enclosure

[jocelys1]

Hi,

Could we design an Altec OB (sort off, for want of a better word), with the speaker basket sealed off like the Klangfilm Bionor, shown here.

What does this sealed off rear do ? Is it similar to an aperiodic enclosure ? GM (who else) has explained this in a post on diyaudio here, but I didn't understand it.

Quoted from diyaudio.com:

Right, damping the vent is for fine tuning the cab, damping the driver is required to make it ~aperiodic and to do that requires the cab be tuned above Fs since adding stuffing lowers Fb, so tuning to ~Fs/Qts with a cab Vb = ~sealed max flat and stuffed with 1-1.5 lbs ft^3 fiberglass acoustic insulation works well, ergo as Vb rises above this point the vent area rapidly increases to where it becomes baffle size and why either using a stuffed to suit open back cab or just a flat baffle with insulation stapled loosely over the back of the driver has historically been the DIYer's default aperiodic alignment.


Will this work for a 416 or 515, used with the 828 flare ? Will it reduce excursion, which seems to be the main problem in an Altec OB ?

thanks,
Sunil

[GM]

Greets!

Based on this response it's obvious that the total system Qt [sysQ] is quite high, i.e. designed for OB, whereas the Altecs clearly aren't, so if made ~aperiodic they wouldn't have any bass to speak of and if they were open back like the Bionor, even some of the horn loading would be rolled off: One Amigo Visits Utah To Hear 5 Reference Turntables-My Step Beyond - Page 9

Best way to acoustically protect the drivers is either to compression load them in a small sealed cab or tune the box real high and in either case XO to a separate sub system.

GM

[jocelys1]

Was hoping you'd pitch in, thanks GM.

You're right about the KFs high QT, making them right for an OB design. Guess there's no free lunch when it comes to Altecs.

Would this work - an enclosure of 12" sides, with the rear open. Cover the insides with damping material and the rear panel with dense cloth.

[GM]

You're welcome! Forgot to answer your Q.......the canvas? tightly stretched over the drivers does act as a somewhat lossy low pass filter cum back box, reducing rear reflections and damping its high [sysQ] against 'ringing' based on the response plot's roll off; notice the 'knee' at ~70 Hz? I imagine there's a high Q spike here without it: http://www.whatsbestforum.com/attach...7&d=1437254836

Overall, a nice design and imagine it sounds more HIFI than competing Altec, etc..

GM

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Would this work - an enclosure of 12" sides, with the rear open. Cover the insides with damping material and the rear panel with dense cloth.

What I said earlier: and if they were open back like the Bionor, even some of the horn loading would be rolled off

The 12" sides are acoustically small enough that they would have little impact on performance unless a > 1 kHz XO point was used. I simmed a similar lossy backbox for someone on the HE forum in MJK's horn software back when it first came out and IIRC it rolled off much of the horn's BW, which ends up around 342 Hz IIRC.

Didn't simm it, but a 420's high Q motor with a 416 or 515 kit might work OK, especially if driven with a high output impedance; ditto an original 803 or 515 [no suffix]. Don't know of any modern prosound drivers with a high Q motor, but could experiment with some relatively cheap wide BW HIFI OB 15".

GM

[jocelys1]

G, off the two designs - canvas stretched over the basket |12" enclosure (rear open), which one is the better compromise considering OB performance and cone excursion ? I'm not looking at real LOW bass, and definitely not for HT.

How do we model this in MJKs sheets ?

- - - Updated - - -

Actually, the 12" dimension was what I remembered from MJKs write-up. We can consider a different size if there is benefit in what we are attempting to do.

[GM]

Dunno, but my SWAG is neither. Anyway, why would you want to if it ruins the horn's response?

At this late date [at least a decade and maybe longer], guessing a customized worksheet I had done by others that was familiar with MC 2k pro or maybe figured a workaround using the BLH. All my old stuff/tech library, myriad design software is locked in a pair of damaged HDs, so been mostly blind, deaf and dumb for many years now, relying on Hornresp, increasingly fading memories and search engines to find old postings to answer folk's Qs.

Hmm, HR can do compound horns [A7 or similar] now, so can do the cab, but not sure its 'filling' feature will work and not really up to date on this and the rest of it's new features, which includes [3] way Synergy concept horns now.

GM

[jocelys1]

Dunno, but my SWAG is neither. Anyway, why would you want to if it ruins the horn's response?

GM

I want to try this to get the midbass OBs are capable off. Keeping with SWAG, I guess neither will work effectively then ? Also, a full blown 828 type horn flare+rear enclosure is large and I don't have the space.

What if we ditch the horn idea and go with an OB, will either design help with cone excursion ?

At this late date [at least a decade and maybe longer], guessing a customized worksheet I had done by others that was familiar with MC 2k pro or maybe figured a workaround using the BLH. All my old stuff/tech library, myriad design software is locked in a pair of damaged HDs, so been mostly blind, deaf and dumb for many years now, relying on Hornresp, increasingly fading memories and search engines to find old postings to answer folk's Qs.

Hmm, HR can do compound horns [A7 or similar] now, so can do the cab, but not sure its 'filling' feature will work and not really up to date on this and the rest of it's new features, which includes [3] way Synergy concept horns now.

GM

I'll have to look at HR again then. Damn.

[GM]

Horn or flat, baffle, makes no difference, only how dense the stuffing and until it becomes as resistant as a sealed rear chamber, won't reduce out-of-band excursion all that much unless peak power requirements are quite low, like maybe 10% of driver rated power, which at this level the horn is the obvious choice and the stretched canvas trumps your modest 12" stuffed box short of using a thick down comforter or similar, which at this point negates any perceived dipole 'magic' beyond having an over-damped response.

GM

[jocelys1]

Thanks GM. That settles it for now.

I have to rework the horn with an enclosure of minimum size ~3C.ft. Will post response graphs soon.

[GM]

Well, try to get a ~1.5 ft^3 net closed rear chamber to ~optimize it, ~80 Hz/f6, easy-peasy 120+ dB peaks above ~100 Hz in theory without exceeding Xmax.

GM