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Defo 21st May 2011 12:16 AM

Foam enclosure behind open baffle
Has anyone tried to encase the bass driver on an open baffle speaker in studio type foam to increase low end a bit while still maintaining a "box free" bass sound?

For example mounting a baffle and a high qts driver to something like this: :)


CLS 21st May 2011 06:31 AM

I once did similar thing.

I used large felt pad covering whole rear side of OB. The intrinsic character of almost all sound-absorbing materials is low pass. So the rear radiation would be restricted in lower range, thus an asymmetry front to back response along the frequency. Yes the overall in room response would be bass heavy in tonal balance (if all other things being the same).

Once the FR is re-adjusted for reasonably flat for the listening position, then the asymmetry of front-rear responses would sound very different from a 'normal' OB -- to my ears, it's inferior.

It's not the 'box' sound of ordinary boxed speakers, but the changing polar responses from low to high would be a bad thing in overall presentation.

I guess the polar response (off axis behavior) contributes to the beloved OB sounds more than the resonance free (box-less) construction.

What I said above is for wide band OB speaker. If in multi-way modular type speakers, such device can be used to tune to fit specific design goal... (cardioid midrange, for example) If so, that'll be another story....

ps. oh, one last thing. So called "box free" is mainly affecting the midrange (or to midbass at max.). While bass to sub-bass is not a major concern, actually.

norman bates 21st May 2011 08:20 AM

I liked it. But if you have the room to have the baffles 6' or greater from the front wall, I'd run without it.

But if you have an open backed box, definitely cover the side walls at least.


JohnL 21st May 2011 12:46 PM

I think what you will get is something like what John K did with his cardoid woofers, or like Hartly did with their "boffle". I think you can achive favorable results, you just have to account for what's going on.

Defo 21st May 2011 03:23 PM

Thanks for the replies :)


Originally Posted by CLS (
It's not the 'box' sound of ordinary boxed speakers, but the changing polar responses from low to high would be a bad thing in overall presentation.

Isnt this the case with normal boxed speakers in general?

The speaker will be a two-way with this woofer and this wideband, with the latter in a 350 hz JMLC front horn.

Will it be best to keep the back of both the woofer and wideband driver enclosed in foam or just the woofer I wonder?

Unfortunately I don't have the space for wide OB's placed far away from the walls.

CLS 21st May 2011 11:31 PM

With horn on the mid-high, it's worth trying cardioid in your bass section. Do it first, at least. Since it's already 'asymmetric' front/rear on your horn.

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