I I have often wondered if anyone has tried slowing down the back wave from a driver by hanging or attaching a sheet of 2 inch foam directly behind the driver? Or how about building a 2" foam box all the way around the back of the driver to enclose it from the rest of the cabinet. This would make the back wave have to slow down going through the foam backwards, then coming back towards the back of the cone again it would have to force its way through the foam again. Yet with foam being porous it should not load the driver differently I would not think. Can anyone share their thoughts on this or results if it has been done?
Interesting idea. I think the foam would act as a lowpass filter of some sort on the energy hitting the rear wall. The thickness and density of the foam would determine x-over. I wonder if it will give some noticable phase change in the time domain.. Would be cool if it had the same effect as having a longer distance to the rear wall! It would also reflect some energy back to the driver but probably neglishable amounts. Will be interesting to hear from someone that has tested this.
I am sure nearly everyone on here is more experienced than I am, and probably way more educated in the scientific side if they are into engineering. But when we design speaker cabinets, we do not allow extra air space for the 2 in pyramid foam we put on the walls. We consider it acoustically transparent. When we stuff a cabinet with fiberfill, we know it fools the cabinet into thinking it is slightly larger, but do we engineer the crossover for this? I've not heard of it being done. The enclosure of foam I'm recommending is not supposed to partially seal the driver, but just to slightly slow the speed of the back wave energy. Not trying to slow it to basic zero, or kill the back wave. Does this change anything, or does it just show my ignorance? LOL
You will have a SLIGHT slowing with no practical effect whatsoever.
FWIW completely filling enclosure with fiberglass or similar will be much better.
FWIW completely filling enclosure with fiberglass or similar will be much better.
Interesting question, though. What about attaching a rubber bladder to the driver on the inside of the cab of a specific volume and resistance, ie: thickness? IOW, tuned to the driver/cab, whatever/however that could be calculated/quantified.
This would seem similar to the old method of reducing driver Q by adding resistance to the basket vents, which is like stapling some blanket material to the baffle cutout and fitting the driver snugly into it.
I wonder if anyone has made an active back wall for a speaker cabinet? A second speaker behind the first that, instead of presenting a stiff boundary like a piece of plywood, it absorbs some of the front speakers back-radiation in the problematic frequency range.
What goes behind that speaker? A small sealed box. Thinking a more expensive speaker facing the listener and a less expensive driver for the "absorber" part. Will require an extra pair of channels to drive the absorbers and HP and LP signal filtering.
What goes behind that speaker? A small sealed box. Thinking a more expensive speaker facing the listener and a less expensive driver for the "absorber" part. Will require an extra pair of channels to drive the absorbers and HP and LP signal filtering.
I have thought about putting a second thin but braced perforated internal baffle roughly halfway along the inner depth of the enclosure, or diametrically oriented between the inner corners, the perforation would be made by ~10mm holes the number of which would sum up to ~ the same area as the drivers Sd.
Damping could them applied on both sides along the cabinets inner walls and top/bottom panels.
I'm not sure why this would solve or accomplish to be honest but I assume a bit more of diffusion and diffraction would come into play, but this is only an assumption.
Damping could them applied on both sides along the cabinets inner walls and top/bottom panels.
I'm not sure why this would solve or accomplish to be honest but I assume a bit more of diffusion and diffraction would come into play, but this is only an assumption.
It's a great idea, but it wont work. If you can get past it absorbing the wanted resonance energy, it also goes modal, which is as detrimental to the success of the active woofer as it is for the original.
[there is a way, but it's needlessly elaborate]
I believe Joe was talking about a speaker mounted on the back wall of the box, whose task it is to respond to pressure within the box, and move to relieve it.. which would make it an absorber. (The back of that back speaker would also be closed off from the room.)
Foam? Probably, though historically, [somewhat folded] sheets of it was loosely stuck in cheap speakers and others with cheesecloth + fiberglass insulation, wool or other high DF materials having been loosely stapled over drivers to form an acoustic 'blanket' or hung from the cab's top plate as a 'fluttering wall' to vertically divide the cab in half [Bozak], but it was RCA's Harry Olson that patented a proper damper/diffusor after WWII as either a secondary driver or as a whole cab, which me and others have built with various coverings for 'FR' drivers, though don't recall any with foam, so not much new in speaker design: US2502018A - Diffraction type sound absorber covered by a membrane
- Google Patents
I can't remember or find RCA's actual speaker, but IIRC from mostly hearsay it was closest to a big PR box alignment WRT to LF output, yet was a relatively small box performing more like an aperiodic one. Quite impressive for the times I'm told, but apparently too expensive for a country recovering from two costly wars.
"The solution proposed by B&W's chief electronics engineer, Laurence Dickie, was to fill the interior of a conventional high-density particle-board cabinet with a matrix structure resembling a honeycomb, with long cells square in cross section. The matrix is formed of a series of interlocking, perforated members having relatively high damping qualities and high stiffness. The inner walls of the cabinet are grooved to accept the horizontal members of the matrix, which is then rigidly bonded to the cabinet. Finally, each cell of the matrix is filled with acoustic foam to damp any remaining resonances."