advice needed on speakers for isolation cabinet

[jrmintz]

Hi,

Anyone have experience with isolation cabinets? Are there some drivers that are better than others for that use? Why would that be?

Thanks - trying to decide what to do.

Seth

[cotton]

No direct experience.  I have two different friends that built Iso Cabs, they both installed an Eminence Governor.  I am not sure why either one of those guys chose that speaker, but there you have it.
Good Luck

[jhadhar65]

I'm very interested in this topic as well.  I don't see any reason why you couldn't use any speaker that's otherwise suited for a closed back cabinet.  I'm curious, though, about internal baffles and ports.  I know there's a compression/rarefaction situation going on with the air inside and outside the cab when the cone moves.  This would be the same with the iso cab, but I wonder if a small port or series of ports somewhere in the baffle to equalize the air would help or hurt?  I don't mean enough equalization to cancel out the signal, but can the frequency response be tailored in the same way it can with a regular performance cab?

Another thing I'm curious about is internal volume.  We generally know that guitar cab dimensions aren't that critical, except as they affect cancellation in an open back cab and bass response in a closed back cab.  If the baffle isn't ported in an iso cab, it's an infinite baffle for all practical purposes.  Does the air volume affect bass response as much as it would for, say, a 4x12 cab?  If so, which space is more critical - the area behind the speaker or in front of it?

I'll search the web on this stuff now that Seth has me thinking, but I'll bet he would like to hear from the voices of experience gathered here as much as I would.

[Frankenamp]

I haven't built any isolation cab's ...yet. However, the same laws of physics will apply in a closed system as an 'open' one. The main difference is that the driver is firing into a space that is a couple of cubic feet instead of 800-1,000+ cubic feet (average room size). The things I would be looking at is Vas. having a driver with a fairly small Vas won't be as affected by the smaller than usual chamber(s). I would recommend a small (6"-8") driver that will handle the power that your amp will be putting out. Don't bother with a larger driver because it is totally unnecessesary. If you are wanting speaker distortion then cruise the thrift shops for stereos or consoles with 8" full range drivers. You don't want screaming efficiency because all that will be hearing it is the cabinet mike. If there is/are too much highs, put a wad of stuffing or fiber in front of the mike, in fact it's best to make it so that the mike can be moved around to find the best place for your 'tone'. If i was going to make one for myself This driver would be a contender: http://www.parts-express.com/pe/pshowdetl.cfm?&DID=7&Partnumber=290-045&ctab=1#Tabs

As far as dimensions: it would need to be about 4 cubic feet total divided fairly equally front to back, keep the dimensions a bit irregular. The inside would be, say, 12" deep, and 24" by 24" with the baffle nearly diagonal to moderate standing waves. Line all the surfaces with fiberglass and or poly batting and stuff the rest of the box both sides of the baffle. Keep the area immediatly in front and to the rear of the driver clear. make the driver access window large enough to access both sides of the driver and to place the mike as needed. Use PL adhesive on every joint. Use weather stripping to seal the access panel. Brace, brace, brace. Use 1/2" (12mm) baltic birch if you intend to move it. Casters are a good idea. Shouldn't need more than 50 watts- or you will have thermal issues in the enclosure. DON'T PORT NUTTIN! Porting the baffle will turn it into a quasi sixth order boom box that will be a one note wonder and you will hate it. If you are curious, cut a hole that is the same size as the baffle hole  on the opposite corner of the baffle and be prepared to seal it up again when it booms. Stuff the hole with fiberglass to make it aperiodically damped if you must.

EDIT:  Checked web, found a couple of ideas: 1) Someone is trying to patent a variation on the above idea- but using two drivers (10" celestion greenbacks) wired out of phase, miking one with a Sure SM57, Sealed, baffle tweaked, dimensions tweaked a little, (reminds me of one of Bob Carver's patents that basically patented a small bass reflex sub woofer enclosure design loosely enough to sue every other manufacturer of small ported subwoofers... The other thing that came up was an invention that replaced the permanant magnet of a conventional driver with an electromagnet! (well the trick is that this electromagnet is ajustable... (vary the volume of the driver by the strength of the magnetic field) Whooie that's an advancement in driver design- all the way back to the roaring twenties ;D I'm sure that PRR can quote chapter and verse!  That trick opens several cans of worms that I don't have time or space to go in to...

[jhadhar65]

Now that's the kind of information I'm talking about!  I don't know what Seth's plans are, but this is exactly what I'm going to do as soon as I can.  Thank you very much and I'll report back on how it turns out.

[jrmintz]

Thanks guys, that is exactly what I was looking for. Frankenamp, I'm having a hard time picturing those dimensions: The inside would be, say, 12" deep, and 24" by 24" Does 'inside' mean the part where the mic/mics are? If I'm understanding correctly, how much volume would there need to be behind the speaker? And it would be stuffed with 'Acoust-Stuff' or something like that?

[Frankenamp]

OK, I might have been a little unclear. The external dimensions will be a bit larger by the thickness of the plywood; say 25*25*13.  "inside" will be 12*24*24 not including the internal baffle which will split the inside into two chambers (front- where the driver fires into, and the mike is, and rear- where the magnet and speaker connections are). The baffle will run on a diagonal, say about six inches down the side from one corner to six inches up from the opposite (catercorner) about 27 inches long. Do a cardboard mock-up to get the dimensions right, or a full size drawing.  since you will be splitting a four cubic foot box in half, you will have two, two cubic foot halves. You have the baffle on an angle to minimize standing waves inside the box, and to increase the functional size of the box by 20-30 percent. Does this help?



Also:
Home Repo, or Lost have a product called "Peel" which is a rubbery stuff like Dynamat that has aluminum on one side and PSA on the other, it will dampen the higher resonances that would otherwise be transmitted through the wood. WallyWorld sells a mattress topper (twin size should be enough) for less than 20 that has egg-carton foam. use a spray glue like Scotch 77 or 99 to stick it on to the inside surfacees (or the aluminum side of the Peel stuff...)  Leave a spot clear of you intend to use a desk type stand for the mike.

[jrmintz]

That's great, Frankenamp. Thank you very much!

Seth