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Damping inside cabs


LITTLEWING
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Does anyone find sound deadening inside bass cabs good or bad? I use a Fender Rumble 100 with a 15" speaker in our rehearsal space (disconnected the tweeter-yuk!!) and the stupid red Leds. (yuk again!!). Anyhow, its always sounded great up to about half volume and then after that begins to sound too harsh as though the bass is 'fighting' itself. The cab isn't exactly very deep so could the waves be reflecting off the back too much?
I was thinking about damping the inside just on the back with something or other to possibly 'tame' the middle boom.

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[quote name='LITTLEWING' timestamp='1419806964' post='2642923']
I was thinking about damping the inside just on the back with something or other to possibly 'tame' the middle boom.
[/quote]

It can't hurt to give it a go. Damping is important, but a lot of mass-produced cabs don't have it - for the usual reason.
You could try a bit of backed carpet or if there's room, some cotton wadding. Stick it with some spray adhesive.

Edited by discreet
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An undamped cab is a defective cab. There's no justification for any manufacturer saving two or three quid by not using damping, other than that they just don't give a flying f..k how it sounds, and telling the consumer "if you want a better amp, spend more money". But as for damping taming midbass boom, it won't. Midbass boom is caused by inexpensive drivers and/or a cab that's too small for them.

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I've found foam packing an excellent damping for cabs - my son's work has lots of surplus packing foam from computer components boxes & some come in useful sizes. I attach it with spray adhesive (£4.99 @ B&Q) & find the black "egg box" shape particularly effective (installed in TE 4x10 & Roland bass combo).

HNY!

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[quote name='Dan Dare' timestamp='1420291390' post='2647566']
It ain't cheap, but proper contoured acoustic foam is best.
[/quote]Actually, it isn't. The main benefit of the contouring of acoustic foam is visual. It looks like it should work better than plain foam, but it doesn't. Compared to an equal thickness of plain foam acoustical foam is a lot less foam, a lot more air, and air has no damping properties.
The reason acoustical foam is made that way is to make it resemble diffraction grids, which serve a totally different purpose than damping foam. It's often called 'eggcrate foam', and in fact decades ago recording studios really were lined with pulp eggcrates. It's often assumed their purpose was damping or soundproofing, but their actual function was to scatter reflected waves. That's not what one wants to do inside a speaker enclosure. When the change was made in studios from low tech egg crates to high tech foam the shape of the crates was retained, but not the function, as foam doesn't scatter reflected waves the way that solid pulp does.

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Bill. Not entirely. The purpose of a contoured surface is not only/simply to scatter reflected waves. It has absorptive properties, too. A flat sheet of solid foam (especially hard foam) will, at least in part, reflect sound waves back at the cone - just the situation one is attempting to avoid. In practice, effectiveness is limited by wavelengths. Foam lining is less effective on lower frequencies.

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I used some candy floss / cotton wool type stuff that was originally packing from something or other at work inside my home-made 1x12" cab. I stapled it to the frame of the cab that houses a 50W no name driver (from a Harley Benton guitar amp, I believe) with a small port in the front. Perfectly adequate for playing at home and turns out it's AWESOME as a guitar cab...

I think it cost me £20 in total. Don't get me wrong, it's never going to compete with a Barefaced / TKS unit, but it does what I need it to.

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[quote name='Dan Dare' timestamp='1420317810' post='2648083']
A flat sheet of solid foam (especially hard foam) will, at least in part, reflect sound waves back at the cone
[/quote]True, hard foam will do so. Acoustical foam isn't hard. Acoustical foam does have reflective properties where the wavelengths are short compared to the solid bits of the material that make up the foam matrix, but those wavelengths are far shorter than those produced by woofers. Even at 4kHz a wavelength is better than 3 inches/85mm.

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[quote name='Dan Dare' timestamp='1420322324' post='2648147']
Looking at the original question, I wonder if amp clipping may be to blame. Most combos will struggle once the master volume goes over the half way mark.
[/quote]Perhaps, but it's a crapshoot with combos which will run out first, the amp headroom or the driver xmax. It's not unusual for combo drivers to run out of excursion capability with even 40 watts input.

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Just a quick update, I took the speaker out and found a large lump of 40 x 40cm wood pinned diagonally across the back panel which looks like there were vibration problems with the cabinet at some point, so as an experiment I screwed a fairly large piece of hard foam to it effectively raising it off the rear panel aiming to kill some box-ey sound waves.

Slap me hard and call me Doris, but it actually sounds like a bloody good quality combo now. Used it yesterday at praccy and I was smilng like a dog with two wotsits.

No more bare wood cabs for me in the future.

(Notes to Rumble 100 owners - 1/ disconnect completely unnecessary tweeter, 2/ disconnect completely unnecessary red led's, 3/ fix a lump of foam to rear internal panel, 4/ snip the spade connectors off the speaker wires and SOLDER them on.

(You will now have a sensible combo).

Edited by LITTLEWING
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A few years back, I had some 15" Tannoy Golds in 2'x2' x 1' Chipboard boxes - the backs simply screwed on, and I landed up fitting them with carpet felt, and a ruddy great 4x2 brace across the middle, the back being screwed to it in three places - used them for about ten years with no issues (apart from the pathetically low wattage handling)....sounded nice, though......

:)

vvvv I bought six at 45 quid each, they'd come from the Blind Beggar pub (Google Kray Brothers) in 1977(ish) - I used them fairly solidly until the mid 80's and sold them, out of the cabinets to a bloke that was exporting UK kit to Japan. I think he paid me a thousand quid for all of them (with crossovers), so I was a happy bunny.....they seem to be about 1k each now, but it's been thirty years......I also sold him a Quad preamp & two amps along with a Thorens deck - I think he gave me 1500 in total (sigh).....

Edited by taunton-hobbit
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  • 2 weeks later...

I recently picked up a secondhand TC Electronics BG500 combo off this here forum. I was initially very impressed with this combo (see my other thread) but the more I got to know the amp, the less I liked the sound coming out of the cabinet. To describe my issues, firstly, it didn't matter how I adjusted the tone controls, I couldn't get rid of that hard edgy mid range sound that really didn't suit my style of playing at all. Secondly, minor tweaks of the tone controls had huge impact on the tone and the overall volume.
So today, remembering this thread, I decided to whip the speakers out and have a look inside to see whether there was any damping material in there. Surprise surprise, when I got the speakers out, I found myself staring into a big empty mdf box with not a shred of damping material anywhere. Having some left over from an earlier project, I set to with the scissors and staple gun covering the back, sides, top and the top of the shelf port. Having reinstalled the speakers and refitted the sturdy grill, I fired up the combo and WOW what a massive difference. The response of the cab is much deeper sounding and very 'natural' with no harshness or brittleness to the tone whatsoever. The difference is so huge I initially thought I must have dialled up the bass control by accident.
Then I got to thinking, this combo wasn't cheap when it was originally purchased, yet a respectable brand like TC Electronics hadn't taken the time to fit 50p's worth of acoustic damping in the cabinet. I hope they were a little more careful with their flagship RS cabs.

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