?? Closed Back Speaker Cab Insulation ??

Discussion:

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Zorrro_2k

2004-02-20 19:50:22 UTC

Would a closed-back guitar speaker cab benefit (or not) from
sound-damping
material installed ? Typically, home sound system speakers are
stuffed with
some type of matting as damping material. But all the closed-back
guitar cabs
I've ever opened have no such material inside. I read on H-C that one
owner of an Avatar 2-12 cab found that the rear panel was lined with
some carpet-type material, and I presume that was for sound
damping/insulation. Just wondering....

Jim Anable

2004-02-20 20:25:13 UTC

Permalink

Post by Zorrro_2k
Would a closed-back guitar speaker cab benefit (or not) from
sound-damping
material installed ? Typically, home sound system speakers are
stuffed with
some type of matting as damping material. But all the closed-back
guitar cabs
I've ever opened have no such material inside. I read on H-C that one
owner of an Avatar 2-12 cab found that the rear panel was lined with
some carpet-type material, and I presume that was for sound
damping/insulation. Just wondering....

There are a couple of reasons to use fill, foam, or other materials.

Loose polyester fiberfill (like pillow stuffing) inside of a sealed
enclosure makes the driver think it's in a bigger box. This will shift
the resonant frequency down slightly. That will give you slightly more
"low lows" or bottom, and slightly less of a mids peak (a more even
response). The effect depends on the volume of the cabinet and the
amount of fiberfill used. I'd experiment with about on half pound of
fill per cubic foot of interior volume. If you don't like it, pull it
out!

Alternatively, an open cell foam can be used on alternating opposing
sides of the inside (the direct opposite surface has no foam). I used
the convoluted gray foam inside of my newer Hiwatt cab. This also has
the effect of slightly boosting the "perceived" volume, and can also
absorb some standing waves and certain frequencies. I like the result
that I got, but I also blocked the front ports at the same time, so I
have not A/B'ed with the foam being the only factor.

You can also do BOTH. Matting on alternating interior sides and fiber
fill.

The good thing about all of these "mods" is that they are completely
reversible. If you like attenuated bottom and a big mids peak, you
probably won't like how this changes tone. Also, changes can be subtle,
you might have to have two identical enclosures (one with mods, one
without) side by side to really notice any effect.

PMG

2004-02-20 21:04:37 UTC

Permalink

I used mattress pad foam on the back panel, and two of the adjacent
sides of my Marshall 4x10 cabinet. I glued it down with silicon
sealant. I've heard of using staples, but the silicon sealant holds
it in place very well. Don't put anything on the baffleboard, because
you don't want to deaden it's resonance.

My VibroWorld built 1936 2x12 cabinet came with all the sides, and the
back panel, all lined with a similar type of foam, but it's flat. I
think it's 1/2" to 3/4" thick. It appears to have been glued in
somehow too. If you wanted to get something like what VibroWorld
used, I've heard it suggested that you could find it at an apulstry
supply place, and I imagine that an art supply store like Michaels
would probably have the same thing.

But my impression is that the foam mattress pad material is at least
as good, if not better. And you can get those at any Kmart, Wallmart,
Target, or whatever store. Less than $20 for a mattress pad, and a
couple bucks for a tube of silicon sealant, and a little bit of time
to do it, and you're done.

fwiw, I let the silicon sealant sit over night after gluing the foam
down, before putting the cabinet back together, just to make sure that
it was on there securely.

Pete

--
Now you know why I used the qualifier "practically" --Bender

Scott Colborn

2004-02-21 04:55:47 UTC

Permalink

Hello,

I would concur with Jim and Pete's comments. I used the foam rubber
mattress pad/sheet that you can pick up at department stores, etc. On my
`74 Marshall 4 x 12 cab, for example, I put the flat side of the foam rubber
(cut to fit) on the interior right side of the cab, and the flat side of a
2nd piece on the interior bottom, and then a third piece on the inside of
the back panel, leaving the other two interior walls uncovered and the
baffle uncovered. The 'peaks-and-valleys' side is towards the interior of
the cab. I used a hand-powered construction-grade stapler to secure the
side and back pieces, making sure that each staple was well-sunk and tight,
and any missed staples were accounted for and removed from the cab. Some
folks use adhesive instead of staples.

The note articulation is more pronounced - that is, each note has a sharper
definition and clarity. The balance from low to high end is better, the
bottom end is bigger and tighter, and there was a slight reduction in the
high end (never a problem with a Super Lead and Celestion speakers). The
amount of volume coming out the front of the cab also increased - it seems
to project more volume out the front. Subjective opinions of mine.

Did I like it? Yes. The small effort was well worth it, and once I heard
the difference it made, I did my other cabs and the adjoining two interior
walls of my Matchless Lightning combo.

I also re-did my wiring harness, using 16 gauge lamp cord, and I tightened
everything I could (baffle, wheels, etc.) while inside the cab. Several of
the castors were bent, so I straightened one and replaced a component in the
other assembly, and now it tracks like it should. Always note that its very
easy to over-tighten the speakers, so just make `em snug so the frames
aren't warped. Do a once-over while you're in the cab - if you've noticed
any vibration or "noise" while using the cab, try to track down where it's
coming from. One of my magnet dust covers on one of the Celesions was
laying in the bottom of the cab, and another was loose and vibrating. I
tracked down a vibration to one of the handle assemblies, and while a friend
was playing at volume, I pushed and pulled the handle assembly and found
what part of it was causing the vibration, and then I jammed a piece of a
paper match book cover in between the handle and the outside tolex and cab,
and tore off the extra. Voila - no vibration. An easy fix that has lasted
for years.

When putting the back panel back on the cab, every one of the back panel
screws should be tight and snug. If any are stripped, either replace the
screw (if bad) or insert a toothpick into the screw hole, and bend it over
and break it off, and then insert the screw and tighten it down. The
toothpick will help it grab and hold. You want that back panel tight and
secure, and not vibrating. Each thing you can isolate and take care of
means that you have more music happening and less extraneous noise and
vibration happening. When that back panel is tight and secure, you have
more pressure and sound coming out the front of the cab (please forgive my
lay-person's terminology - I'm not an expert, just a player).

All the small stuff we can do as tweaks and the fine-tuning we can give our
rigs, all add up. When you hear the before and after of a cab that is
vibration and noise-free, and the result of your foam rubber tweaking is
evident to your own ears in a pleasing way, you'll be proud of your work and
glad you did it. Some guys don't care - I do, and I was rewarded by a
better sounding cab (to my subjective ears).

All the best.
Walk in Beauty, Peace. Scott

Zorrro_2k wrote in message

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Chris Berry

2004-02-21 09:40:43 UTC

Permalink

There are 2 ways to look at damping.
The first way is to consider how it breaks up internal standing waves and
increases the theoretical volume of the cab.
The second is to damp the panels themselves against vibration. Both are
valid in my books.

A little background as to why I think both are valid.

I did a few experiments with 2 cabs of identical internal volume (125
litres) and one smaller one. (90 litres).
the 125's were loaded with 10"s and a 12" respectively. The cabs were for
bass.
the 10"s cab was made of 18mm ply and he 12" made of 12mm ply.
There was a differrence in construction as well. the 10"s had a 4 slot vent
along the bottom and no internal bracing while I used a single cross brace
for the 12" running front to back, left to right and top to bottom -
effectively dividing all the panels into 4 and supporting the baffle very
well. the 10"s had no other bracing apart from the vents.
Both cabs were covered with acoustic foam on the inside.
Result: the 12" was a lot less resonant at the same volumes as the 10"s.
Next I made a 15" cab with the same height and depth but reduced the width
to bring down panel sizes and volume.
I covered the inside of the cab with low density fibreboard and the cab was
made of 9mm baltic birch (YES 9mm).
Thick acoustic foam on the back was used and no bracing.
Well, this cab was pretty dead compared to both the 12" (slight improvement)
and the 10"s so I decided to draw some conclusions and work from these.

1. Panel size plays a huge difference.
2. How you either stop a panel from vibrating (bracing) or damp a panel's
vibrations is important.
3. Foam only affects the lower frequencies. you need something more dense to
kill midrange resonance.

So it was time to kill the resonances (predominantly midrange) on the 10"s.
First step - brace each panel leaving rectangles with dimensions less than
10" X 10".
Next - use bitumen mat on the panels - glued on with carpet double sided
sticky tape.
Next reuse foam on cabinet.

Result: resonances dead and gone. cab very heavy but works really well for
bass.

Now for guitar you've got different requirements - you want a bit of
resonance but not too much - so...
Work with pine or ply and keep the panel sizes down, where you've got a long
panel - brace it. If there's too much resonance - use the fibreboard/bracing
or heavy rubber backed carpet (bitumen mat) for the midrange resonance
(foam doesn't do squat above 800Hz).
Just remember that there are no fixed rules just ways of achieving your
objectives...
cb