Discussion in 'Amps & Cabs' started by smitty_p, Sep 2, 2013.
Just play a little Jailhouse Rock and it'll be fine.
Or if it's a C&W gig, Folsom Prison Blues.....
Then, suddenly...I was done!
It's been awhile since I posted and realize I just finished my cabinet, with one major change from the original plan. I have decided not to use the wedge foam. I may still add it later, but I've decided to forego it, for now.
Here's why I made that decision. Essentially, acoustic foam is only effective at higher frequencies. It doesn't really absorb sound. It can be helpful in diffusing some frequencies. However, the Rockwool I've put in is pretty much already absorbing more than the foam would, anyway.
So, instead I just covered the rockwool with a fabric called "Osnaburg." It is a thin, loosely woven 100% cotton fabric. I use it pretty much just to protect the rockwool and to contain fibers. One fabric that some folks use is burlap, but the burlap I found seemed to be too loose and could let small rockwool fibers out. The osnaburg is thin and loose enough to easily blow air through and should be acoustically transparent.
Plus, at less than nine dollars for three yards, it is WAY less expensive than true acoustic foam!
Here is a shot inside the cabinet.
I chose white fabric on purpose since it would allow me to see more easily in dimly-lit situations. I covered the floor with a shag carpet. I also put a black felt border all around the edges. In the shot above, you can see how I made corner fillers for the front corners. The idea here was to lessen any standing waves that I may get in the corners, though, admittedly, there is not enough space in the cabinet to build a truly functional bass trap. This is really more just to break up sound waves.
Here's another shot inside the cabinet. Incidentally, I used 3M 77 spray adhesive to glue the black felt onto the edges of the rockwool and I used contact cement to affix the felt to the wooden edges. I also used the 3M 77 to fasten the cotton fabric to the face of the rockwool, but I only applied adhesive to the edges and the corners. I did not spray the entire surface of the rockwool. I wanted to keep the rockwool porous and was concerned that too much adhesive would seal the porous texture.
This next shot shows some corner detail. I applied some shellac to the edges of the fabric to keep it from fraying. You can also see the weather strip along the edges to help seal the lid. I was going to use a fuzzy strip, but I couldn't find anything. The weather strip seems a decent alternative.
This shot is just a shot of the whole cabinet.
Finally, here's another pic with my Fender Champ.
I'm really looking forward to using this, now that it is done. If I decide I still want to add wedge foam panels or some additional diffusing, I have a plan for how I can add that later without messing things up.
Very nice. You thought through it thoroughly. Looking forward to a review. Incidentally, what does it all weigh?
It was a very interesting project. I did a lot of study prior to beginning and I continued to research as I proceeded through the build.
One thing became apparent to me as got into the build. The box is really serving two purposes. First, there is the obvious purpose of isolating the sound. This would suggest that you would do anything and everything to absorb sound.
However, there is a second, less obvious purpose. That is, since I have a microphone in the box picking up the sound of the amp, the box is also a miniature studio. Therefore, the sound has to be absorbed in a balanced way. On one forum (it may have been GearSlutz - I don't recall for sure) one individual noted that the highs and mids are easy to tame. Focus on the bass frequencies and you'll often end up taming the highs and mids in the process. This contributed to my decision to forego the foam, for now. In this sense, I was thinking less about how it would sound outside the box, but more how it would sound inside the box.
Believe it or not, but I've already thought about how I could add more Rockboard 60 insulation panels, if necessary! I'm even contemplating some sort of acoustic baffling mounted in the box made from some of my left over Rockboard. But before I do all that, I want to see what it sounds like now.
I would have preferred to build it larger. However, where I will use it the most, I have a narrow doorway to fit it through behind the stage. I made it as wide as I could, though it is still narrower than I would have preferred. I will post pics in a few days to show what I mean.
As for its weight. I don't know, but I'd estimate it at 80 pounds (36 Kg) or more!
Well, today I took the cabinet to see how it would fit. In the pic below you can see the challenge I had. I had mentioned in an earlier post that I had a narrow doorway to fit it through to get behind the stage. You can see what I mean. This door leads behind the stage.
I haven't shown this, yet. This is my Marshall Class 5 in the cabinet. It fits with only a 1/2 inch (about 13 mm) space on each side. This is also the first pic where I've shown the mic in place. In this shot, the mic is as far as I can get it from the front of the amp.
This is essentially the same shot, but I moved the mic much closer.
Having a lot of flexibility in mic placement was pretty important to me.
The next thing I want to do is to get some audio level comparisons. I have to wait until next week for that, though. I'll also hear it for the first time through the house mix, too.
If you drive a valve amplifier hard in such a cabinet for longer sessions it will probably make sense to open and check the temperature once in a while.
Install a quiet fan for ventilation if needed.
Good points, both.
I think I'll be fine. I have ventilation slots cut in the top and bottom. The top slot is directly over the amp and the bottom slot is just behind it. This will allow heated air to exit the top and cooler air to enter from the bottom.
If this passive, convection approach doesn't work, I can investigate fans. But, I think it will be okay.
Well, the saga continues...
Today, I actually put my cabinet into use. Here it is in place behind the stage.
To give a sense of perspective, this is a shot of the stage, itself. The cabinet is on the other side of the wall behind my SG in this shot. But, as I stand on the top tier, the top of the cabinet is about at my foot level. In fact, the power outlet you see in the first picture comes out right behind my SG in the picture below.
But, as you could see from the first shot, there is only one layer of drywall. During sound check I could barely hear anything from my amp! The cabinet cuts down on the sound pretty nicely.
BTW, that single-cut guitar on the far left is not a Les Paul. It's a single-cut PRS. It belongs to the other guitar player. He saw me take this picture and wanted to be sure I got his guitar in the shot! He also has a '72 black Les Paul Custom, a '78 (I think) Telecaster, and a double-cut PRS.
At this point during rehearsal it sounded pretty good! I use a wireless and walked out into the audience area to listen and was pleased.
I decided to experiment.
I have several pieces of wedge-shaped packing foam. I had a few minutes and cut some to shape and put it in my cabinet. You can see how I just fit them into place in this shot. They are not glued or fastened onto the cabinet.
Here's a shot with some foam cut to shape on the lid. It is just held in place with some pieces of duct tape.
Actually, I'm beginning to think I may go ahead and either leave this in place or get some foam actually designed for acoustic purposes. I think the foam did help attenuate the sound a little better and it did not seem to have a negative effect on the sound in the house.
At any rate, I'm pretty happy with the results. I also had absolutely no issues with heat. the vents seem to be doing their job, too.
The results are in. I ran a few tests on my cabinet this evening using an SPL meter.
I ran a tone into my amp. I used an app called "Pitch Perfect." I have this app on my phone and on my Kindle Fire. It generates a tone for all 88 keys on the keyboard. I ran tones into my Marshall Class 5 and recorded the following results. The SPL meter was 10 feet away from my amp when I took these readings.
BTW, for some frequencies it may not look like much, but a decibel is a logarithmic comparison. It is not linear. Every 6 dB drop represents HALF the sound pressure.
As I suspected would be the case, there is less of a drop with the lower frequencies.
My test with and without foam involved using or not using the gray, wedge-shaped foam in my previous post.
Good job Smitty.
Thanks for the data, Smitty. You also keep the cabinet behind a wall, so you have even more sound suppression than this, I suppose.
That is correct. I pulled the cabinet back into the audience area and did my sound tests in the open.
Here's a pic of the test in progress. For the tests with the amp un-isolated, I simply set the amp on the floor right beside the cabinet. In this shot, you can see my Kindle Fire on top of the cabinet and plugged into it.
Also, if you zoom in on the photo, you may be able to see "55" on the display of the SPL meter. This was my last reading, as shown in my previous chart.
Very nice, Smitty. A natural use of the music stand, too.
Do you plan to compare the frequency response of the mic'ed amp in the cabinet to the amp in the open? (You could use the same mic for both, I suppose, but maybe it is more convenient to just use your SPL meter and the house PA.)
Thanks, Dorian. I appreciate the kind words.
I probably won't do the frequency response tests in and out of the cabinet. It would be a pretty interesting set of tests to run, though.
Cool project, Smitty. Keep us posted as you use it more!
Yes, please do. This was a fun project to watch. What I like is how you planned this isolation cabinet for your particular needs as a musician. Please update us, certainly in a few months, to let us know how you use it. I'll bet you will find some interesting tricks.
I'll definitely post updates!
I can tell ya'll this, right now...my little '73 Fender Champ sounds nothing like a little amp when mic'd up inside the cabinet. When mic'd through the house PA, it sounds big, yet clear. You'd never guess it's a little 6 watt amp.
The one change I may make is to install real acoustic foam, like Auralex, since even packing foam showed some improvement, however slight. That may be awhile, though, because Auralex is rather pricey.
As promised, I figured I'd give an update.
I've used this cabinet for a few months and it is a fantastic thing to have. I've swapped different amps in, as necessary, and it really works nicely. Right now, I'm using my modded Vox AC4TV in it. As I've said before, you'd never imagine I'm running small, 4-6 watt amps in it.
About the only thing I would have done differently is to install larger wheels. I rarely have to move it, but if I did, larger wheels would work better when pushing it across parking lots and such. I installed 2" wheels; 3" wheels would probably be better.
I haven't swapped the foam I have in it with Auralex, yet. Though, I still plan to do that. Frankly, its working pretty nicely, as is.
For small to medium venues where stage noise can be an issue, an iso-cab is great.
Very nice work and construction.
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