Discussion in 'Amps & Cabs' started by smitty_p, Sep 2, 2013.
Or thunder FX. Old foley trick.
another one -
"He rode into town at sunset....."
"Quick! Get the coconut shells!"
I did a little more work between yesterday and today. I hinged the lid. This is another thing I wasn't going to do, but decided it would be worthwhile.
In the process of working on the cabinet, I've taken the lid on and off quite a bit. I realized that whenever I remove the lid, I end up looking for a suitable spot for it. So, I decided it would be good to hinge the lid.
In this shot I just finished installing a piano hinge:
Well, obviously, I need to be sure the lid doesn't just flop open. So, I needed to come up with some sort of retainer. I did not want to use a rigid retainer as it could interfere with the acoustic treatment and it could vibrate, causing noise. I had considered 1/2" (approx. 1 cm) wide nylon strap, similar to what is used on bicycle helmets. This would have worked great. But, in looking around my garage, I noticed some nylon cord. This stuff is strong and very flexible, so I decided to use that.
In this shot, my son is showing how I use a flame to cut the cord. Using a flame melts the cord and keeps it from fraying:
Now, the challenge was how to attach the cord. I chose to make some loops at one end that would screw to the inside of the cabinet, and knot the other end to the flange I put on the underside of the lid.
This picture shows how I looped the cord and wrapped magnet wire very tightly around the ends of the cord to make a loop. When I finished the wire wrap, I twisted the copper ends to together and soldered them so the wire wrapping will never come loose. Magnet wire has a coating, so I had to remove that from the ends so the solder would bond to the wire.
Also shown are wood screws and washers I used. The round head wood screws are good since they have a smooth shank near the head that will not fray the nylon cord as the lid is opened and closed.
This photo just shows one of the loops screwed to the cabinet side.
The next step was attaching the other end to the lid. I drilled partway into the edge of the flange under the lid, and then drilled from the inner side to form a channel. Then I used a length of magnet wire as a fish tape to pull the nylon cord through. This shot shows just after I pulled the cord through.
The next step was just to knot the cord at an appropriate length that would allow the lid to stay open of it's own accord, but would not allow the lid to open too far.
Here's one side with the cord knotted though the flange. By the way, the pic makes it look like I drilled way to close to one side of the flange. Actually, I didn't. The hole is centered, but the flange is chamfered, so the angle of the picture doesn't really show the rest of the flange thickness.
Then, I installed the other cord at the other end of the box. Here, the biggest trick was to tie the knot so it was the same length as the other end.
This last shot shows the completed lid installation with both retainer cords installed. The angle of the shot makes the left one difficult to see.
It's looking good.
At this stage it kind of looks like half of the magician's "saw the woman in half" trick. Or is that just me?
I'm thinking along the line of putting a First Act Wallmart special amp in it. Close the lid. Recite the magic words, and open the box with a vintage Fender Princeton amp appears inside...
Works for me!
My hardware and other items arrived a couple of days ago, so I've started the process of getting those installed. From this point on there a lot of little things to do and its a tap dance as to which step to do next.
So, today I did mostly prep work. In this shot I have just finished routing the recess for the handles.
The next step was to make the mounting holes for the audio interfaces. In this pic, I've just mocked up the cabinet with one of the handles and the audio jacks. The handle is just held in place with duct tape in this shot.
I've centered the XLR as that will give me the most flexibility with mic placement in the cabinet. The 1/4" jack is offset as that cable will run along that side of the cabinet to plug into the amp.
Finally, here is just a shot of the cabinet with a coat of primer. I will be applying at least three coats of black paint.
Once I've finished painting, I'll attach the handles and start the wiring.
Well, boys and girls, the saga continues.
This pic shows several things. The cabinet is painted with three coats of satin black acrylic enamel, I have the handles installed, and I have the microphone and instrument jacks installed.
The 1/4" instrument jack is kind of cool in that it locks onto the plug, similar to an XLR. There is a little release that you push to pull the cable out of the jack.
Anyway, this next shot shows the back of the jacks just after I finished soldering the cables into place. That gray stuff you see is JB Weld that I used to glue the jack mounting nuts into place.
I decided I needed some way of physically protecting the back of the jacks, so I fabricated some small sheet metal shields. I used some contact cement to glue some rubber onto the back of the shields. This would assure I would not get any shorts. I cut the rubber from an old bicycle inner tube.
Finally, this pic shot shows the shields screwed into place. I elected to make them from sheet metal instead of wood since wooden shields would have required me to cut more out of the sound absorption when I install it.
At this point, I am finally ready for the reason I made this in the first place...that is to install the sound absorption, itself. It's been a long road getting here, but I'm almost done.
Thanks, smitty_p. This is an interesting project so far, and I’m sure it will be even more so once you get it going. I’ve thought about doing an isolation cabinet (probably with a built-in speaker, though), so thanks for showing how it is done.
One question. Have you thought about cross-bracing the large surfaces on the sides? They might have resonances that color the sound. Of course you could wait to see how they vibrate, and perhaps this will be a non-issue.
I really hadn't thought about cross bracing the sides. I don't know if that will be an issue with the absorption I'll be putting into it.
smitty_p – Well, I don’t know either. I do know that at sufficiently low frequencies the sound absorber is not effective. If the sound pressure is changing really slowly, it is like pumping air into the box and pressurizing it. What this low-frequency cutoff is, though, and whether it might resonate your sides, is not simple. The last part of this page is relevant, and might even be partly correct:
Auralex Acoustics - Frequently Asked Questions - General Acoustics Answers
Good points. You're correct about low frequencies. I don't expect to be fully able to control them. I may change the first layer of absorption from Owens Corning 703 to Roxul Rockboard 60. The Roxul product has slightly better absorption than the 703, especially in the 125 Hz area.
As I have ventilation slots in the top and bottom, I'll not likely have pressurization issues. But, this obviously already compromises the performance. My goal is not 100% sound-proofing - I had to let that go when I decided to ventilate the box! My goal is to be able to attenuate the sound enough where it doesn't create issues with stage volume.
smitty_p – I look forward to hearing your thoughts after you compare the sounds from the amp in the box and outside it.
The ventilation holes might serve to tune box as a ported enclosure, although using the isolation cabinet with an open back speaker will cause some differences. I’m not sure you need to worry about tuning the openings, but perhaps my old post on port tuning might interest you. This cabinet has worked out very well for me.
Slight change of plans.
As I suggested in an earlier post, I've decided on a different material for the first layer of sound absorption. Instead of the Owens Corning 703, I've decided to use Roxul Rockboard 60. The Rockboard 60 is only very slightly outperformed by the Owens Corning 703 in the mid and upper frequencies. In fact, the ratings are so close as to be essentially the same. But, the Rockboard is decidedly better in the lower frequencies.
The other good point is that the Rockboard is less expensive, though it is heavier.
I just ordered a six pack of the stuff. Hopefully, it'll show up in a few days.
Well, the home stretch has begun.
Today I installed the first layer of acoustic treatment in the cabinet. This is the layer that will actually provide most of the sound absorption. As I mentioned earlier, I was originally going to use Owens Corning 703, but I changed my mind and decided to use the Roxul Rockboard 60.
This picture just shows all the stuff just before I began installing it. I ordered a box of 6 panels of the Rockboard 60 from ATS Acoustics. This pic also shows the "Projects" Liquid Nails I used to glue the panels into place. This worked well as an adhesive.
This next shot just shows what a Rockboard 60 panel looks like. I bought the 2" thick panels.
The name "Rockboard" is a little misleading. It gives the impression that it has a hard surface. It doesn't. It is really a compressed mineral fiber board. It is firm and holds it's shape well, but it is soft. It can also be easily torn, so care is necessary when working with it. Speaking of which, it is a lot like fiberglass panels, so all the same precautions of protecting yourself from the fibers apply. Use respiratory and eye protection and protect your skin.
As I said, I used the Projects Liquid Nails as adhesive. This picture just shows the glue applied to one side just before I put the panel into place. I bought six tubes of the adhesive ($1.83 per tube at Home Depot), but only used five for the whole box.
This shot just shows the first panel glued into place. You can see how I've notched the ends to go around the vertical strips in the corners. To cut the board I just used a pocket knife. A long razor knife would probably work well, too. I did have to sharpen the blade frequently. The panel cuts very easily, but being a mineral fiber it dulls a blade quickly.
I installed the sides first, then the floor.
This shot just shows how I cut little recesses in the panel to fit around the sheet metal shields I put over my connectors in the end of the box. It was easy to just poke a hole through the panel with my knife and push the cables through.
Finally, I have all the panels in place in this shot. For the lid, I screwed through the panel and installed some thin wooden strips as supports. Being compressed fiber, the panel is stratified. I don't want part of the panel to peel off since there is no other support for the panel in the lid. So, in addition to the adhesive, I installed these supports to physically hold the panel in place. I'm not worried about this with the sides as I cut them to fit pretty tightly into place and they sit directly on the floor of the cabinet.
Even though it is not done, yet, I did a little sound check with a couple of my amps. It REALLY cuts down on the sound! Of course, I can still hear the amp, but it is pretty effective. When I am all done, I will run a tone at a few frequencies into it and take a reading with an SPL meter to see how much it really attenuates the sound.
I still have a few tasks remaining. As the Rockboard is fragile and flakes easily, I will be using contact cement to apply felt to all the surfaces of the Rockboard that would otherwise be exposed. This will protect it from flaking. I also will be adding my second layer of treatment. This will be acoustic foam wedges. Lastly, I will be putting fuzzy fabric tape on the edge of the box and on the underside of the lid where they meet when the lid is closed.
I have discovered that the cabinet is now rather heavy. The Rockboard has a density of 6 pounds per cubic foot. I'm glad I installed handles because it is definitely a two-man lift. I'm also glad I installed the casters.
For a minute I was wondering if this was a casket or a cooler!
Just messing with ya Smitty. You're doing good!
That's funny. Actually, the cooler idea isn't so far-fetched. The Roxul Rockboard 60 is also used as a high-temp insulation. It is fireproof. It is essentially compressed rockwool insulation.
I just did a little work on the cabinet today. All I did was terminate the cables.
This picture just shows the terminated cables. I elected to go with right-angle connectors as they present less strain to the cable. Also, the right-angle XLR connector for the microphone cable will allow me to push the mic all the way to the end of the cabinet if I want.
If anyone is interested. I bought all the connectors and the recessed handles from Parts Express. All the connectors except for the right-angle XLR are Switchcraft. The XLR is a Neutrik connector. Parts Express didn't list the Switchcraft XLR, so I bought the Neutrik, which is still a great connector. I liked the idea of getting all my hardware from one place to save on shipping.
This next shot just shows my Vox AC4TV in the cabinet. The benefit of the right-angle 1/4" instrument connector is apparent here. The cable is long enough to fit around to the back side of the amp and plug in very cleanly.
This next shot is pretty much the same, except I have my Fender Champ in the cabinet.
Finally, this last pic shows the rear of the amp. I will just be dropping the power cord for the amp down through the lower vent hole. I didn't want to hassle with installing an AC power interface like I did with the audio connectors. I'm not nervous about my ability to do it safely, but I don't want a power cord coiled around inside the cabinet with the audio lines, so this seemed like a simple way to get the power out of the cabinet.
That's all for now. There's more to come!
It looks like that amp has been confined to solitary in Amp Prison.
I guess that's the point......
I guess it does seem a little cruel to confine the amp to a box with no one to keep it company but a microphone. It never gets to see the audience that hears it.
Ah well, I'll be sure to reassure my amp that it is appreciated!
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