I’ve been asked to design and probably build an enclosure for a gas-powered generator to hold down the noise.
It’s fairly large - roughly 2x2x3 feet, and powered by an engine in the 10 hp neighborhood. Sorry for being vague - this is how it was described to me, and I haven’t seen the actual generator yet. This is generically what I’m dealing with.
The concept I’ve got for this is to build a large plywood box and line it with unfaced fiberglass insulation. For the sake of conceptualizing, let’s say I make the ends of the box 4x4 feet and cut a 1x1 foot hole in them. a 3x3 foot baffle panel is covered with fiberglass front and back and spaced about 6" back from the outer panel.
I was thinking of using flexible foil-faced fiberglass heating duct material to supply fresh air to the carburetor, so the engine isn’t re-breathing its exhaust.
Am I going in the right direction here?
Once I have the actual dimensions of the generator, how tight can be enclosure be to it? Obviously, I can’t have plywood and insulation right on the muffler - is a foot away enough? For the rest of the unit, how close can I get?
Per Padeye’s note if the intent is to baffle the noise with a box enclosure I think you will make the generator cook itself as these things need as much air cooling as they can get to operate properly. There are enerators engineered from the ground up to be quiet(er) but they are usually higher end models.
If you’ve got to use the existing generator, a good sized brick shed with plenty of active airflow around the unit is the only way I see to enclose it safely.
In addition to insulating the walls of the box, you’re going to need to build a vibration dampening mount for the generator itself. All the insulation in the world will do you no good if the generator is bolted down so as to turn the box itself into a sounding board.
For what it’s worth, I’m slowly getting around to building a house for my compressor. It’s not exactly the same project, but I think the requirements are similar enough that it’s worth mentioning the details.
Basically, I plan to build a box inside a box. All walls and the ceiling will be isolated from the studs and rafters by isolation channel (the z-channel used to stand drywall off of walls.) The floor joists will be sandwiched between two sheets of plywood, with the bottom sheet held off with joists of 2X material laid flat, so that there is not contact with the floor.
All walls, door(s), floor and ceiling will be filled with dense-pack cellulose. Fiberglass does not provide sound isolation. Cellulose does if you pack it densely enough.
Air intake and cooling flow will be through labyrinths lined with dampening material, with an intake fan providing positive pressure inside the box.
I am worried about one aspect of your project. By enclosing a generator you could have gasoline fumes collecting in an enclosed space shared with several potential ignition sources. These include sparks from the generator and the engine’s ignition and heat from the engine’s exhaust. If the output shaft of the engine was long enough, I might try to put the generator windings outside the “house” and I’d probably try to have a spark-proof fan vent the engine house. Even then, I’d set the fan up to blow into the box to keep the fan upstream of any fumes and to create a positive pressure inside the box. I’m no expert though; it’s your job and your liability. I don’t mean to be cynical about this, I just thought that there might be a risk and thought that it was worth mentioning.
I don’t see any means of getting the exhaust out of the box. Did I overlook something? You just plain have to get the exhaust out. As another poster said, the exhaust contains some unburned fuel. In addition it is hot. The engine might very well overheat if you don’t provide outside air to cool it and get rid of the exhaust heat from inside the box.
And as another poster wrote, the exhaust is probably the worst noise source. Work to improve the exhaust muffler systerm first and see if that does the job.
Two 4x4 foot plywood panels at right angles and a triangular “roof” spanning them. Visualize a cube cut in half on the diagonal. Air can circulate, exhaust can leave. The open end of this will face away from the campsite and into open, empty land. The whole aim of this was to keep the generator’s noise away from a campsite. We really don’t care about our neighbors as there aren’t any - across a fire road from our site is acres of farmland.
**cornflakes ** says that fiberglass doesn’t work well, but dense-pack cellulose is good. OK, I’ll bite. What is dense-pack cellulose? On this not-really-a-box revision, I envisioned putting the plywood on a 1x4 frame and laying in the fiberglass batts and holding it in place with chicken wire or something similar.
BTW - I at least know the exact generator I’m working with. It claims to keep noise to “minimum” levels, but you still can’t have a conversation near it.
Consider egg-carton foam as a noise absorbing material. It’s a dirt-cheap and readily available substitute for professional anechoic wedge acoustic foam. Layers of carpet padding foam are also quite effective, and can be had for free from any carpet store.
Ive actually got a Generac 6500 model. Little bit noisier than yours probably. They are WAY too loud. I looked into quieting the thing down and the best I could do was to order some SuperTrap mufflers for the thing. The same design the autos use cept smaller. Generac didnt offer any aftermarket quieter mufflers and neither did Briggs and Stratton. Both of these companies are local for me (the Milwaukee area) so you would think if something else was available Id be able to find it around here.
I think with the right exhaust method you could get that thing to purr. Honda generators are VERY quiet, so I know it can be done.
If you could use aftermarket parts (SuperTrap, etc.) to route and quiet the exhaust and then direct the remainder of the exhaust into a ‘quiet box’ you would be successful.
Start here and look up the correct muffler and parts you will need for your model.
Take a wall finished on both sides and use a blower to pack it full of cellulose. On the first read, I assumed that you were going to use fiberglass to stop sound from being transmitted through the walls, and I’ve always heard that fiberglass does not have enough mass to block sound transmission. The dense cellulose does, and I think it keeps the walls from vibrating at their resonant frquencies. It also helps block any cracks or holes where sound could be transmitted.
On the other hand, if you leave an open side I’d assume that your first job will be to dampen sound, not prevent its transmission. I’ll agree with everyone else though that your first job will be to quiet the exhaust. After that, you may want to check into sound absorbing panels, but I’m not sure where you would look to find them.