I have a Generac Wheelhouse 5550W portable generator. I bought it for use as an emergency generator to power essentials in my house in the (unlikely) event that a major catastrophe should occur. Chances are, I'll never use it. Still, having an emergency generator around nowdays seems like a good idea.
To quiet it down I put in in a hole that measures about 4.5' x 4.5' and is about 40" deep.
After I put the generator in the hole and tried it, I fully expected the noise to all but disappear. But, that didn't happen. Here's the measurements I got using a Radio Shack meter:
Out of the hole, 7 meters
84 db(C)
81 db(A)
In the hole, 7 meters
87 db(C)
72 db(A)
Out of the hole ~60 feet
77 db(C)
73 db(A)
In the hole ~60 feet
79 db(C)
63 db(A)
In otherwords, using the A-weighted measurement, I got about a 10 db reduction by placing the generator in the hole.
I then moved the generator from the center of the hole to as close to the east wall of the hole as I could get it and measured (to the east) again:
In the hole ~60ft (Generator on East Wall)
76 db(C)
60 db(A)
So, moving the generator against the wall of the hole yielded a small, but generally insignificant (3db) reduction.
I then lined the opposite side of the hole with six, 20 lb bags of cellulose that I borrowed from a friend. Cellulose is well known for it's accoustical absorbency characteristics. However, there was absolutely no difference in the 60-ft measurement with the bags in the hole. It was as if they weren't even there!
So, the question is what's the theory here? Or, in otherwords, what's the method of propagation of sound out of a hole? My idea was that the sound waves bounced like a ping-pong ball against the sides of the hole until they reached the top and made their exit. But if that's the case, how come the cellulose didn't absorb some of the sound as the sound waves bounced back and forth?
Now, I'm starting to wonder if there is some other sort of propagation mechanism at work here. For instance, could there be a "skin effect" taking place where the sound waves crawl up the side of the hole and then along the ground? Or, is the sound wave simply using the earth as a ground plane and ignoring the hole altogether? Since there is no improvement in the C-weighted measurement, one could certainly make the argument that the hole is being mostly ignored by the sound wave.
The bottom line is that I only have a 10db reduction with the hole and I would like to get at least another 15-20db noise reduction.