Generator In a Hole--How Does the Sound Get Out!?

[james]

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.


















 

[johnbergstromslc]

The problem is that the sound is probably heavily weighted in the lower frequencies.  Low frequency sound is omnidirectional - it diffuses around obstacles and corners.  You need a fully contained enclosure to completely kill the noise.

P.S.:  If the generator is only for rare emergencies, why sweat it if it's a bit loud?

[james]

I've thought about ignoring the noise issue since the generator is only for emergency use.  I wonder, for instance, if I would be the only one in the neighborhood with the lights on. Or, would there be a half-dozen of them running? Right now I'm still playing with the idea of trying to make it quiet since I'm retired and have the time.



I think you are right about complete containment. I've come to the conclusion that there's no way that you can finesse this problem.

[johnbergstromslc]

And if the power goes out, nobody can complain about the noise cause their cordless and cellular phones won't work!  Too bad for them....

[skip]

James:







We know of many users of generators that have used the following method of reducing sound from the generator. They build a box that totally encapsulates the generator. Then, they line the box with a sound absorbing closed cell foam material. The box has a removable top or door so that one can service the generator when needed. The generator in a box can be in a building, on the back of a truck or anywhere you wish. It need not be in the ground.

[johnbergstromslc]

I say don't worry about it.  If the power goes out, I doubt the neighbors will see the noise as a nuisance.  They might find it as a reassuring sign that civilization hasn't collapsed....

[joel]

James,

The closed cell foam that Skip mentioned in his reply is our Super Soundproofing Mat.  It  is the first product listed at http://www.soundproofing.org/sales/prices.html  This material absorbs AND blocks sound in a contained space.  And as Skip and John also mentioned, you need to fully enclose the generator.  The reason sound "gets out of the hole" is that sound "bounce" at a ledge or through a hole is not proportional.  This phenomenon is discussed on page 18 of the "Quieting in the Home" EPA reprint available on line at http://www.soundproofing.org/infopages/EPAmanual.htm  Lots of great, complere info in that book.

Joel