The Onan Emerald III propane generator in our '89 GV makes an acceptable noise level inside the coach, but, WOW, the noise outside is unacceptably loud in my opinion. The noise is not coming from the exhaust but straight down under the generator itself. It seems almost as if the large cooling fan makes most of the noise.
I have read about attenuating the sound by placing shag carpeting or similar stuff on the surface/blacktop/concrete to absorb some of the sound, but am wondering if all that noise is typical and if there isn't a better solution. There seems to be enough fan forced air to blow the carpet away.
When we had our 93, U225 with the Emerald III I took some of the reflective bubble wrap and make a three sided box under the coach to direct the sound away from where we were sitting or away from a neighbors camp site. I simply wedged it under the coach and let friction hold it in place. It did help some but the Emerald III is just noisy and you are correct, I don't think it was the exhaust but rather just the noise of the air cooled motor.
Thanks for the reply, Kent.
You have given me an idea! (maybe another Rube Goldberg contraption) When we get the coach back out of storage I think I will make a box similiar to what you did but a bit longer, extending toward the rear of the coach. Line it like you did, but put in two removeable baffles and exhaust out away from the side of the coach. As you know sound travels in a straight line so the sound would hit down, then have to traverse up and over a baffle, then down and under another baffle, and then out the side. Just like a muffler!
What do you think?
Norm
Maybe some useless info, being in the generator game for many years, have always been told by the bright bulbs that every time you change the direction of noise, it gets reduced 3db, that is the basis of sound attenuation I am told. I do not know that for fact, but there does seem to be some truth to it.
Maybe there is a audio engineer who can straighten us out ?
Anyway good luck
Dave M
3dB is 50%, that sounds like a lot for just one elbow/bend.
Jim, I agree on the face of it, but changing direction using baffels, and changing direction 90 deg, not thru a pipe, is said to be 3db/50% indeed, I have never really gotten into this, but know there is some truth to it. Using a sound box type works, On generators with the best sound attenuated cabinets, the intake air and radiator discharge air change directions many times and run thru sound deadening baffels, seems to be a real science to it. For sure over my head.
Have started up large gensets that when they are running, and you close the cabinet, put your back to the cabinet, and listen, you feel the very light sound you are hearing must be the air conditioner at the building. Just crazy how that works.
Of course your talking real money here for a real quiet outcome.
Dave M
A friend of mine who had a career on "Boomers", aka nuclear submarines gave me a tidbit of info on sound attenuation which they take quite seriously in their business. For airborne sound, change the direction of travel three times to minimize it. He gave me the db reduction but don't recall.
Following this idea, I inserted baffles inside a very noisy return air plenum below an air handler in our house. This thing made so much racket my wife was threatening to call a real ac contractor (as opposed to the jake leg who installed it, ahem) and have the system ripped out and replaced. After placing the baffles it was very close to silent. Absolutely amazing improvement.
The way I inserted the baffles the air direction changed three times, 180 degrees each time.
Chuck
Chuck,
I am having trouble visualizing this. Sounds like you might be slowing down the air flow.
Would you please describe the material you used and the size and placement of the baffles?
Thanks,
Trent
I'm surprised there was room to make 3 turns in ducting. Was it tricky to do that inside the gen compartment? How big was the ducting and what was it made of? Any pictures?
Jim
I'll try to describe it, the only way I can post a drawing is as a dfx file which most people won't be able to use. The plenum was about 40" tall, 24 wide, 36 deep. It was lined with duct board. I put two "ledges" that extended the width of the plenum on the back wall. Each extended from the back wall half the depth of the plenum. On the front wall, evenly spaced between them was one ledge the same size and extending half way into the plenum. The air then, instead of coming into the plenum thru the filter and directly to the fan, now has to follow an S route.
Ledges were made of duct board. Airflow velocity was actually increased but volume was most likely reduced some. It made no difference in AC performance other than it was now super quiet rather than loud. I calculated the effective opening sizes for airpath and was careful to keep it larger in area than the relatively small opening for suction fan. I'm sure a good AC man could find plenty wrong with the idea regarding static pressure and such but in my experience most residential AC units are so poorly installed in the field this would be picking nits.
Don't know how this would be applicable to genset fan noise. I suspect it could be quietened by using a thermostatically controlled variable speed drive, looks like way more fan than is actually needed to me.
Chuck
Chuck,
Your description is very good; anyone should be able to reproduce it!
Thank you,
Trent
Chuck, thanks. Pretty much the design principle I was thinking of doing. My problem now is to come up with a design that will accomplish the sound reduction but also be small enough or collapsible enough to take along in our GV coach. There isn't a lot of storage ans what there is is not very large.