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From: John De Armond
Subject: Re: Noisy generator
Date: Tue, 08 Feb 2000 16:58:52 EST
Newsgroups: rec.outdoors.rv-travel

Dick Johnson wrote:
> The Onan generator on my '78 Class C sounds like a B-17 warming up.
> Is there a way to somehow muffle this racket? I see there is nothing in
> the compartment  but bare metal, which serves as a sounding board.
> I have been told by RV servicepeople that it is the nature of the beast
> in old generators.
> If I line the sides with some kind of fiberglas to somewhat deaden the
> noise will it serve to overheat the engine? There is plenty of space to
> do this.
> Any suggestions?

Sure.  Been there, done that with the generator in my 85 Itasca. 
Because of space limitations they had to put a high speed (3600 RPM)
Onan in my MH.  You probably have the same thing.  These generators
will never be as quiet as the 1800 RPM or variable speed units but a
lot can be done.

First off, identify the sources of the noise.  If the generator is
as old as the MH, then it is likely that the engine is worn and that
you're getting some piston slap and perhaps valvetrain noise and
maybe even some rod knock.  I notice a LOT of older generators with
piston slap.  This is a sharp, metallic noise that sounds somewhat
like tapping the cooling fins with a hammer.  It happens at the
rotational speed (3600 dings per minute) of the engine.  It will be
twice as fast as the exhaust note.  Heavy oil will help quiet this
slap, though the only real solution is to overhaul the engine.  I
have a little slap in my engine and 20W50 made a significant dent in
the noise.

Valvetrain noise can be addressed by properly setting the clearance,
or, if your engine has hydraulic lifters, cleaning or replacing

Another source of noise is the cooling shroud sheet metal.  Older
systems didn't seem to pay as much attention to this source of
noise. Careful listening and feeling around will identify these
sources.  Touching the vibrating metal will dampen the vibration so
it is easy to find.  Addressing this noise is the same as addressing
the housing vibration addressed below.

Another source of low frequency noise/rumble in old generators is
the motor mounts.  The rubber gets worn and hard and no longer
dampens the vibration.  Sometimes the mounts rot so much that the
generator is actually sitting directly on its steel base plate. 
Changing out the motor mounts will almost always improve the

Make sure, of course, that the exhaust system is in good shape and
any heat shields aren't vibrating.  Also take a look at the intake. 
A lot of noise originates from the intake system. New generators
have intake mufflers but older ones frequently don't.  Mine had a
simple paper element open to air.  I found a lawn tractor air
cleaner housing that would fit (no idea from what - junkyard score)
and that has a pipe for the inlet.  This pipe can be routed via a
flex hose outside the box toward the ground where the residual noise
is least noticeable.

Once the generator itself has been muffled, it's time to take care
of the enclosure. If your box is like mine, welded steel sheeting,
it resonates and amplifies the noise.  3M makes an automotive
product designed to dampen the vibration of sheet metal panels in
car doors and fenders.  It is what gives a lightweight door that
nice "thunk" sound.  It is available from body shops in the form of
thick high hysteresis rubber sheets backed with adhesive.  This
stuff can be plastered to the walls of the box (inside the cabin if
accessible so the rubber is protected from the heat and fumes) to
dampen the vibration.  The effect is remarkable.  In my MH, the
sound in the cabin was reduced by 6 db (X4) with just this
addition.  The stuff is moderately expensive - about $50 for a box
of several sheets - but well worth the money.

One last note.  Make sure the exhaust pipe extends beyond the body
of the vehicle.  If it ends under the MH as mine did, the exhaust
pulses can resonate with the space under the vehicle and make a
booming noise.  Simply extending the  exhaust to beyond the vehicle
makes a remarkable difference.


From: John De Armond
Subject: Re: Noisy generator
Date: Wed, 09 Feb 2000 01:04:50 EST
Newsgroups: rec.outdoors.rv-travel

Alan Robinson wrote:

>     My suggestion, FWIW, is to concentrate first on getting the gen running
> as well - and as quietly - as possible, then worry about adding noise
> insulation to the compartment. It's always more effective to prevent noise
> from being generated than it is to try to suppress it afterward.
> HTH,
> Alan

YES, YES, YES, mainly for the sanity of others!!!!  Last weekend we
pulled in beside another MH in the parking lot of a flea market we
were going to visit the next day.  He had his generator running. 
Normally gensets don't bother me at all.  Got ready for the sack. 
Genset must have known that, cuz it started missing.  Only missed a
stroke every 10 seconds or so.  Still didn't bother me much.  But
that damn thing kept me in a stupor all night.  Not enough to wake
me up enough to get up and move but enough not to get any sleep. 
I'd rather one be LOUD than miss.  I can get used to almost any
noise as long as it is steady.  But the random nature of missing
will drive you crazy!!!


From: John De Armond
Subject: Re: Help on generator purchase
Date: Wed, 09 Feb 2000 14:06:34 EST
Newsgroups: rec.outdoors.rv-travel

David W Sutton wrote:
> Well I thought I would ask the old pros a question. I have a Toyota Huntsman
> 19'MH. This was purchased to tailgate at Tennessee Titans games and take on
> a few road trips to away games. I have been looking into generators as the
> Toyota is limited on space I will be mounting the generator outside on the
> rear on a small platform welded to the rear steel bumper. With that said,
> heres my problem. The only thing I am concerned with it the AC - a Brisk Air
> 13500 BTU unit. I have a power inverter to run all the other AC items. The
> two considerations I have would be noise level and enough wattage to run
> this one AC unit. I have had problems trying to figure out the actual
> generator size needed to run the AC unit. I have been leaning towards a
> Honda portable for the quiet factor but what would be the minimum wattage
> requirements?  Honda came out with a small super quiet model the EU3000is
> that produces AC Output: 120V 3000W max., 2800W rated and seems to be the
> answer but my concern before spending two grand is that the power is not
> enough. 

That generator should have enough grunt to run your AC.  I have an
Onan 3kw unit in my MH and it will run my 13500 btu AC and a
microwave at the same time (a tiny bit overloaded but doesn't hurt
the generator). The only way to know for sure is to take your RV to
the generator place and try it.  The issue will be how the inverter
inside the EU3000 reacts to the momentary, highly inductive overload
that the AC starting inrush constitutes.  If the situation is
borderline, then there are some tricks (such as a kickstarter pack)
that can be applied to the AC to help it draw less starting
current.  No substitute for testing.

>Anyone else use a Honda and what king of noise level is acceptable
> in these units? This one is super quiet- 49-53 dB but I was wondering if the
> ones in the 59 to 69db were ok for camping without folks having to shout or
> them being that noticeable while running. Any help would be greatly
> appreciated.

Remember that 3 db increase, the sound level is DOUBLED.  The honda
is quiet enough to use almost anywhere.  Very few of the others meet
that standard. 

Might I suggest that you make a mount where you can easily remove
the generator from the RV?  My previous RV was a Keystone
Toyota-based MH that was too small to include a generator.  I towed
a light trailer behind it to carry my generator, an ice box and
other light stuff.  It was wonderful to be able to unhook the
trailer, roll it 50 ft away and get the generator noise out from
under my bedroom window.  IN addition to the noise, with the
generator attached to the bumper, you will experience conducted
vibration which may be louder than the radiated noise from a very
quiet generator like the Honda.

I strongly recommend that you trundle down to your local Radio Shack
and buy yourself a db meter.  Cost is about $40.  Considering that
you're about to spend several thousand $$$ on a product that will
affect the quality of your life for years to come, spending a Grant
so that you can evaluate products is a small price.  With the db
meter, you can evaluate noise and how much it bothers you and
compare it to a measured value.  You'll find that the stated sound
level will almost never correspond to what you measure.  That is
because the method used to measure SPL (sound pressure level), while
useful to compare numbers against each other, is not representative
of service conditions.  You'll find that the sound will be
directional and that you may be able to orient the generator to beam
the worst noise away from you.

Beaming the sound away also works when a generator would otherwise
be inappropriate.  I've put my smaller Honda inside a cardboard box
so I could use it at craft shows where generators were not allowed. 
No one even knew it was there.  In other instances where my
concession trailer had to be in close proximity to others, I put my
larger generator inside a large, acoustic-foam lined plywood box. 
This absorbed much of the sound and beamed the rest up and away from
people.  This cut the radiated sound by over 6 db.  If you fabricate
a bracket so that you can easily remove your generator, then you can
use these same techniques to reduce noise.


From: John De Armond
Subject: Re: Help on generator purchase
Date: Wed, 09 Feb 2000 14:32:23 EST
Newsgroups: rec.outdoors.rv-travel

Erich Coiner wrote:
> Neon John wrote:
> >
> > Remember that 3 db increase, the sound level is DOUBLED.  The honda
> > is quiet enough to use almost anywhere.  Very few of the others meet
> > that standard.
> >
> You are correct that 3db is a doubling of the sound pressure level. But
> your ears hear on a logarithmic scale, and a 3db difference is a "just
> noticeable" difference in sound.
> A 10 db difference in sound is perceived as being Twice as loud even tho
> it is 10 times as much pressure.

1 db is the just noticable threshold.  3 db does, in fact, represent
a doubling in perceived loudness.  Easy enough to prove to yourself
if you have the instruments.


From: John De Armond
Subject: Re: Sound deadening: fiberglass or carpet squares?
Date: Wed, 05 Apr 2000 23:59:53 EDT
Newsgroups: rec.outdoors.rv-travel

Dick Johnson wrote:
> I'm getting ready to apply sound-deadening material to my genset
> compartment.
> I can use fiberglass insulation or carpet material. Does anyone have
> experience with such a project?
> I should think the pile elements in the carpet would be a pretty good
> sound absorber. Since the Onan generator is forced air-cooled, I don't
> think heat would be a problem.
> Any ideas?

You have two issues to address:  Radiated sound from the generator
and sympathetic vibration from the generator box.  Treating one may
not treat the other.  Fiberglass might do a fair job of damping
radiated noise but it won't do much of anything for quieting the

For dampening the box which will produce the best results for you in
the coach, 3M and others make a high hysteresis rubber-like sheeting
designed to be fastened to the inside of vehicle panels to give them
a more substantial sound and feel, e.g., to dampen the tinny
vibration.  It is adhesive-backed and is simply stuck on.  I get
mine from a local auto parts store that caters to body shops.  I
recommend sticking this stuff on all walls of the generator box. 
Back it up with a few broad head pop rivets just in case the
adhesive fails.

For absorbing radiated noise, a good material is that cheap
multi-colored crumbled foam carpet pad. I saw a couple of different
brands of semi tractor diesel gensets at the recent Mid America
Truck Show that were manufactured using this stuff.  It resulted in
a very quiet generator.  It appeared to be simply glued on the
inside of the generator box using contact cement.  Don't forget to
deal with any air vents.  There should be no direct path to the
outside world.  Any opening should be baffled with sound absorbing
material.  The result is startling.  My genset box had a louvered
door to the outside door.  When I fabricated a piece of metal
overlaid with foam and stood it off the louvers a couple of inches
on the inside of the box, measured sound 10 feet away from the MH
dropped 6db.  That's a LOT.


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