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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Propane generator question
Date: Sat, 12 Jun 2004 19:44:02 -0400
Message-ID: <>

On 12 Jun 2004 18:03:10 GMT, (SOROBON) wrote:

>>There is almost NO chance that adding isulation will have a helpful
>>effect.  It cannot be put INSIDE the enclosure without voiding the
>>warranty and risking engine damage by interfering with air flow.  The
>>construction of the compartment will make putting insulation on the
>>outside all but impossible - and in any event insulation will not
>>affect the majority mode of noise transmission - which is conduction.
>>(Conduction means vibration mechanically transmitted - it is felt as
>>much as heard). Onan has very competent noise control experts, and
>>when they design RV gensets they do an excellent job of balancing
>>reliability, performance, and low noise.  User attempts to "improve"
>>are almost always doomed to failure.
>>Will Sill
>I met someone this winter that installed a small Onan generator in a
>compartment in an airstream TT. They used some very expensive acoustic
>insulation, and mounted the generator on a double system of rubber mounts and
>had a special door made that would alow air in but also acted as a noise
>baffle.  They also opened a door on the bottom of the comartment when the
>generator was running.  It worked great, but he told me he spent more on the
>mounting of the generator than the generator cost.

As usual sill pontificates from a position of ignorance.  It takes only a
modest effort to manage sound if the generator is in an enclosure.  One only
need look at what Generac did with their QuietPack line.  heavy metal
enclosure, compressed fiberglass lining (the same stuff used inside central AC
units), baffled air inlet and outlet, a muffler as large as the engine and a
well baffled and muffled intake.

I've given my generator box similar treatment.  3M automotive sound dampening
compound on the outside of the metal box.  Compressed fiberglass on the
inside.  All air ports baffled with undercoating-coated (heavy rubber) sheet
metal.  A super quiet muffler from  An automotive exhaust flex
coupler to isolate vibration from the exhaust system.

I paid $175 for the muffler and probably $50 for the rest of the stuff.  The
Generac Impact inverter gen I use has more than adequate vibration isolation
as it is so nothing to do there.  I spent a long afternoon with the

I haven't measured this setup yet but subjectively, it is much quieter than
the Onan Emerald in my mom's MH.  Both are a somewhat more noisy than my
QuietPack that sets the standard.  From inside my rig I can't here the
generator run over the wind noise of the AC.  With the AC off, the generator
noise is a low mechanical type that is not objectionable at all.

If someone didn't want to do the work I've done, he could do much worse than
to simply order the QuietPack housing as a spare part and put his generator
inside.  The box looks large enough to house the smaller Onans.  Order the
muffler along with the box and you have the makings of one of the quietest
generators out there.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Champion Generator review
Date: Thu, 02 Nov 2006 12:56:41 -0500
Message-ID: <>

On Thu, 02 Nov 2006 06:01:25 -0700, boondocky@no.spam wrote:

>Will Sill <will@epix.anet> wrote:
>>I see where "Victor V" <> contributed:
>>>Anyway, I wanted the lightest thing I could get, and the cheapest. I got
>>>this thing at Kragen Auto Supply for $180. . . .

Glad my advice worked out :-)  I always like to get feedback.

>You're making assumptions about how the wooden box will be
>constructed, Will.  You're assuming that it will be too small, that
>its top or sides will be too near the exhaust, and that it will be
>inadequately ventillated.  The important thing is to balance weather
>protection against heat buildup in a way that is safe.  Certainly it
>can be done in a way that is dangerous, but it can also be done

I've done several boxed generators and amazingly enough, none have
burned up.  The key is to arrange the box so that the engines cooling
air INTAKE gets fresh air from outside and to arrange a path for most
of the hot exhaust air to exit.  And, of course, get the engine
exhaust out safely.  Most small engines let cooling air escape from so
many places that it is difficult to duct it out so just let it escape
into the box and provide plenty of ventilation.  Getting good cool air
to the intake is the key, making sure the exhausted air doesn't
short-circuit back in.

Make sure each vent is optically opaque (borrowing a term from high
vacuum work which means that light can't shine through.)  Sound
doesn't turn corners very well so if the vent is opaque - a baffle,
preferably coated with some open cell foam - air will come in/out but
sound will mostly stay in the box.  A little angle on the baffle,
maybe like this: /\ (but shallower than ASCII art will permit) will
help deflect the sound to the side so that it won't bounce around -
basic stealth techniques :-)  Many of the techniques that work for
radar also work for sound.

An improvised method that worked amazingly well involved an open top
wooden box laid on its side, the generator sat inside and a hunk of
plywood propped at an angle over the opening.  No sound absorbing
materials, no nothing, just a box.  Amazing results.  A more formal
arrangement could be a box with a side that hinged from the top and
had something to prop it open a few inches at the bottom.  You might
try something really simple like that before engineering something
more complex.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Champion Generator review
Date: Thu, 02 Nov 2006 22:07:35 -0500
Message-ID: <>

On Thu, 02 Nov 2006 18:18:29 GMT, "Victor V" <>

>"Neon John" <> wrote in message
>> An improvised method that worked amazingly well involved an open top
>> wooden box laid on its side, the generator sat inside and a hunk of
>> plywood propped at an angle over the opening.  No sound absorbing
>> materials, no nothing, just a box.  Amazing results.  A more formal
>> arrangement could be a box with a side that hinged from the top and
>> had something to prop it open a few inches at the bottom.  You might
>> try something really simple like that before engineering something
>> more complex.
>Well, that was my original idea. I already have the box, with lid and
>handles and everything. Its a tight fit, but has a couple of inches top and
>sides have about 6". I thought I'd cut maybe a 3" hole for the exhaust, and
>maybe a 5" for the intake, and use a muffin fan that will run off the 12 V
>tap. All this in the back of a pick=up, tailgate down. Dangerous, you think?

I'm afraid that an ordinary muffin fan would be like peeing on a
forest fire :-(  Maybe several or a squirrel cage blower.  Problem is,
you're robbing power from the charging process no matter which way you
go.  You have a relatively powerful fan at your disposal in the form
of the engine cooling fan that won't cost you an extra watt to use.
All you have to do is arrange some baffling so that it sucks outside


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Champion Generator review
Date: Thu, 02 Nov 2006 22:19:27 -0500
Message-ID: <>

I've seen a similar arrangement at a client's site on equipment
cabinets.  I'm not sure if it was acoustic or electronic emission
suppression or both.  Kinda a glance walking by but the image stuck
because of my work with high vacuum.

I'm not sure all that work would gain one much over a simple single
chevron baffle as I mentioned previously.  Especially since some sound
is going to be re-radiated by the box.

I think the 80/20 rule will apply here.  80% of the possible
improvement for the first 20% of the work, etc.  I know that when I
had to hide a generator at craft shows, simply placing the Honda
EX-350 in a heavy cardboard box with a shut lid and a few
strategically placed vent openings (baffled with simple cardboard
baffles) made the thing so quiet that one could not hear it standing
in the booth if one didn't know to listen.  The EX is fairly quiet by
itself but the cardboard made it practically silent.

Many/most  times when I use my little 1KW 2-stroke generator while
boondocking, I can simply place it in a heavy open-sided cardboard box
and aim the opening away from people (out toward the woods, etc) and
get the job done.  I have a 100 ft roll of light gauge (16 or 18
gauge, I forget which) extension cord that will let me position it
farther away if I can't aim it completely away from others.

Never had a complaint nor even a funny look from other campers.  I
HAVe had people who strode up to chat be extremely surprised to learn
that I had a generator running.  Even more so when they learned that
it wasn't a kilobuck Honda :-)


On Thu, 2 Nov 2006 14:22:34 -0500, "Eisboch" <> wrote:

>"Neon John" <> wrote in message
>> Make sure each vent is optically opaque (borrowing a term from high
>> vacuum work which means that light can't shine through.)
>Also (and more commonly referred to) as being "Optically Dense".
>You caused me to think about something.  If the openings include a properly
>sized chevron type, 2 bounce trap, it will be optically dense. If the inside
>surfaces of the chevrons were covered with a sound absorbent foam (glued on)
>then the sound would be effectively dampened.  Probably need to have forced
>air cooling though, with a fan or something.   The end view of the chevrons
>would look like an inverted "V" with the point of the V on each row
>positioned slightly within the top opening of the V in the next row.
>Make sense?

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Latest on this prototype....with pictures!!!
Date: Mon, 22 Jan 2007 14:49:06 -0500
Message-ID: <>

On 22 Jan 2007 00:32:58 -0800, "Dene" <> wrote:

>Spent the part of the day on the transient dock, experimenting with
>this prototype.  Bad news is that after 20 mins. of running on a medium
>load, two of the four inner cooler walls were unacceptably
>like a Texas sidewalk in July.  The following link reveals some
>pictures along with captions which further explain the problem.

Now that all the childish name-calling has settled down I thought I'd
chime in since I've designed several successful generator boxes.  I've
also written about it fairly extensively so you ought to go to google
and look up my old posts.

By now you've discovered that an insulated box makes a lousy generator
enclosure.  You can continue smearing lipstick on that pig but it
won't get any prettier.  Write this off as a learning experience, toss
the box and start over.

Rigid polyurethane is a lousy sound suppressant even if it wasn't
encapsulated in even harder plastic.  Sound has to be dissipated
ultimately as heat and that means that the materials the sound
impinges has to be acoustically lossy.  Closed pore foam isn't.

Open pore foam, especially that designed for sound damping, works
well, as does fiberglass.  Generac took the fiberglass batting
approach with my QuietPack 55G, a stunningly quiet large generator.
Onan and most everyone else does the same.

Even better is lead.  One of the best sound absorbers going,
especially in spun fiber form.  There is at least one company that
makes spun lead batting for marine silencing.  They're on the web so
go looking.

I suggest either a sheet metal or plywood enclosure, lined with
fiberglass or lead batting.

The next issue is heat.  You MUST get ALL exhaust - engine and cooling
air - out of the box.  The generator should be surrounded by ambient
air drawn in by the generator's cooling fan.  This means welding on an
exhaust extension.  It also means ducting the exhausted hot air out of
the box.  The tricky part is doing it in an acoustically opaque
manner.  This usually involves baffling made of absorbent materials.

Lose the notion of muffin fans.  They work in their designed
environment - high volume and very low static backpressure - but will
be lousy for this application.  By the time you provide enough
openings to reduce the static pressure on the fans to something they
like, you'll have a huge noise port.  Plus muffin fans are noisy of
themselves, especially when working against backpressure.

If you properly duct the hot air out then you should not need any aux
cooling fans but if you do, a radial squirrel cage fan is probably the
best bet, decent performance against moderate pressure, quiet and not
too thick.

Lastly, forget all this piddling around with eco-throttle and such.
The only way to test your creation is at full load.  This is
especially true with inverter generators that vary the engine speed
with load.

You must also simulate summer heat.  One or more refrigerator boxes
makes a good, very cheap test chamber.  Again the engine exhaust has
to be removed from the box but the hot air can be allowed to remain,
at least partially.  Let the inside of the box heat up to, say, 100
deg and regulate it there with variable venting while the generator is


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Generator Noise (was Re: The Honda EU2000i..where to buy it and for 
	how much?)
Date: Thu, 17 Jan 2008 17:55:31 -0500
Message-ID: <>

On Thu, 17 Jan 2008 13:06:48 -0800 (PST), "Chuck Wilson (Los Angeles Area)"
<> wrote:

>And you are right about many Coleman generators.  They make you think
>you are living near an  However, to "accurately" make
>statements and not just assumptions, wouldn't one need to compare the
>db level of two "particular" generators before making a blanket
>statement (i.e. assumption) about all non-inverter-type generators?
>Indirectly, you may have been comparing your Coleman to the Honda
>EB3000 and without specific db ratings, perhaps there is no comparison
>at all.  Just a thought that the only truly accurate and valid method
>to measure the difference is to select the particulat EU model and
>choose the particulat EB, EN or EWhatever other model and do a side-by-
>side noise comparison.  Not trying to argue here or accuse anyone of
>malfeasance, simply erring on the side of logical and precise

The problems with that method are several.

First, orientation makes a LOT of difference.  Do you orient both generators the same
(exhausts pointing the same direction, for instance) or do you orient each for the
best or worst sound production?  If a generator emits strongly from one side then I
can turn that side away from me or whomever it might bother.  Does the generator get
credit for that?

Next are surroundings.  A generator that emits much noise downward would not be
nearly as loud sitting on dirt or grass as it would be in a truck bed.  Similarly, a
directionally emitting generator aimed into a truck bed would be louder than if it
were aimed out.  Same situation if the generator is in a generator box.

Third, and probably the biggest problem with simple db measurements, the sound
pressure spec tells nothing about the quality of the sound.  Two sounds of equal db
can vary widely in their ability to irritate.  My Generac Impulse, for example, makes
slightly more noise at moderate load than does my Onan Emerald but the Generac noise
is much less irritating.

There are other measurements that more accurately characterize perceived sound.  One
is the Sone or unit of perceived loudness.  This takes into account the
physioacoustic properties of the human ear and brain.  If you look at the displays of
bathroom fans at Lowe's for instance, they all have Sone ratings.

What we're really interested in is how much a given sound mixture annoys.  Acoustical
engineers have quantified that too.  FAA developed a VERY complex model for
calculating Perceived Sound Pressure Levels (PSPL) from a spectral analysis of the
sound.  It works well for airport kinds of noises but maybe not so well for noises
with strong frequency components such as generator noise.  Strong components are
things like 60 and 120 hz hum, 3600 RPM vibration and the exhaust notes.

There is a unit of measurement for annoyance too, called the "noys".  Yep from
"annoys".  This model takes into accounts psychological factors.  The inputs are
sound pressure in each fractional octave as well as the time-varying nature of the
sound.  For a generator, that might one that has a hunting governor.

For more than you ever wanted to know, see General Radio's "Handbook of Noise
Measurement".  It's available online as a large PDF.

I'd love to see generators have published Noys or Sone values but unfortunately we
don't get that.  All we get is some db number measured at some undefined distance and
undefined orientation.  While I can confidently say that one generator that has an
SPL 20db higher than another will sound louder, I can't say how much louder.  I have
to rely on my standard issue Mk I ears and a little intuition and experience.  Or my
General Radio noise measuring instrument :-)

Here are some things that I've observed over the years about generators:

Mechanical noise followed by intake roar are the most irritating.

Exhaust noise alone isn't irritating until it gets fairly loud.

Metallic ringing from resonant muffler walls is very irritating.

Synchronous speeds and harmonics thereof are more annoying than sounds made by
generators running at other than 1800 or 3600 RPM.  That probably has to do with us
being literally immersed in low level 60 and 120 hz sound most all the time in our
daily lives.

Honda obviously used psychoacoustic engineering principles in designing the EU
series.  The result is the sound it does produce is not very annoying.  Ditto for the
Generac QuietPact and the Yamaha inverter generators.

Other generators with apparently larger SPL production (db rating), however, don't
necessarily annoy any more.  That ubiquitous little $100 ChiCom 2 stroke generator
that I talk about, for instance.  Even though it tickles my db meter a bit higher
than an EU, the sound is roughly equivalently tolerable in my subjective opinion.
That is, once I damped the ringing muffler wall with a little hunk of lead!

In lieu of actually going around and listening to generators yourself, the best you
can do is listen to people who have heard the generators of interest side by side.  I
have, for example, operated my QuietPack sitting right beside an EU3000i.  They were
about equivalent at intermediate loads but at near full load, the Honda is louder.
Intake roar dominates. The QuietPack barely changes pitch.

Generac Impact vs Onan Emerald is another personal experience.  The Impact is much
quieter at part load but at full load the Emerald takes the prize.  The reason is
that the Impact is a variable speed inverter generator.  At full load it is running
something like 4200 RPM.  This comparison was with both generators tucked away in
their compartments.

Of course, we all know what contractor generators sound like.  We don't even need to
go there :-)


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: genset exhaust temperature
Date: Mon, 04 Aug 2008 15:50:56 -0400
Message-ID: <>

On Mon, 4 Aug 2008 10:57:05 -0400, "tim fm ct" <> wrote:

>Thanks for your replies.
>John, that must be quite a temperature gizmo you have. Thanks.

You're welcome.  Yep, a high end Omega/Wahl unit that I dropped about 6 bills
for.  (more accurately, a client did :-)  It has a USB interface.  I could log
the temperature rise over time if you wished :-)

>I'll look for my wife's oven thermometer. I has a long cord on the sensor.

If it's like mine, it probably runs out at about 500 deg.

>I'll have to check the upper temperature limit.   - - - of course.
>Because I'm interested in reducing noise, I just don't think the "genturi"
>will do the job.

I think you'll be surprised.  Cooling the exhaust also quiets it by absorbing
the energy carried in the sound pulses.  Plus the sound is very directional.
It's aimed out of the genny exhaust up the Genturi.

why don't you buy a stick of PVC sewer pipe and give it a shot?  Jerry-rig it
in place using cable ties or whatnot.  See what the result is.

>For safety, maybe I'll paint it hi temp neon orange. Naw, maybe a hardware
>cloth heat shield.

The paint will burn off.  If you're going that route, why don't you visit a
trucker's chrome shop and buy a "straight pipe" stack that already has a nice
chrome heat shield attached?

>I'll have the parts on hand by Wednesday and I'll let you know. As you can
>see, I got replies indicating quite a temperature range.

I didn't see any inconsistency.  The dull red that I observed on my old Onan
was probably in the 900-1000 deg range.  The temperature that I recorded on
the bottom of my muffler was after the gases had expanded and cooled, plus
there was a fan blowing relatively cool air across the other side.

The exhaust pipe on the way back to the rear of the rig can act like a mighty
fine radiant heater (like the overhead radiant tube heaters in Home Depot and
similar places) if it's painted with a high temperature flat black paint.  "A
few hundred" degrees, "few" being as low as 2, might be possible.

If you went up a tube size or two and had a muffler shop bend some zig-zags in
the under-carriage pipe, you'd get even more heat reduction.  Approach this as
a system and not just a bolt-on appendage.

Finally, don't expect all that much.  Very little of a generator's noise is
actually exhaust noise.  I installed a second glass-pack muffler on mine. This
cools and smoothes out the exhaust pulses so that the flow is almost
continuous with very little pulsing.  The sound is but a low chuff.

The intake emits most of the noise, especially at full load.  It's also the
hardest to silence.  On my homemade diesel generator, I used a large
industrial air compressor intake filter/muffler that uses tuned "organ tubes"
to cancel the sound waves.  It only works at one speed but then, one speed is
what most generators run at.  Mine's an inverter generator so the engine speed
varies but still, the speed range over which it operates at wide open throttle
(where the noise is) is narrow.  I'm going to try a similar approach or maybe
a Hemholtz resonator.  GM used Hemholtz resonators on the intake of my hotrod
Caprice's LT1 engine.  Almost no intake sound, even at WOT.

The bulk of the noise will still be mechanical - parts thrashing around
inside.  Generator box sound damping can get rid of a lot of that noise, as
can eliminating optical paths. (that's a vacuum technology term that means
eliminating straight-shots where light could shine though if there was any
present.  Gas molecules in a high vacuum - and sound waves - behave somewhat
like light in that regard.)  An optically opaque baffle might look something
like this:

+-----------------               |
|                                |
|                                |
|               -----------------+
|                                |
|                                |
|-----------------               |
|                                |
|                                |
|               -----------------+

Do you have a decibel meter?  Rat shack sells a decent one for about $40.  You
need one if you're to eliminate noise.  Your ears can get overloaded and can't
tell where the noise is coming from.  The meter won't.  You can also
objectively judge the effectiveness of each thing that you do.


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