On Tue, 19 Jan 2016 20:44:21 -0500, "Jim Wilkins"
wrote:
"Larry Jaques" wrote in message
.. .
On Tue, 19 Jan 2016 17:44:32 -0500, "Jim Wilkins"
wrote:
Build a double baffle box around it to quiet it down, Jim.
Add a hose for clean air intake. 3-sided+top boxes with carpeting
tacked on both sides works unusually well. A larger one over the
other makes the double. A third larger one really quiets things.
I did make one, from fireproof acoustic ceiling panels rimmed with 2"
strips of old steel pool liner bent into U channel. I also made an
extended muffler from fiberglass pipe insulation.
The box cut the sound level about in half from the back side and
reduced its pitch, the muffler showed that the factory one is quite
effective already and the sound radiates from the engine instead. The
box can't enclose the generator too much because it needs a free flow
of cooling air.
-jsw
We can pick up some lessons from the way they enclose marine engines
in boats, and backup generators for large buildings. Some of this also
applies to today's cars, which use some of the same principles.
There are three basic principles: Increase the mass of the enclosure;
use non-ringing materials that are inherently resistant to vibrating;
and kill the energy with friction.
The first is done in boats and some generator enclosures with sheet
lead or, in the case of some large stationary generators, like the one
we had at McGraw-Hill in NJ, to back up our IBM 360s, masonry. The
lead gives you mass and is non-ringing.
Friction comes into play with viscoelastic materials that turn
vibrational energy into friction and heat.
A common enclosure for marine engines (it has a name, which I forget)
is two sheets of lead sandwiching a plastic foam that is somewhat
viscoelastic. My uncle's 42-footer with CAT V8 used this material,
about an inch and a half thick. The foam decouples vibration from one
lead sheet to the other, by killing the energy in friction.
In cars, that was once the real purpose for undercoating, but today
they often use a material, originally from Sweden but now produced
around the world, that is two layers of steel sheet sandwiching a
layer of solid viscoelastic polymer. The total sandwich is less than
1/8" thick, and it's very effective. It's used between the passenger
compartment and the engine compartment, in the firewall, and you may
never see it because the edges of the sandwich are either welded or
turned over. You can spot-weld right through it. Volvo used a lot of
it.
So, disregarding the muffler and assuming a stationary setup, the
trick is to get as much non-vibrating mass as you can around the
engine; and decouple the sound from the insde of the enclosure to the
outside. Use some of that marine sound enclosure material if you can
get it. You can buy open-cell foam made for decoupling sound, such as
Antiphon:
http://antiphon.se/en/products-appli...bing-products/
Using baffles and so on is another approach, but it involves some
acoustical trickery. You may get lucky, or it may do nothing. Muffler
design, for example, has gotten pretty fancy, tuned for destructive
interference of sound waves. But that's over our heads unless you do
it for a living.
May you have peace and quiet...
--
Ed Huntress