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Subject: TWL: Engine noise control
LRZeitlin@XXX.XXX
Date: Mon Dec 02 2002 - 12:41:11 EST
The recent thread on noise treatment of engine compartments piqued my
interest. In my youth I trained as a physicist specializing in acoustics and
vibration control. I was a member of the Acoustical Society of America and
have published about a half dozen articles on sound and vibration control
methodology.
I like my boat engines to be completely as completely unobtrusive as my
housecat. To paraphrase the old Rolls Royce commercial, the loudest
mechanical noise aboard should be the ticking of the chronometer. But I
suspect that most trawler manufacturers are going about sound and vibration
minimization the wrong way. You can't just muffle a noise producing device
cheaply. It takes sound absorption and mass, correctly applied in layers of
insulation. Lead loaded foam is one way of going about it but best would be a
12" thick brick wall around the engine compartment.
Automobile manufacturers faced this problem years ago and came up with other
solutions. Look under the hood of your car. You will find very little
insulation to contain the sound and vibration of a lot of moving machinery.
What the auto makers did was to develop ways of decoupling the noisy and
vibrating components from the structure of the auto body. Only a small amount
of acoustic insulation is necessary to keep the airborne noise out of the
passenger compartment. Most of what we think of as noise in a fiberglass boat
is really structure borne vibration which forces the relatively thin boat
panels to resonate. Wooden boats have less of a problem since the structure
is much thicker and moist wood is fairly effective at damping acoustical
noise. Minimize structure borne vibration and you go a long way toward making
a boat quiet. And it doesn't require $10,000 of after the fact noise
insulation. Just a little attention to the physics of vibration control.
The MOST effective technique for noise and vibration control is a well
designed flex mounting of the engine. Mounts should be chosen so that the
vibration isolation from the boat structure is at least 90% at the usual
cruising RPM. This requires a little hunting through the catalogs of flexible
engine mount makers and some simple calculations. Anyone capable of filling
out an income tax form should be able to do it without difficulty (as well as
designing an atomic bomb and doing the NY Times crossword puzzle). But I
digress. Essential to a properly flex mounted engine is a good flexible
coupling to the drive shaft. The AquaDrive, Scania, or Evolution couplings
all work well. We have covered the mechanics of engine flex mounting on the
TWL several times and a lot of information is available in the archives.
After the engine is softly mounted and isolated from the structure, you can
worry about airborne acoustic noise control.
Basically, all methods of attenuating noise work by converting sound waves to
heat. Absorbing high frequency engine sounds like valve clatter and gear
noise requires only a couple of inches of fiber or foam. Lower frequency
sounds reqire some mass, either as a continuous layer or dispersed through
the sound absorbant material. Lead is probably best, but sheet metal covered
with a mastic material such as automobile undercoating will suffice.
Commercial noise control materials work well but the engine room must be
totally covered. A surprising amount of sound will leak through any opening
or vent. In my boat, simply covering up a 3/4" finger hole in an engine hatch
made an audible difference in the noise transmitted to the pilothouse. Since
engines must breath to run, air ducts should be lined with sound absorbing
material and contain several bends or turns so that sound doesn't have a
clear run to the outside. Best is to isolate the engine with a sound shield,
then decouple that shield from the boat structure by flex mounts. That's how
they design anechoic chambers and low sound research studios. But this is
overkill in boat situations.
Most naval architects do not consider noise and vibration control high on
their list of requirements. The topic is scarcely mentioned in standard mar
ine reference material and not covered at all in Skene's or Calder's books.
On the other hand, it is of great concern in industry and commercial
architecture. So if you want to get hints on how to make your boat quiet,
eschew the usual marine sources and do a Google search on noise treatment and
control.
Larry Z
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