UNDERSTANDING that what is a pretty
sound to us as modelers isn’t necessarily a
pretty sound to our neighbors is a reality that
we all must come to grips with.
The largest contributor to the loss of flying
sites, by a wide margin, is the noise we
produce. Sooner rather than later, someone is
going to build his or her dream country home
within earshot of your flying field and, if you
don’t have noise under a certain amount of
control, you will quickly become a target.
This should help you to be reactive to a
noise complaint. You should use these
guidelines to become proactive in getting
sound under control before problems arise.
After the authorities are alerted to a noise
problem is not really the best time to handle it.
The best time is long before there’s an issue,
and if it’s done sincerely and adequately, there
may never be a problem.
There is a difference between what a
guideline is and what is legal for a particular
piece of property. I’ll discuss this difference
later.
What is “dB”? It is the abbreviation for
decibels, which is a measurement of sound
wave impact; it is also a measurement that
increases exponentially on the scale. In layman
terms as the number increases, the impact
increases by several times over. To bring it
down farther, 45 dB is just above a whisper,
96 to 98 dB is tolerable, and at 107 dB and up,
noise starts to do damage to your hearing.
The easiest way to measure sound/noise is
with a decibel meter. They are now relatively
inexpensive and are mostly battery-operated,
so using them at remote locations is easy.
The one most often used is a small handheld
model sold at the nearest Radio Shack for
less than $50, so all clubs should have at least
one. They can also be mounted on a camera
tripod which will make testing and consistency
simple.
Sound Limit Recommendations
96 dB measured from 20 feet distance over
soft field (sod).
98 dB measured from 20 feet distance over
hard surface (pavement/concrete).
The decibel meter should be held
approximately 2 feet above the surface in the
direction of the aircraft and take a few
readings at different angles to the aircraft
(front, back, side, etc.) and take an average.
This method should adequately measure the
real sound level being put out by a particular
aircraft/engine/muffler combination.
These recommendations are not quiet by
any stretch but are normally tolerable to most
people in the vicinity, pit area, and spectators.
These recommendations should also be
adequate for controlling the sound level
outside your overfly area while in the air.
What Is Actually Legal
Unless you live in the middle of no-man’s
land, all property will fall under the
jurisdiction of a set of ordinances of some
type. Whether it is state, county, township,
etc., there will be a set of rules for what can
and can’t be done with or on a particular
piece of property. In these ordinances there
are rules for fence lines, building set-backs,
types of building, and so on.
There are also rules concerning how much
noise (dB) is allowed to cross the property
line. It may be hard to find, but if you look
deep enough or ask the right person, you’ll
find it. Normally somewhere between 50 and
65 dB is the amount of noise that’s allowed to
cross a property line.
After you find the information pertaining
to your property, checking to see if you’re
legal is easy by setting your dB meters at
different places on the property line and
taking the readings. Other factors can add to
the dB readings you get. If your field is
located close to a major highway or busy
road, the tire noise and other road noise can
make a big difference.
This may sound trivial, but knowing
where your flying site stands legally regarding
noise can and will give you a leg up if and
when the need arises. Think of how handy it
would be if you were to be confronted with a
complaint, and you know and are able to state
that your flying site is within the noise
ordinance limits.
Ways of Abating Noise Problems
Try a three-bladed propeller. The most
important thing to remember about airplane
noise is that most of what you hear does not
come from the engine; it comes from the
propeller. What you hear when your engine
unloads in the air is the propeller tips going
supersonic, so getting the engine RPM down
will limit much of that.
So you think that having a bigger twobladed
propeller will slow the engine down? It
will slow the engine down, but now you have
longer blades and the tips are actually going
faster (physics). The answer is to add blades
so you keep the diameter down while also
reducing the rpm.
The reason you see most of the big
International Miniature Aerobatic Club
(IMAC) pilots now using three-bladed
propellers is not for performance, but noise
control. If you don’t know how to figure for a
three-bladed propeller, you might ask your
engine manufacturer for a recommendation. A
rule of thumb is to reduce diameter by 1 inch,
pitch by 1 inch, or both if needed.
Use as large a canister-type muffler as
possible. Nearly all glow and gas engines sold
in the US market today are supplied with a
large canister-type muffler. Most of these
supplied mufflers will do an adequate job of
controlling the combustion noise coming from
the cylinder. Adding a piece of rubber hose to
the outlet pipe can also help stop some of the
ringing sound that the muffler can produce.
The rubber hose doesn’t have to extend past
the pipe, just cover it like insulation.
Soft-mount your engine. Often there can
be quite a bit of noise coming from your
airframe, especially if it contains a lot of
fiberglass or is an open structure covered in
plastic film. When the engine is hard-mounted
to the firewall, the vibration will be carried
back through the airframe and essentially act
as a drum unless there is some type of
absorption material in there to soak it up.
Many of the newer 30% and 40%
aerobatic aircraft have foam turtledecks and
other foam parts that absorb much of the
vibration noise; if not your beautiful machine
will sound more like a drum kit than a purring
kitten. There are several soft-mount systems
on the market today.
As far as I am aware, all full-scale piston
engines are soft-mounted. Take a lesson from
the big boys. MA
Edition: Model Aviation - 2011/12
Page Numbers: 156
