Understanding the fact 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 and deal with. The largest contributor to the loss of flying sites by a wide margin is the noise we produce. Sooner than later someone is going to build their 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 document should help you to not only be reactive to a
noise complaint but also 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 start getting a handle on 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. Keep in mind also there is a difference between what is a recommendation and what is legal for a particular piece of property and we’ll discuss this difference later in the document.
What is “dB”?
It is the acronym 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, 45dB is just above a whisper, 96 to 98dB is tolerable, at 107dB and up it starts to do real 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 hand held 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 tri-pod which will make testing and consistency simple.
Sound Limit Guidelines:
96db measured from 20’ distance over soft field (sod)
98db measured from 20’ distance over hard surface (pavement/
are normally tolerable to most people in the vicinity, pit area and spectators. These recommendations should also be very 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 will also be a rule for how much noise (dB) that’s 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, and normally that number will be somewhere between 50 and 65dB that’s allowed to cross a property line. Once you find the info on your property checking to see if you’re legal is easy by setting your dB meters at different places on the property line and take the readings. Keep in mind that 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 all may sound a bit trivial but knowing where your flying site stands legally with regards to 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, to know and be able to state that your flying site is within the noise ordinance limits.
Ways of Abating Noise Problems
1. Try a three-blade prop.
The most important thing to remember about airplane noise is
most of what you hear does not come from the engine, it comes
from the prop. What you hear when your engine unloads in the air
is the prop tips going supersonic, so getting the engine rpm down will limit much of that. So you think bigger two blade prop will slow the engine down right? wrong! Yes 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 IMACC guys now using three blade props is not for performance, but noise control. If you don’t know how to figure for a three blade prop, you might ask your engine manufacturer for a recommendation or a rule of thumb is to reduce diameter by 1 inch or pitch by 1 or both if needed.
2. Use as large as possible “canister-type” muffler.
Just about all engines, glow and gas sold in the US market today are supplied with a large can type muffler. Most of these supplied mufflers will do a very adequate job of controlling the combustion noise coming from the cylinder. Also adding a piece of rubber hose to the outlet piped can also help in stopping some of the ringing sound that can be produced by the muffler. The rubber hose doesn’t have to extend past the pipe, just cover it like insulation.
3. 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 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 turtle decks 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. BTW, to my knowledge, all full-scale piston engines are soft mounted. Take a lesson from the big boys.
96 dB at 20 Feet - Soft Surface 98 dB at 20 Feet - Hard Surface
Sound Meter to Aircraft 20 feet Distance
concrete)
The dB meter should be held approximately two 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
Distance for Sound Measurement Sound Meter
do an adequate job
of measuring the real sound level being put out by a particular aircraft/engine/muffler combination.
These recommendations are not quiet by any stretch but
24 inches
Sound Meter Setup