AMA News: Focus on Competition-2011/12
AMA sound/noise-abatement recommendations
Understanding that what is a pleasant sound to us as modelers isn't necessarily pleasant to our neighbors is a reality 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 or later someone is going to build a home within earshot of your flying field and, if you don't have noise under control, you will quickly become a target. These guidelines will help you be proactive in getting sound under control before problems arise. After the authorities are alerted to a noise problem is not the best time to handle it — the best time is long before there's an issue. 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. That difference is discussed below.
What is "dB"? It is the abbreviation for decibels, a measurement of sound level expressed on a logarithmic scale. As the number increases, the perceived loudness increases significantly. For reference, about 45 dB is just above a whisper, 96–98 dB is tolerable, and at 107 dB and up noise can begin to damage hearing.
The easiest way to measure sound is with a decibel meter. They are relatively inexpensive and mostly battery-operated, making them easy to use at remote locations. A common handheld model sold at electronics stores for under $50 will suffice, so all clubs should have at least one. They can also be mounted on a camera tripod to 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).
Hold the decibel meter approximately 2 feet above the surface, pointed toward the aircraft. Take a few readings at different angles to the aircraft (front, back, side, etc.) and average them. This method should adequately measure the real sound level produced 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. They should also be adequate for controlling the sound level outside your overfly area while the aircraft are in the air.
What Is Actually Legal
Unless you live in the middle of nowhere, all property falls under the jurisdiction of some set of ordinances — state, county, township, etc. These ordinances set rules for fence lines, building set-backs, types of buildings, and so on.
There are also rules concerning how much noise (dB) is allowed to cross a property line. It may be hard to find, but if you look deeply 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 whether you're legal is easy: set your dB meter at different places on the property line and take readings. Other factors can add to the dB readings you get. If your field is located close to a major highway or busy road, tire and road noise can make a big difference. Knowing where your flying site stands legally regarding noise can and will give you a leg up if and when the need arises. Think how handy it would be to be confronted with a complaint and be able to state that your flying site is within the noise ordinance limits.
Ways of Abating Noise Problems
#### Try a three-bladed propeller Most of what you hear does not come from the engine; it comes from the propeller. When the engine unloads in the air, propeller tips can approach supersonic speeds, so reducing engine RPM will limit much of that noise.
Using a larger two-bladed propeller will slow the engine down, but because the blades are longer the tips may actually travel faster. Adding blades lets you keep the diameter down while reducing RPM. Many large International Miniature Aerobatic Club (IMAC) pilots now use three-bladed propellers primarily for noise control rather than performance. If you don't know how to size a three-bladed propeller, 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 U.S. market today are supplied with a large canister-type muffler. Most of these supplied mufflers will do an adequate job of controlling combustion noise from the cylinder. Adding a short piece of rubber hose over the outlet pipe can also help stop some of the ringing that a muffler can produce. The rubber hose doesn't have to extend past the pipe — just cover it like insulation.
#### Soft-mount your engine A lot of noise can come from the 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, vibration is carried back through the airframe and can make it sound like a drum unless there is absorption material to soak it up.
Many newer 30% and 40% aerobatic aircraft have foam turtle decks and other foam parts that absorb much of the vibration noise; without them a machine can sound more like a drum kit than a kitten. Several soft-mount systems are available 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.
Transcribed from original scans by AI. Minor OCR errors may remain.
