babflyer-

The problems with AMA (or anybody else) mandating limits on sound level are manifold. Here are just a couple of items to consider:

1) A limit on sound source level is ultimately a limit on power delivered to the prop. Engine intake and exhaust noise can be limited to any arbitrary level without much difficulty - there are no limits imposed by physics. Not so for the levels generated by the prop. The prop blade displaces air, and no matter how you want to slice it, displacement of air is the mechanism that causes sound. AMA for many years recommended a limit of 90 dBA at 9 ft for models flown at sites where noise is an issue. That effectively limits power absorbed by the prop to less than about 1 BHP, i.e., the power delivered by a decent .40-.46 2-stroke engine. It doesn't matter what engine/motor is turning the prop. If a brushless electric motor with good bearings is the prime mover and it is putting out on the order of 750 watts into the prop, the 90@9 rule will be approached or exceeded. This is a rule-of-thumb and not an absolute, as some mitigation is possible without giving up power in direct proportion to noise level reduction. the A-weighting in the dBA measurement is a frequency compensation intended to model human hearing response. Hearing response peaks at about 1 kHz, and the further below that frequency the power plant is operated at, the lower the perceived level, and accordingly the dBA reading, will be for a given physical shaft power level. This is why 4-strokes typically have an advantage when it comes to noise; the engine's power is usually generated at a lower RPM (and so frequency of engine and prop sounds), hence perceived sound levels are lower though actual physical levels may be similar.

2) Not all model flying fields are created equal. The 90@9 rule may make sense if the nearest neighbor is 1/4 mile away, but what if the separation distance is much greater? A doubling of the separation distance reduces the received level by 6 dB, due to spreading loss. That allows for up to about 4 times the engine power, with judicious prop selection. The only AMA rule-book event AFAIK that puts a limit on sound level is pattern aerobatics. The limit of 96 dBA @ 3M is really a driver as to who can fairly compete in these events. A higher level would limit venues where competitions could be held, and would eliminate many potential competitors because they would not be able to practice at their home sites. Everybody wants more power of course, but allowing more power effectively means fewer available venues to accommodate the sport. Look at the relative exclusivity of pylon racing for the why - think about why the top competitors always seem to come from Nevada.

I am not an advocate of broad, uniform regulation of sound emissions. Better to be aware of what standards apply in the locale where you fly, and do what it takes to conform to them.

btw, suppression of intake noise and reducing RPM aren't new. Back when pattern rules limited engines to 10 cc, reduction gearboxes and rear intake (where it could more readily be baffled) were common amongst the top competitors.

Abel