ORIGINAL: 50+AirYears
We bought one of the "Cheap" RS instruments about 8 years ago. After a complaint from a large scaler about an airborne reading on his Byron P51 of 118 dbA at 100 feet, I took it into my lab and checked it against our B&K lab standard, used for our ISO 9000 compliance calibration of some expensive lab equipment. At a lab temperature of 79 degrees, the RS checked out within a +/- 0.5 dbA, fast response, from 70 to 120 dbA The cheaper price just means it can't be banged around the way the $1500.00 B&Ks we use in the lab can.
There's also a lot of subjective "Perception" about how loud one engine is compared to another, and a lot of people will trust their subjective opinion over an actual objective measurement. Regardless of facts! Just like people saying their brand X 60 turning an 11-8 prop at 10500 rpm puts out more power than a brand Y 60 turning the same prop at 11200 rpm. Folks, it ain't necessarily so!
50+AirYears-
Thanks for bring this topic into the real world. The RS meter is good enough. Its tolerance is well within the variance in levels that cause people to be annoyed, and also the error band that accumulates from other conditions. Here are a few:
* By convention in the US we measure at a standard distance of 9 ft (old AMA rule) or 3 Meters (later change in competition rules to get with the rest of the world). This is on the order of a wavelength of significant engine and exhaust harmonics, so direct and reflected waves sum as vectors to produce a resultant that can be significantly different from the direct path level that is of primary concern (as it is expected to dominate while the model is airborne).
* The level of reflected waves is an uncontrolled variable, dependent on the surfzce over which the measurment is taken. In the US, a hard reflective surface is specified by ANSI (and in turn AMA and many others), which is unfortunate as this is the most likely to produce corrupted readings in the case of our primary concern, which is the direct path to the receptor while the model is flying.
* Applying sound level rules intended for competition to the sport flying situation is mixing apples and oranges. In aerobatic competition, one model is flying at a time. Normally sport flying has several in the air simultaneously. Equal sounds summing incoherently increase by 3 dB for every doubling in the number of sources. For example, 4 sport models at 90 dB produce about the same perceived level as one competition model at 96 dB.
* The human hearing apparatus tends to integrate annoyance level with sound over some time period. Standards set by agencies concerned with environmental quality (HUD, ANSI, ISO, et al) usually specify tolerable sound levels as a running average over some interval, usually one hour, as an equivalent level Leq, the propensity to disturb being 'equivalent' to constant noise at the specified level. Most localities that specify objective levels follow the guidance of these agencies, and the bottom line is that compliance with the law requires giving the time factor due consideration. Example: 90 dB instantaneous level that persists for a total of 1/2 hour in any hour is 84 dB Leq.
* Sound emissions from model airplanes are directional. Prop emissions are greatest in magnitude a few degrees aft of the plane of the prop arc. If the prop emissions are dominant, then the AMA specified procedure of measuring broadside to the model produces reasonably accurate results. With glow engines fitted with stock mufflers, however, the exhaust and intake sound levels are usually above the prop generated levels by several dB, so the maximum level could well be emitted from another aspect.
There are many more variables that are ususally uncontrolled outside of the laboratory, but these should be sufficient to conceptualize the relative accuracy of the procedures used vs the accuracy of the meter. You're kidding yourself when you use a micrometer to measure the length of a rubber band.
Abel