The noise that model airplane motors make is a frequent topic of discussion these days. Many magazine articles about it have been published; most clubs enforce strict "muffled engines only" field rules; and the AMA even has a special committee on engine sound levels.
This subject's quite important to us modelers. "Excessive noise" complaints lead to the loss of flying sites. That's why it's unfortunate that most of the information being discussed and published about engine noise is incorrect and misleading.
The decibel level *numbers* involved in this subject are accurate enough; there's little question about that. But the meaning of these figures has been grossly misunderstood.
First, decibels aren't really units of sound energy. Second, there isn't much relationship between decibels and the irritating quality of noises. And third, the "rule of thumb" that a drop of three decibels means a 50% reduction in sound output, while objectively true, is false at the subjective level -- that is, what the hearer experiences.
Now I'll explain these surprising statements. Decibels are actually *relative power factors*, on what's called a logarithmic scale. On a *linear* scale, twenty is twice as high as ten, while thirty is three times as high. But in the *logarithmic* decibel scale, twenty decibels is ten times as high as ten; and thirty is ten times higher than twenty.
The original power factor unit was the "Bel" (named for Alexander Graham Bell) and was used in analyzing transmission problems in early telephone systems. A loss of one Bel represented a drop in power level to one tenth what it was originally. A gain of one Bel (by amplification, say) meant an output power ten times as high as the input.
However, the Bel turned out to be too large a unit for convenient use in modern technology. The "decibel" took its place. A decibel is one-tenth of a Bel, just as a decigram (a seldom-used metric unit of weight) is one-tenth of a gram. Because the "Bel scale" is logarithmic (involving multiples of ten rather than a direct numerical relation) the decibel scale is also logarithmic. For example, one decibel (dB) of gain represents a power increase of 1.259 times; two dB indicate a boost of 1.585; and three dB up means an increase of 1.995. That's why a three decibel drop equals a 50% power reduction.
If these figures seem confusing and illogical, try looking at them this way. A gain from, say, 50 to 60 dB is a ten decibel change, or one Bel. By definition, that's a ten-fold power increase.
Subdividing that 10 dB difference: from 50 to 53 is three dB up, 53 to 56 another three, 56 to 59 three more -- and one further dB makes it 60. Work it out on your calculator: 1.995 multiplied by itself three times (representing three successive boosts of 3 dB each) equals 7.94. That times 1.259 (for the last dB) gives you 9.996 -- mighty close to a total gain of 10.
Did you notice that in this explanation of decibels I haven't mentioned sound at all? That's because decibels are no more a measurement of actual sound output than watts are of light output. We use watts to specify sizes of electric light bulbs merely because watts are easy to measure, not because they define any quantity of light. After all, a hundred-watt bulb painted black emits no light whatever, but still consumes its full 100-watt quota of electricity whenever it's turned on.
Watts are *absolute* units of power. They can be used to specify the amount of energy used to produce light, heat, motion (as by an electric motor), or sound. Decibels, on the other hand, are *relative* units of power. They don't tell you directly how much energy is involved in whatever's being measured. They only let you know how many times more (or less) power there is than whatever standard value has been selected as a starting place.
Long ago a standard was chosen for the decibel scale as it relates to sound measurement. It's a pressure of 0.0002 dynes per square centimeter at a frequency of 1000 hertz (which is two octaves above middle C). An arbitrary value of 4 dB has been assigned to this, which represents the lowest level of sound audible to normal human ears. That appears like a straightforward enough standard. But unfortunately our ears are more sensitive to some sound frequencies than to others. Two noises, each registering 85 dB on a "sound meter", can seem very different in loudness to a listener if their frequencies are dissimilar.
Most decibel meters (such as my own Radio Shack unit) incorporate "weighting". This attempts to simulate the frequency sensitivity of the human ear. A meter so equipped registers decibels on what is termed the "A" scale, and its readings are often specified as "dBA". However, this isn't a great improvement, as engineers involved in noise reduction programs learned years ago. Trying to better define the problems they have to solve, these engineers came up with new units of sound measurement. One of these they call "Perceived Noise" decibels; another is appropriately named "noys". These new units depend more on human judgment than measurement by scientific instrumentation. A "jury" of several dozen people rated sounds of various pitches and intensities according to their degree of unpleasantness, and the units were defined through a consensus of the jury members' opinions.
What it all boils down to is this: personal reaction to noise turns out to be far more a matter of subjective impression than it is of readings on a meter.
I asked a dozen people to tell me what kind of noise they found most irritating. Their answers were nearly unanimous: the sickening screech of hard chalk scraped down a slate blackboard. A friend then located a big old-fashioned blackboard for me in a local church. With my sound meter we took decibel readings there of the loudest screeches we could manage to produce -- noises my friend said were literally unbearable for more than a second or so. (I couldn't tell myself, because I've been totally deaf since 1959: mostly as a result of overexposure to jet fighter engine noise.)
With my sound meter less than a foot from the blackboard, the highest reading I got was only 83 dB! That's the same as the meter indicates inside a VW "Super Beetle" at 55 mph, or on my front porch when my next-door neighbor mows his lawn. We also tried scraping our fingernails down the blackboard. That, too, produced extremely irritating noise -- but the meter needle never passed 74 db!
Some highly bothersome sounds won't register at all on a decibel meter: the whine of a mosquito, for example, or the drip-drip-drip of a leaky faucet when you're trying to get to sleep. No, there's no direct relationship between decibel levels and irritation. Think of a musical tone played by an accomplished trumpeter versus the "sour note" of a beginner on the instrument. They'll read much the same on a meter -- but what a difference to the listener!
After reading my description of how the decibel scale works you may have wondered why such a complicated way of rating sound was thought up in the first place. Why not measure it in linear units, like electricity's volts and amperes?
Insofar as it affects people, sound HAS to be evaluated in non-linear terms -- because human hearing itself is non-linear! Think about it. The loudest non-harmful sound level is regarded as being around 110 dB, while the lowest audible sound is defined as 4 dB. That range represents a difference in power output of FORTY BILLION TO ONE.
Does a hi-fi playing rock music (or Stravinsky) seem forty billion times as loud as a baby's sigh? Of course not -- and that's the reason for the decibel scale. It's an approximation of the way the human auditory system responds. The math earlier in this article showed how decibels relate to the power output of sound. However, the way our ears and brains react to sound is something else again. In this area the decibel scale IS approximately linear with respect to loudness.
A noise that registers 60 dB seems only about 20% greater to us than one of 50 dB, even though the power involved is ten times as high. That's why a model engine muffler that cuts exhaust noise from 90 dB to 87 -- a sound-power reduction of 50% -- doesn't appear to be especially effective to the average bystander. It isn't lowering the sensation of noise he experiences by more than a few percent. High dB levels aren't in themselves unbearably bothersome. Loud sounds are everywhere these days. Aside from rock music, truck engines, and chain saws (all of which frequently top 100 dB), people are regularly exposed to high noise levels from power tools and household appliances. An automatic washing machine puts out close to 80 dB; my Hoover carpet sweeper registers 90 dB, and my shop vacuum turns up 95. In fact, only one power tool in my workshop runs under 90 dB: my disk/belt sander at 88.
Recently I took my sound meter out to the local R/C club flying field. Decibel readings at the runway edge while three .40- and .45-powered airplanes were simultaneously flying overhead produced a meter reading of just 72 dB maximum!
No: decibels are NOT the problem we need to solve to safeguard our model flying sites. The Penn-Ohio Radio Kontrol Society (PORKS) got evicted from three club flying locations in a period of just a few years. Each time the reason was supposedly excessive noise. But one of the club's top officers later investigated all the circumstances "in depth". He came to the conclusion that the real reason the PORKS lost their fields was irresponsible behavior of a few inconsiderate club members. This led to personality conflicts with property owners, who retaliated by complaining about "excessive noise" to local officials.
Three very good sites were closed to all model flying -- even gliders! -- as a result. The only part noise really played in the field shutdowns was that it was much easier to offer that in evidence against the model club than to put on display the outraged personal feelings that were the real issue.
In contrast, the "Flying Dutchmen" club members have been using a hilltop site only three miles outside the city of Reading, Penn- sylvania for over twenty years with no noise complaints whatever. The club has more than 40 active members, who fly U-Control there. That's the most irritatingly noisy type of model activity of all, because of the monotonous Wow-Wow-Wow-Wow sound the airplanes make going around and around and AROUND. Few of the "Dutchmen" use mufflers. They power their models with engines of all sizes, including some .60's. They even do a bit of control-line speed flying at the club field, which is only a mile or so from homes and businesses.
How do the Flying Dutchmen get away with this? According to Bob Diefenderfer, the club's publicity director, it's mostly good public relations. The club puts on many public flying demonstrations and model displays; they welcome youngsters and newcomers to model flying; and in general they're good neighbors and nice people to have around. Nobody wants to evict folks like that just because they happen to make a bit of a racket with their hobby....
Summing it all up: to save ourselves from having our flying sites closed to us, our personal attitudes and behavior seem to be more important than the number of decibels our motors produce. I've seen arrogant, insensitive, and dangerous behavior by some modelers myself such that if they were flying from a field in my neighborhood, I'd do anything I could to have them stopped -- even (totally deaf as I am) complaining about excessive noise!
