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Sound - Noise |
R. W. Stuart Over much of the human auditory amplitude range subjective loudness is proportional to the physical sound intensity raised to the 0.3 power. L = I.3, where L is perceived loudness and I is physical sound intensity. This response to sound allows us to have high sensitivity to minute sounds, but protects from very loud sounds. To double the perceived loudness of a model airplane engine we must increase the number ten fold or provide 10 equally loud engines. If we want to halve the loudness of a particular sound we must reduce the acoustic output of that sound tenfold. A tenfold increase in average intensity of engine noise caused by a tenfold increase in engine numbers might raise the rate of complaints perhaps a hundredfold. A loud sound every five minutes is tolerable, but one every 5 seconds is not. Aircraft suffer this same perception phenomena, and little can be done about it. For trucks, tire noise is a real culprit, but tire noise intensity is nearly proportional to the fourth power of speed - speed reduction pays off in a big way - can not be done with airplanes. Model airplane noises are relatively high pitched, high amplitude noises with many irritating harmonics. Model airplane sounds to the "average human" are aggravating merely because the source is an activity from which the "average human" unconsciously feels excluded. As humans we inherently dislike what we do not know or understand. Model airplane noise is composed of engine noise (2000 - 12000 cps), prop noise (6000 -18000 cps), air noise (600 - 1800 cps), and air frame noise (50 - 500 cps). Engine noise results from throttle inrush noise which can be decreased by inlet filters or muffles, and exhaust noise which can be reduced by silencer or muffler application. Prop noise - a very high frequency noise caused by tip shape and tip speed can be effectively reduced only by reducing the prop speed and thus reducing turbulence, but since the output of our engines depends on speed ( rpm ) times torque, we must run at high speed to get horsepower. The increase in sound pressure level is equal to 30 times the log (base 10) of the rpm ratio increase. Doubling the speed produces an increase of about 9 dB. Air noise over the airframe is generally not a serious problem unless panels or parts are flexible enough to allow harmonic vibrations to occur, said vibrations being self- correcting in that they will eventually destroy the airframe and may be avoided by changing the stiffness, area, or mass of the part- or by changing air speed which may not be safe. Propellor sound- on a 72 degree day the speed of sound in air is about 1130 fps. Tip speed can be calculated (see Stuart editorial last year on tip speed) and changes in rpm will result in airspeed changes at the tip further increased by the airfoil of the tip. As the tip airspeed approaches the speed of sound, the sound increases exponentially and eventually becomes a sonic boom. The German-American lingo around the space center after WWII would describe it as "das iss an eargeshsplitten loudenboomer". A decibel ( dB ) is a number - the logarithm of a ratio used to describe how one sound compares powerwise with another sound as sensed by the human ear. It describes the ratio of two power levels, and is logarithmic because the human perception system responds to sound as the .3 exponent of the absolute power. If one sound level is twice as much as another the dB = 10 log (base 10) of P1/P2 (log base 10 of 2/1 is 0.3); doubling the sound level is a 3 dB increase. A dB is very close to the least change (change being a small ratio of two powers) in audio power that can be detected by the average human. The "bel" part of it comes from honoring Alexander Graham Bell by naming the function after him. One author has stated that the increase in sound pressure level, dB, is equal to ten times the log (base 10) of the horsepower (any power- RWS !) ratio increase. So far, so good! If the power is doubled it produces a 3 dB rise in sound level. With a 2 microwatt sound level, power doubling produces 4 microwatts of sound level- a 3 dB rise in power level. With a 2 watt sound level, power doubling produces 4 watts of sound level- again a 3 dB rise in power level. 3 dB tells us only that we have doubled the power. We still have no idea that we may be talking microwatts, watts, or megawatts. So talking Db without some absolute reference (dBA) is meaningless. Nothing from the AMA beyond the 90 dB requirement-- do they mean 90 dBA? Do they realize that 90 dB is only a power ratio and 90 dBA is a true sound level ? The noise requirement (restriction) imposed by the AMA is published as 90 dB which means that the noise produced by our power systems should not be more than 90 dB more than some reference noise, but the reference noise is not defined when using the term dB. In order to give 90 dB a useful meaning we must compare our noise to a reference noise of known intensity and when done the dB changes to dBA - "A" for absolute and for "A" weighted to the frequencies most easily heard by humans - centering around 3200 cps. Some sound meters are calibrated so that 0 dB = 0.0002 microbars where 1 bar is a pressure of 29.53 in. of mercury column- very near one atmosphere of pressure. Sound pressure is the proper term to describe sound. 1 dBA is at the threshold of human hearing. Equivilents are 1 dBA = 1 Newton/ sq meter or 0.001 K Pa (Pa is a Pascal) which means that a Pa = N/sq M). 1 bar = 100,000 N/sq M or 100,000 Pa. To bring it back to English units 101,300 Pa is 14.7 psi- 1 atmos, another pressure. 0.00002 microbars, 20 micro Pa, 0.000,020 N\sq meter, and 1 dBA are all the same pressure. The term "sound pressure level" is used to describe sound. Sound pressure level is actually the pressure amplitude of the sine wave produced by the sound in air as the sound enters our ear or as it enters a detector. Again, if described as dB, it represents only a ratio of one power to another, and unless one of these powers is a dBA value the actual sound level is not defined. Sound decreases in power level as the square of the distance from the source. To get some idea of the meaning of sound levels we can review a few typical sound sources as OSHA sees them. I know that OSHA has a rather comical reputation, but at least they know that a Db and a dBA are two different things.
20 micro Pa--------1 dBA--------Threshold of hearing
100 micro Pa-------14 dBA--------Bedroom or deep timber
1000 micro Pa-------34 dBA--------Library
40 dBA--------Living room
10000 micro Pa-------54 dBA
60 dBA--------Conversational speech
63 dBA--------Office
100K micro Pa-------74 dBA
83 dBA--------Average street traffic
90 dBA----Long term exposure is
damaging to hearing
90 dBA--------Heavy truck
>> 90 dB or dBA?-AMA restriction << 1M micro Pa-------94 dBA-------Same as 1 Pa 100 dBA--------Air hammer chipper 110 dBA--------Rock group 10M micro Pa-------113 dBA-------Same as 10 Pa 128 dBA--Jet, take-off, 100 meter distance 100M micro Pa-------134 dBA-------Same as 100 Pa 140 dBA-----Jet engine, 25 meter distance 140+ dBA----Threshold of pain
This paper uses the terms sound and noise interchangeable, but from the purist's point of view sound is the preferred term-- sound can be measured and defined where noise is purely subjective. Locally there is an individual who produces very powerful sounds (church bells at 6 AM) which could be called noise- depends on point of view.
Galileo Galilei--
Born: 1564
Censured: 1616
Died: 1642
Tomb built: 1737
Honored with monument: 1842
Exonerated: 1982
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