Our club recently received a noise complaint (that may not have been caused by a model) and there has been a knee jerk reaction to initiate noise testing.

I have looked at the BMFA rules and they have no limits for gas turbines. I can't find anything from the SAA and the IJMC rules appear to have been removed.

There are a couple of previous threads on this but they are a little old. I am quite happy to tell the club that the rules to not apply to gas turbines but would like to suggest some form of test to soften the blow.

Does anyone else here have to undergo any form of noise test for their model? How do you do it?

Thanks

Let sleeping dogs lie.........

don't poke the bear........

jets do not have noise issues.....


I love the sound a jet is supposed to make!!!!

Call on the folks down in Austin, Texas for this information. I remember reading on this site a couple of years ago, that Patrick Fernandez (hope the spelling is correct)

Did some testing at a jet rally with Ducted Fan engines, and Turbine Engines. I believe the decibel level was on par with 110 or about that of a lawn mower from 10 feet away at idle.

I'm sorry I do not remember his user name here.

Pat's username is patf

90db @ 900ft

As they say in the north of england, somebodies got to piss on your bonfire, they nothing to worry about, sad!

Darren,

The IJMC rules are still there under "F4J Rules" as they have always been. Did you have problems finding them?

Cheers

Davie.
IJMC web guy

My club has a 96 db @ 3 metre noise limit measured from the side opposite the exhaust. We tested a Wren 44 in a Fivejet at the side opposite the exhaust, the front. The results were not encouraging, the limit being exceeded at about 3/4 throttle. BUT - a prop driven model's noise increases greatly once it's in the air & the RPM speeds up. Of course a turbine's RPM is limited by the ECU so it does not speed up & make more noise in flight. That's the excuse we use. [sm=idea.gif]- John.

Davie,

I found the rules but the noise test has been completely removed. I think it was rule 3?


I think I will offer a comparison test. I will ask for a prop model that has passed the ground test to be re-tested in the air. I will then offer to have my jet tested in the air and see what happens.

Hopefully it won't come to this.

The reason the noise limit was dropped by the BMFA was because it was found turbines tested under the piston engine rules will ALL fail. It was though noted that turbines were significantly quieter in the air, they also produced a less monotonous annoying sound so, the noise rule was dropped to allow them to be used.

Having said all that we need to be careful because the recent new breed of more powerful engines are by their nature noisier. So personally I recommend a little prudent throttle control whilst flying, to help divert attention away from "the noise". Obviously a model will be noisier when being flown away than coming towards yourself. Therefore I consider the future of turbine flying to be in the hands of the pilots.

m

Due to the high frequency content, only children, dogs & bats can hear our turbines ! They don't pay taxes so can be ignored.
The apparent noise soon falls off with 1/distance or is it 1/(distance squared)

Also, certain models are more noisier than others. For example a lot of Avonds F15's are really loud, I presume due to the bifurcated duct.

Hi siclick33,

The rules on noise, (rules 3.0.1, 3.0.2 & 3.0.3), were entirely deleted back in 2002.

Davie.

Mick,

We need someone who knows a bit about the physics of sound to chip in here and explain why different frequency sounds have different attenuation rates...but the bottom line is that a high frequency sound (eg jet engine) dissipates faster than a low frequency sound (big 4-stroke, prop, etc). It'll be interesting to see if the BMFA ever come up with a credible testing regime/sound limit for gas turbines.

On the same subject and getting a bit booring ('cos some people think personal safety is booring!) hearing degradation can take place over a long period of time and can result from the cumulative effects of repeated exposure...how many times have we run our engines at max power for a long period and put up with the discomfort and inability to hear properly for a short time afterwards...I know I have. Trouble is, in time, the ability to hear (maybe not all sounds/frequencies) is compromised and things like tinnitis can result (and that can, I'm told, drive you to distraction if it's bad). So, moral of the story...wear hearing protection when doing high power engine runs! End of soapbox bit!

Cheers,

Dick
England

I whole heartedly agree Dick, sitting on the start line for at least one minute with four FAI pylon motors singing in my ears over a period of ten years have completely stuffed my hearing, tinnitus and all,, pardon, sorry thought someone spoke!

m

I am trying to remember more of the details from when we did the testing. Essentially we found that exposed turbine engines, and those with biforcated exhaust ducts were equally loud and in some cases louder to a noise meter as a ducted fan. Those fully ducted bypassed with a straight pipe were quieter, but not that much. I agree that someone with some real knowledge about how sound works needs to be contacted because what the meter was telling us was a little different than what was percieved. in addition, once the aircraft was in the air with a little distance, the percieved noise is drastically differnt.

Yep, agreed. The engine in my sport model is fully enclosed and is much quieter with the hatch on & secured, no whistle from the compressor at all. Not a lot you can do about the other end though.

Right again. And the percceived noise is quieter & less annoying than glow powered models, particularly at 'cruise' power settings, lots of club members have commented how turbines make your ears ring on the ground but once flying are surprisingly quiet. - John.

Absolutely true! Two years ramping Boeings back in the early 80's took a big chunk out of my upper range....and that was with ear protectors!!!

Our club has very stiff noise rules, but for what ever reason exempted turbines (I'm not complaining). The test is done at 25 feet from the right side of the model with the soundmeter set for "A" rating, slow resonse. The max limit is 92 db. Just for fun, I tried one of my turbines on a test stand. It blew by 92 db at half throttle, then went way beyond that....

I think, as has been mentioned, the fairest test would be to do the sound test in the air, using a prop plane as a bench mark......

Attenuation of noise is subject to distance as well as to absortion through the atmosphere as described below:

1. Distance Sound Attenuation

Sound waves that spread into the atmosphere loose more of their intensity, i.e. the sound amplitude decreases. One reason is the geometrical spreading since the emitted sound energy is distributed over a larger and larger volume. Therefore the local sound energy flux (sound intensity) decreases with growing distance. If the sound originates from a point source (e.g. a single car), the waves are spherical and spread into all directions. In this case the sound level reduces by 6 dB as the distance from the source doubles. If, on the other hand, the sound originates from a line source (e.g. a busy road), the waves are cylindrical and the sound level reduces only by 3 dB as the distance from the source doubles. Model aircraft generally fall under the spherical category.


Distance Sound Attenuation (dBA) Examples

3m 6m 12m 24m 48m 96m 192m 384m 776m

95 89 83 77 71 65 59 53 47

98 92 86 80 74 68 62 56 50

101 95 89 83 77 71 65 59 53

110 104 98 92 86 80 76 70 64



2. Sound Attenuation through Air Absorption

A further effect that leads to an attenuation of sound pressure is the atmospheric absorption. As the sound waves propagate through the air, sound energy is lost due to friction between air molecules and because of further properties of the molecules. This loss depends on the air temperature and humidity. High frequencies are much more affected by atmospheric absorption than low frequencies. Therefore, one can hear only the bass tones at greater distance from an open air concert.


Absorption Attenuation Examples - Weather Conditions 29.92” Hg @ 20ºC and 70%RH (ISO9613-2)

83.3 Hz 0.15 dBA/km

100 Hz 0.22 dBA/km

125 Hz 0.3 dBA/km

250 Hz 1.1 dBA/km

500Hz 2.8 dBA/km

2000 Hz 9.0 dBA/km

2666 Hz 12.74 dBA/km

The base attenuation is governed by distance, and air absorption attenuation is additive so a jet producing 110 dBA @ 3m would be attenuated to 64 dBA @ 776m, and given a frequency of 2666 Hz the effects of air absorption would further attenuate by (12.74dBA x 0.776) = 9.88 dBA giving a noise level of about 54 dBA. In other words, less than normal converstion sound levels!

That's the simplified version - sound levels are determined by a number of other factors. An excellent paper on the subject may be found at: http://www.nonoise.org/library/envnoise/#measuring

Dennis