Atmospheric effect on Engine noise level

[Marion]

Can any body here direct me to a source of factual information related to the subject line?

Reason : I measured an engine noise level as being 94-95 db. Next day I could get only 90 db from the engine. Best I could tell the only thing different was the temperature and humidity. Is a 4 or 5 db decrease too much to be attributed to increased temperature and humidity ??

If this is not in the correct forum, please Mr. Moderator move it to the correct place.

Thanks for any helpful info.

[vintage1]

Sound certainly travels best in cold clear air: but t standard test is at a meter or so, and it shouldn't make much odds.

However 405dB is fairly close to 'not worth talking about' - a slight difference in position, or use of a different weighting scale could certainly make that much difference.

[groundfx]

Each 3 dB requires a doubling of the sound power source, so it's not insignificant.

A lot of sound can come for the propeller. What's your propeller tip speed? If it's high (e.g. greater than .65 mach) you might get a sound level change as the speed of sound changes with air temperature... just a thought.

[MarkusN]

Also, what were the surroundings? If you measure over wet vs. dry grass, that can have a significant impact.

Then, engine performance depends on temp. and humidity. So if engine speed was affected by the conditions, you have another big contributor.

[Batmanwpg]

You should probably use a tach along with db measurement just to make sure you have the same prop tip speed

[Marion]

Thanks for all the thoughts and ideas. Your input is appreciated.

We have a strict limit of 94 db, thus my interest in a good measurement. This is a fail or pass requirement to fly at our flying fields around here.

Yes, we tried our best to duplicate the testing conditions -- several different (all at the same time) meters, same engine RPM, always over grass, within an inch of the same measuring distance. We were always at 11,400 RPM with the same 12 x 6 prop, so I don't think Mach number is a factor. Can somebody verify that ??

Wet vs dry grass... Obviously high humidity in the morning can make the grass wet. And the temperature and humidity can affect how the sound propogates.

Thus I am back at my original question --- Can any body here direct me to a source of factual information about how atmosphere conditions affects engine noise levels.

We were hoping to find a "correction factor" so we could demonstrate that our engine noise was within requirements.

Any more comments on this matter ?? It is very important to our model clubs around here.

Thanks again, Marion

[yoyoML]

.

Yes, we tried our best to duplicate the testing conditions -- several different (all at the same time) meters, same engine RPM, always over grass, within an inch of the same measuring distance. We were always at 11,400 RPM with the same 12 x 6 prop, so I don't think Mach number is a factor. Can somebody verify that ??


11400 (rpm) * 2pi / 60 (sec/min) * 12 (inch)/2 * 0.0254 (m/inch) = 182 (m/s)

Oh great you're way below Mach 1 !!!

[groundfx]

= 402 mph

You're just in the danger zone of increasing noise. Since you're so close to the edge... why not just tackle the problem and take a few steps to make your plane quieter. For example, you might try a smaller prop with greater pitch (or a 3 blader) as a quick and cheap way to get the noise down.

See also: http://www.bolly.com.au/book/content/22.htm

[hul]

a bigger prop makes a glow motor quieter, not a smaller one. Or it is a very noisy electric at 90dBA.

Hans

[groundfx]

a bigger prop makes a glow motor quieter, not a smaller one. Or it is a very noisy electric at 90dBA.

Hans
Not if that bigger propeller causes the tip speed to start going over ~400mph.

You might be thinking that a bigger prop causes more load and hence the engine turns the prop slower. This is not true in general either. For instance, a smaller propeller but with greater pitch (or more blades) can load the engine just as much (or more) and hence cause the engine to run at the same RPM (or even slower.)