Noise

[EHaury at aol.com]

What Troy has said about props being the major noise generator is accurate, 
especially when measuring with the same (or similar) airplane. There are other 
considerations though.

While the sound meter measures max sound pressure, the response is not 
linear, but logarithmic. Meaning that sound pressures at different frequencies will 
be "weighed" (logarithmic scale dBA in our case) to represent the human ear 
response. Therefore, a sound pressure of some absolute value, lower than another 
of equal pressure but different frequency, may drive a meter higher.

I've had the opportunity to observe the noise spectrum generated by our 
systems using a digital spectrum analyzer. This is an expensive lab gizmo that 
presents data on screen much like an oscilloscope, except that it presents 
amplitude vs. frequency. You can observe the magnitude of noise as generated at 
various frequencies. 

I experimented by blocking the sound path between the noise generator (engine 
system) and microphone with foam blocks. This allows one to observe the 
portion of the sound spectrum generated by various components by comparing readings 
with and without isolation. 

As Troy stated, the prop is the major offender and is variable basis speed 
and prop design. The exhaust, noise radiation from parts (pipe wall, engine, air 
inlet, etc.) are all portions of the spectrum and their reduction is obvious 
with the analyzer, but only on the meter if the major noise source is isolated 
(prop). I suspect that it's a reduction of total spectrum that causes some 
airplanes to be quieter in the air, although they measure the same (or higher) 
on the ground.

The trick in all this is that a high frequency noise source (maybe the gear 
box on an electric) that wouldn't seem significant, because of the weighing 
scale, may be a major offender. In the case with Sean at the TT, his engine was a 
tad lean and detonating. The meter picked up the high freq detonation and he 
failed the test. Richening the engine very slightly (no noticeable rpm change) 
dropped the noise measurement 2+ dB.

One last point, an airframe that resonates more than another will generally 
both sound louder in the air and measure louder on the ground. Apparently the 
resonating airframe amplifies (much like a speaker cone) the vibratory energy 
generated by the power system. I've a glass Hydeout and one with a scratch 
built wooden fuse that are both equipped the same. The glass version is 
consistently 2+ dB louder than the "Woody."

Earl
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