An update on testing the baffle box insert concept...
The PLA 3D printed box has been left insitu in the airbox all week without
degradation with the extension tubes being replaced prior with PETG fuel
resistant units.
A further round of ground testing was carried out over the weekend to
gather data from the external far-field and compare those to the internal
tests. All up now about three hours of flight time has been completed and
data captured.
External far-field tests revealed similar broadband frequency response to
the internal tests with the exception of low end passes being stronger with
the internal run tests. This would be indicative of low end panel drumming
through the fuselage, however the weighted frequency response is now
fairly flat and uniform from 70hz through to falloff beginning at 3000hz...
The other exception with the external testing related to propeller tip noise
which falls off cleanly at 3000Hz by about 10db. Other distinct frequency
peaks were observed at 94/141/193/270 Hz, which will likely be exhaust
tone related, however these peaks and troughs are again all fairly flat and
uniform from 60Hz through to 3000Hz with select weighting applied.
With the far-field external tests completed and recorded the results were
fed into a matrix to calculate the farfield SPL (sound Pressure Level) at
select distances... At the initial distance of 4.5m at the port wing tip max
recorded SPL's were all around 70db-75db across the broadband 60Hz to
3000Hz at 4000rpm; which are correlate with the 100db internal recorded
peak SPL's on WOT. Extrapolating the SPL's and applying distances the
matrix returned SPL's below 45db beyond 500' and 37db at 1000'... In
short the aircraft is now exceptionally quiet for both operator and
observer on the ground.
Other observations... with the baffle tuned specifically for the existing
airbox and installed the SPL's in the cockpit have been reduced by some
20db to 24db in the target frequency spectrum... Operational perceptions
were met with the mitigation of engine howling previously observed at
4100rpm to 4900rpm; slight residual resonance remains between 4050rpm
to 4100rpm only... Below 4000rpm the engine is extremely quiet for the
operator... Measured broadband weighted SPL's in the cockpit are 66db at
2000rpm, 76db at 3800rpm, 80db at 4200rpm, 85db at 5200rpm.
Conclusions findings and assumptions:
Reflecting upon the results achieved and the data gathered through the
alternate forms of analysis applied it would be acceptable to assume that
the forecast analysis results are aligned accurately with the measured field
observations. It would also be correct to assume that the testing matrix is
able to provide realistic and accurate forecasts.
Findings have prompted further study toward sound attenuation of
specific frequencies for the purpose of improved performance. Results
so far have provided a sound attenuation environment that is able to be
tuned to specific broadband frequency ranges; allowing the end result to
be applied to exhaust mufflers. Results so far forecast SPL attenuation
levels of 80db - 147db across a frequency spread on average of 2500Hz.
In conclusion the "Shut That Noise Up" project has been a success in its
ability to meet all required targets so far, and further ongoing observation
is required to ensure and record the operational integrity of the 3D printed
components. Although far better results can be achieved, in meeting the
specified target qualitative levels it is a great position to arrive at.
A final note upon the simplicity of attenuating air filter and exhaust noise...
It is not very difficult to achieve brilliant results with just a bunch of fine
stainless steel mesh either rolled or scrunched up and stuffed into a box.
The only caveat would be to do this upstream of an inlet air filter rather
than downstream; unless the quality of the mesh can be relied upon
without question... There are many ways to attenuate exhaust and
induction noise with the mesh method being one of the most effective and
efficient. The baffle box and other select modes applied in this chapter
have simply been an exercise in applied design and engineering, and
should not be regarded as the only or best method to achieve similar or
even better results.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=510955#510955
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