Maybe this is why the turbo is quieter than the 100?
At least according to unscientific ear data
William Daniell
+1 786 878 0246
On Tue, Jun 13, 2023, 04:01 Area-51 <goldsteinindustrial@gmail.com> wrote:
> goldsteinindustrial@gmail.com>
>
> 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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