Brian Perry wrote:

Hello All !
Looks like you are working on a very interesting project!

My name is Brian Perry.  I am an NVH engineer at Ford Motor Co, Dearborn MI.
(Noise Vibration & Harshness)
I have become interested in Gyroplanes and the Mazda 13B rotary engine.  I'm
in the planning stages of building
a Barnett J4B2 with a 13B & misc NVH improvements.

If you are interested, I would like to consult wih you and at the very
least, learn from your experience.  Here are
some quick pointers that may help you on your way to taming the "raspy
snarl" of the 13B.

First, the basics....
A good rule of thumb for muffler design is to use 10 times the engine
displacement as a starting point for your
muffler volume.  I believe the 13B has a displacement of approx 1.3L, 1300
CC or 78 cubic inches.  This
guideline would indicate that your muffler volume should be on the order of
780 cubic inches.  Therefore,
your 3" diameter tube would need to be 83" long, vs the original 36 inches!

How you get the volume is not particularly important, but the MORE THE
BETTER.  You may even want to
consider increasing this to 12 or 13 times engine displacement, depending on
the severity of your noise problem.

However, the question still remains, even if you can find a way to make 780
cubic inches of muffler volume fit in your
plane, will it be light enough to leave the ground?  Will it be properly
balanced?  It may not be PRACTICAL to
meet the above guideline and still have an aircraft that meets other
functional requirements.

The above comments are equally relevant to the air induction system.  A
significant percentage of the total noise may
be coming from the air intake system.  People often times confuse exhaust
noise with intake noise, because they
often-times sound so much alike!  Even so-called experts, like me, just
can't tell the first time they evaluate the vehicle.

The comment regarding noise from the propellor is equally relevant.  It will
probably be a large percentage of the
total noise the pilot hears.  There may not be much you can do about this,
other than to try other propellors.  There
are experts in propellor design and manufacturing out there that may be able
to help you with this.

Your approach to diagnosing the nature of the problem is correct.  Use of a
frequency analyzer with FFT capability
will help you pinpoint the objectionable frequencie(s).  Ussually, the
tallest peak is the one of greatest interest.

A good rule of thumb is that 3 dB of improvement provides a noticeable
improvement for most listeners.  A 6 dB
improvement will be noticed by all listeners.  If you can find a way to
lower the amplitude of the tallest peak at
the frequency of interest so that it is the same amplitude as surrounding
peaks, you will effecively "mask" that
frequency with "noise" from the rest of the system.

As a practical matter, the best way to make a root cause determination of
your problem frequency is to run the
engine at constant RPM on a dyno, test-stand, or under actual operating
conditions.  If the problem is most noticeable
at a given engine RPM, then you may have a problem with a single frequency
and should test AT THAT RPM.

If the noise is objectionable at all RPMs or over a wide range, the system
is just TOO LOUD!  Addressing the
volume consideraions for intake and exhaust, as well as proper attention to
the propellor noise should help you
resolve the issue.

Good luck with your diagnosis and please keep me apprised of your progress.
I will likely be heading down
a similar path in the near future.

 
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