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I'm interested because I have been studying ANR and I fly almost exclusively
with it.  In about 1000 hours with my O540 in the Dakota I have discovered
that there are certain unmistakable sounds I can hear with my Lightspeed 20K
(or my modified old Flight Com) that I cannot hear either without a headset
or with my stock Peltors.  For instance when the oil level drops down below
10qt, I have discovered that I can tell by a faint metallic rattle related
to prop setting (It holds 12 and 9 is published minimum... for the constant
speed prop).

Most of the ANR manufacturers publish frequency analysis charts showing what
you can expect from their products.  These reveal that the ANR electronics
are essentially a smart notch filter centered on the most prevalent low
frequencies in the GA cockpit.  If you examine these charts, you can see,
that the industry has chosen to optimize for the 3-bladed props (at the
expense of performance for the 2-bladed world; if memory serves there is
about a 30 Hz difference).  Along the same vane, because the filter work is
done with some flavor of Digital Signal Processor, there just isn't the
processing bandwidth to handle frequencies of more than a few hundred Hz.
Since metallic rattles usually have a high frequency component (beyond the
capability of the DSP to filter), these tend to stand out more when using
ANR.

Additionally, the DSP bandwidth limitation is also the reason why I am so
concerned about the wind noise that comes from a poor door seal.  From
conversations with the engineers at Lord (they do cabin-based ANR for biz
jets as well as engine mounts) I've learned that wind noise, which is rich
in high frequency harmonics, is one of the biggest problems of the noise
environment.  The best way to deal with this is good door seals, sound
insulation, and thick windows (the latter of which may not be realistic).

So that's my experience and a bit of the theory behind ANR.  All of that
aside, I still can't help wondering if I'm missing something...

For more than you want to know about the topic, check out these sites:
    Virginia Tech Acoustics & Vibrations Lab: http://www.val.me.vt.edu/
    Acoustics & Vibrations Lab at MIT: http://web.mit.edu/org/a/avlab/www/
    Acoustics & Vibration WWW Virtual Library: http://www.ecgcorp.com/velav
    Lord Corporation NVX Home Page: http://www.lordcorp.com/NVX/NVX.html


Regards,

Joe Stack
983SC - XL/RG