In article ,
David Findlay wrote:

I'm in the early stages of the homebuilding disease :-P and still don't
know exactly what I want to build, just that I want to build something.
I'm interested to know what people have done in their aircraft to reduce
the interior noise level? Can such noise insulation be added to plan or
kit built aircraft without too much difficulty? Is it possible to reach
car or commercial airline jet noise levels in a homebuilt? Thanks,

David

There was a great article in the April 2005 issue of Sport Aviation (EAA
publication) entitled "Taming the Noise." It's a great article,
explaining the acoustic principles at work, and offering insights into
how to deal with many different types of noise in different ways.
Insulation is only one tool in the battle against noise. Taming the
source (i.e. prop and engine balancing,) preventing the transmission
(isolation mounts,) and absorbing the reflected noises (carpeting and
other upholstery) are some of the other measures discussed. The author
and his associates were able to reduce the noise in their "test
airplane" by almost 8 dB fairly readily, at a modest penalty of 23 lbs.
It's also important to have a well-sealed cockpit. Stop the air leaks
and you stop all of the direct noise leaks. The rest of the noise is
"just" transmitted noise that comes in the form of vibration.