I'm not experienced at sound deadening in automobiles, but I do know
a bit about sound otherwise:
Sound comes through a car floor in one of three ways:
a.) going around it through open holes
b.) going right through it
c.) causing the surface to vibrate, thus re-radiating the sound on
the other side.
In soundproofing rooms and houses I tell people to solve (a) before
considering the others, but in an auto with a steel floor the
biggest source is usually (c). The solution for both (b) and (c)
is to secure something limp and of high mass to the floor.
Asphalt based roofing materials seem to fit the bill perfectly,
and also have the advantage of being reasonably weatherproof.
To a smaller extent, the quality of the sound in the car will
influence how loud it seems to be. An echo-ey cockpit will be
noisier than a 'damped' one, and this is where carpet and jute
padding can improve matters.
(Sound transmission decreases with increased mass on the barrier, but
often increasing the mass will increase the stiffness, either causing
more vibration or moving the resonant frequency to a pitch where it is
more noticeable. Multi-layer constructions tend to have different
resonant frequencies in the different layers, and so reduce the
re-radiated sound problem. Things that are limp will not vibrate. Note
that fuzzy insulation does not enter the equation...)
Douglas McKinnie
(working towards a PhD at the Institute of Sound Recording, University of
Surrey.)
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