Dan wrote:

You really want to dampen the vibrations you have. i.e. reduce the Q
factor of the resonator. You can do this by introducing a dissapative
element (e.g. sand) or by coupling to a resonator that is not a
harmonic of what you have. I suspect the foam might do well at
reducing the Q factor. Getting the expanding foam in to the middle of
a 78 inch bar might be a bit tricky,.

I've seen contractors use expanding foam inside the hollow rungs of
aluminum ladders. (This is definitely a case where large amounts of
added mass is not an optimum solution.)

Michael Daly wrote:


Use the expanding foam. Fill one end with foam and then put the end
cap back on. The foam will expand to the other end. You may want to
have something to catch the excess so it doesn't get on you car.

You could easily test the sound deadening properties on a small length
of pipe before filling your car rack. But I have seen it used to stop
noise on metal stairs and around drain pipes. Should work fine on the
inside of pipes.


--
Gordon Niessen
If you aren't on the bleeding edge, you are history.