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:

On 22-Mar-2004, "Gordon Niessen" wrote:

The expanding foam would stick to the inside of the tube and become
rigid. This should stop the vibration. And would be much lighter then
sand

Take the foam's Young's modulus and calculate how much steel it is
equivalent to. Then determine how much it increases the stiffness
of the combined tube + foam. It won't do much.

You don't want to make it stiffer. You want to reduce the resonant
frequency, not increase it. That means adding mass. Check any
book on structural dynamics.

Mike