Looking at the data again I think I've actually missed the first resonance mode, which is at about 35Hz on both beams. I just assumed it was noise at first. It is much lower amplitude than the second resonance modes around 138Hz.

Unfortunately, my kit is not really designed for very low frequency data collection so there is not a lot of resolution down at 35Hz to get a nice waterfall plot of decay.

I've already cast concrete in the beams so I can't go back and run more tests but in the future I will try to run multiple measurements and average the data to get more resolution.

Thin Tube, hollow. Note I have adjusted the vertical axis from previous plots.
Attachment 444044

Thick Tube, hollow.
Attachment 444046

I think it is important that we differentiate between 'damping' and 'loss'. If we look only at the amplitude response, added mass seems to damp the resonances, but this is only spreading the energy through time as we see in the waterfall plots. I'm not sure this is actually an increased 'loss factor'? When I think of damping what comes to mind is lossy damping that converts kinetic energy to heat, such as visco-elastic material properties. I believe this will provide a damped amplitude response as well as a faster energy decay, that would be quite desirable.

Pete, I'll have a read of those papers. Quite a lot to take in there.