Cardboard is not a good speaker baffle material. Cardboard is very noisy in and of itself. Just putting my mouth near a piece of cardboard and humming lowly makes the cardboard vibrate like crazy, adding all kinds of resonances.
Just a side note, I was trying to replicate an experiment where you stretch a balloon tight over the top of a glass to make a trampoline surface that a ball will bounce on for a long time. That didn’t work out very well. But what it did do well was add a lot of reverb to my voice if I talked in to it. The kids were highly amused. So stretched balloons also do not make good speaker baffles.
Maybe something more dead, like a piece of carpet tile or mass loaded vinyl would be better for the experiment.