[jajjguy]

If you fill the sealed airspace behind a panel with fiberglass or other absorbing material, you "damp" the resonance of the system, which means you widen the effective frequency range, possibly as wide as one octave. So if you target 70Hz, you could damp the resonance and achieve absorption in the range of say 50 to 100Hz.

Of course Ethan is right that a resonant chamber acts on a narrower range of frequencies, but this isn't necessarily a bad thing. If your aim is to absorb certain bass freqs, you may prefer not to absorb high freqs at the same time. Not everyone likes the "dead" sound that can create.