Absorbers treating one frequency band also lower reverberation time at other bands

Evidence

However, the overall reverberation time has gone down, especially at the lowest frequencies, because of the effect of the wood paneling at frequencies other than the one being concentrated on. Thus in practice, an iterative approach Which single material could be added to the room in Example 6. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1903