Resonant panel absorber formulas lose accuracy for thick panels because stiffness is ignored

Evidence

However, this equation must be applied with some caution because it assumes that the panel has no stiffness. This assumption is valid for thin panels but becomes less applicable as the panel becomes thicker and thus stiffer. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2058

Related

• Low diaphragm mass does not improve transient response in panels (similarity 0.78)