  We experimentally and analytically report broadband and narrowband perfect
absorption in two different acoustic waveguide-resonator geometries by the
mechanism of critical coupling. In the first geometry the resonator (a
Helmholtz resonator) is side-loaded to the waveguide and it has a moderate
quality factor. In the second geometry the resonator (a viscoelastic porous
plate) is in-line loaded and it contains two resonant modes with low quality
factor. The interplay between the energy leakage of the resonant modes into the
waveguide and the inherent losses of the system reveals a perfect and a
broadband nearly perfect absorption. The results shown in this work can
motivate relevant research for the design of broadband perfect absorbers in
other domains of wave physics.
