  By using a structured tungsten-polyurethane composite that is
impedance-matched to water while simultaneously having a much slower
longitudinal sound speed, we have theoretically designed, and experimentally
realized, an underwater acoustic absorber exhibiting high absorption from 4 to
20 kHz, measured in a 5.6m times 3.6m water pool with the time-domain approach.
The broadband functionality is achieved by optimally engineering the
distribution of the Fabry-Perot resonances, based on an integration scheme, to
attain impedance matching over a broad frequency range. The average thickness
of the integrated absorber, 8.9 mm, is in the deep subwavelength regime
(~{\lambda}/42 at 4 kHz) and close to the causal minimum thickness of 8.2 mm
that is evaluated from the simulated absorption spectrum. The structured
composite represents a new type of acoustic metamaterials that has high
acoustic energy density and promises broad underwater applications.
