sentences:  Resonating absorbers can be broadened in the frequency range of efficacy to some degree by either introducing porous absorptive material to the interior of the vessel, by constraining the vibrations of the panel or membrane, or by installing an array of resonating devices each tuned to adjacent frequency ranges so that collectively the array functions over a broadened range of sounds.Such devices can be enormously effective over their tuned range, but can take up a great deal of space, especially when installed in arrays, and thus are sometimes not a practical solution.A simple panel resonator can be built to hang on a wall by building a wooden frame, adding several inches of mineral wool to the inside, and fitting a sheet of plywood over the top attached only at the edges.