  We present the design of a structured material supporting complete absorption
of sound with a broadband response and functional for any direction of incident
radiation. The structure which is fabricated out of porous lamellas is arranged
into a low-density crystal and backed by a reflecting support. Experimental
measurements show that strong all-angle sound absorption with almost zero
reflectance takes place for a frequency range exceeding two octaves. We
demonstrate that lowering the crystal filling fraction increases the wave
interaction time and is responsible for the enhancement of intrinsic material
dissipation, making the system more absorptive with less material.
