  We report the experimental realization of perfect sound absorption by
sub-wavelength monopole and dipole resonators that exhibit degenerate resonant
frequencies. This is achieved through the destructive interference of two
resonators' transmission responses, while the matching of their averaged
impedances to that of air implies no backscattering, thereby leading to total
absorption. Two examples, both using decorated membrane resonators (DMRs) as
the basic units, are presented. The first is a flat panel comprising a DMR and
a pair of coupled DMRs, while the second one is a ventilated short tube
containing a DMR in conjunction with a sidewall DMR backed by a cavity. In both
examples, near perfect absorption, up to 99.7%, has been observed with the
airborne wavelength up to 1.2 m, which is at least an order of magnitude larger
than the composite absorber. Excellent agreement between theory and experiment
is obtained.
