  The advent of acoustic metamaterials opened up a new frontier in the control
of sound transmission. A key limitation, however, is that an acoustic
metamaterial based on a single local resonator in the unit cell produces a
restricted narrow-band attenuation peak; and when multiple local resonators are
used the emerging attenuation peaks -- while numerous -- are each still narrow
and separated by pass bands. Here, we present a new acoustic metamaterial
concept that yields a sound transmission loss through two antiresonances -- in
a single band gap -- that are fully coupled and hence provide a broadband
attenuation range; this is in addition to delivering an isolation intensity
that exceeds 90 decibels for both peaks. The underlying coupled resonance
mechanism is realized in the form of a single-panel, single-material pillared
plate structure with internal contiguous holes$-$a practical configuration that
lends itself to design adjustments and optimization for a frequency range of
interest, down to sub-kilohertz, and to mass fabrication.
