  We demonstrate a ventilated metamaterial absorber operating at low frequency
(< 500 Hz).With only two layers of the absorption units, high-efficiency
absorption (> 90%) has been achieved in both simulations and experiments. This
high-efficiency absorption under ventilation condition is originated from the
weak coupling of the two identical split tube resonators constituting the
absorber, which leads to the hybridization of the degenerate eigenmodes and
breaks the absorption upper limit of 50% for conventional transmissive
symmetric acoustic absorbers. The absorber can also be extended to an array and
work in free space. The absorber should have potential applications in acoustic
engineering where both noise reduction and ventilation are required.
