Acoustic metaliners for sound insulation in a duct with little flow resistance

We propose a metaliner that can insulate the duct noise for various grazing flow speeds with little flow resistance. The metaliner, whose unit cell consists of two different Helmholtz resonators with subwavelength scales, is placed underneath the duct. In order to predict sound insulation and absorption of metaliner in a duct with flow, an effective impedance model of the metaliner is established by considering the effect of flow. We present a design procedure for a metaliner with high transmission loss for various flow speeds. Experiments show that designed metaliners exhibit transmission loss above 45 dB/m within ±5% of the target frequencies of 500 and 1000 Hz for different flow speeds of 0, 17, and 34 m/s. We investigate the effects of flow on sound absorption as well as sound insulation for probing the applicability of the metaliner to various environments where noise should be suppressed at both the upstream and downstream sides in the ducts with flow.