Broadband and broadangle extraordinary acoustic transmission through subwavelength apertures surrounded by fluids

We present a mechanism for ultra-broadband transmission of acoustic waves through subwavelength hole arrays. Different fluids surrounding (or filling) the holes are considered, which allows tuning of the band of maximum transmission to different angles of incidence. For certain configurations, this band of total transmission may appear at very small incident angles, making the system 'invisible' to sound at almost normal incidence. Analytical expressions for the specific incident angle, and for maximum transmission at that angle, are provided for any fluid-system configuration. (paper)