Ultra-broadband symmetrical acoustic coherent perfect absorbers designed by the causality principle

We report a methodology to reach the symmetrical broadband coherent perfect absorption in an air tube whose reliability and feasibility are demonstrated experimentally. Through rigorous analytical calculations, we discover that the causality principle provides a minimal volume requirement based on absorption capability, while the idealistic coherent symmetrical perfect absorption cannot be achieved but can be approached within a finite volume. As a demonstration, a side-loaded near-perfect absorber is designed and realized working at a broad bandwidth from 300 to 5000 Hz without blocking the tube. The size of the absorber is also super-subwavelength, only 1% of the longest working wavelength. By experimentally measuring the absorption in both frequency and time domains, broadband and tunable near-perfect coherent absorption properties are demonstrated. In addition, we achieve the manipulation of sound waves in a ventilated tube by using sound pulses.