Tunable acoustic absorbers with periodical micro-perforations having varying pore shapes Shuwei Ren, Xuewei Liu, Junqing Gong, Yufan Tang, Fengxian Xin, Lixi Huang and Tian Jian Lu EPL, 120 4 (2017) 44001 Published online: 05 February 2018 DOI: 10.1209/0295-5075/120/44001