  For most acoustic metamaterials, once they have been fabricated, their
operating frequencies and functions cannot be adjusted, which is an intrinsic
barrier for development of realistic applications. The study to overcome this
limit has become a significant issue in acoustic metamaterial engineering.
Although with the advance of metamaterials in the past two decades, a series of
methods such as electric or magnetic control have been proposed, most of them
can only work in the condition of no fluid passage. Some metamaterials with
large transmission losses have been proposed, but the sounds are essentially
reflected rather than absorbed. Here, to overcome this intrinsic difficulty, we
propose a ventilated sound absorber that can be manually tuned in a large range
after being manufactured. During the tuning which is achieved through an
intricately designed slider, high-performance absorption and ventilation are
both ensured. The tunable ventilated sound absorber is demonstrated
experimentally and the effective model of coupled lossy oscillators can be
employed to understand its mechanism. The manually tunable ventilated
metamaterial has the potential application values in various complicated pipe
systems that require frequency adjustment and it also establishes the
foundation for future development of active tunable ventilated acoustic
metamaterials.
