Wave-based active control for nonreciprocal sound transmission and absorption

Recent interest has arisen in the realisation of nonreciprocal acoustic devices that achieve unidirectional sound transmission. In other words, these devices allow perfect transmission from waves travelling in one direction, whilst blocking waves travelling in the opposite direction. These devices have thus been described as the acoustic analogue of an electrical diode and could be useful in many applications. There are a variety of passive and active devices that have been designed based on the concept of acoustic metamaterials to achieve unidirectional transmission by introducing non-linearities or breaking spatial and temporal symmetries. This paper will investigate how unidirectional transmission can be achieved using active control within a one-dimensional duct. The proposed active control system has the capability to manipulate either the positive or negative travelling waves within the duct in order to achieve zero transmission in one direction, whilst allowing perfect transmission in the opposite direction. The advantage of this system is not only its capability to achieve nonreciprocal sound transmission, but it is also fully adaptable