A compressor for compressing a gas. One embodiment takes the form of a method that involves a system detecting a triggering event, and responsively identifying a current access-channel occupancy. Alternatively, the acoustically coupled IDT’s may include a series IDT to which the phase rotating element is connected and which is connected in series between the input and output terminals, and a branch IDT acoustically coupled to the series IDT, and disposed on a transmission path which branches from a transmission path between the input and output terminals to a reference potential. This method lays a fundament for automatic identification of harmful objects, such as explosive, drugs, etc. The straddling member are configured to correspond to the surrounding soft tissue.