A flexible passive acoustic driver for use in an acoustic driver system which applies an oscillating stress to a subject undergoing a magnetic resonance elastography (MRE) examination which includes receiving acoustic pressure waves from an active driver through a tube and imparts pressure waves to a subject of an imaging procedure. In one configuration, the passive driver includes a flexible bag that forms the walls of an acoustic cavity, and a structure filling material located inside the acoustic cavity provides support for the flexible bag. The flexible bag conforms to the shape of the subject and may be held in place by an elastic band. The passive driver can have an integrated or detachable non-active push-on compartment which is rigid or semi-flexible to improve the human-driver mechanical coupling and the driver energy efficiency of converting acoustic pressure to mechanical vibration applied to a subject.