An acoustic device with an enclosure and a first passive radiator structure which includes a first passive radiator diaphragm. The first passive radiator structure has an effective radiating area and a first mass, and is mounted to the enclosure such that its diaphragm can vibrate relative to the enclosure. There is a second passive radiator structure which includes a second passive radiator diaphragm. The second passive radiator structure has substantially the same effective radiating area as the first passive radiator structure, and is mounted to the enclosure such that its diaphragm can vibrate relative to the enclosure. At least one active electro-acoustic transducer is mounted to the second passive radiator structure such that it moves when the diaphragm vibrates. The second passive radiator structure and the active transducer together have a second mass that is substantially greater than the mass of the first passive radiator structure. Passive radiators with the same effective radiating area results in force balancing of the device.