A loudspeaker system with an enclosure, an electro-acoustic transducer mounted in the enclosure so as to leave space inside of the enclosure that is unoccupied by the transducer, and an air-adsorbing material in the space inside of the enclosure that is unoccupied by the transducer. The air-adsorbing material includes a silicon-based zeolite with a small amount of a second metal. The mole ratio of silicon to the second metal is at least about 200 and is less than 400.