An enclosure with a substantial rectangular configuration, adapted to contain an apparatus sensitive to acoustic vibrations, the enclosure comprising walls and acoustic damping material located within the wall, wherein the acoustic damping material comprises at least one absorbing body of acoustic energy absorbing material located adjacent to a rib of the enclosure.The acoustic vibrations most disturbing the processes in the apparatus within the enclosure are caused by standing acoustic waves within the enclosure with frequencies in the range between 50 Hz and 1000 Hz. These acoustic waves are efficiently damped by the provision of a block of acoustic absorbing material adjacent to one of the ribs of the enclosure, to such an extent that the need for thick walls of the enclosure is substantially obviated, leading to a less voluminous enclosure.