and the face 101 is mounted a gasket 17 of thin rubber or the like by which the structure is given resilience and by which vibration between the different components is damped various different types of panels are provided as will be explained hereinafter but each is mounted in the manner shown in most detail in fig3 a basic panel indicated at 18 comprises simply a plain sheet which closes the area between the frame elements a second type of panel indicated at 18 includes an access door the access door is square and only slightly smaller than the basic panel with outside flanges 19 see fig4 for cooperating with inner flanges 20 of the access door panel 21 the access door includes a handle 22 which can be actuated to lock or release the access door and pivots on pins 23 arranged at the bottom of the door for cooperation with side edges of the access door panel portion 21 a third type of panel is indicated at 24 in fig1 and 2 and comprises an inlet panel having flanges for attachment to a square feed duct particularly of clean outside air thus the panel 24 comprises the flange 15 as previously explained a narrow surrounding strip portion 25 at.