Perforated steel sheets that are used for loudspeaker grille material directly in front of the speaker or to cover loudspeakers in built constructions (video-conference furniture etc.), have to have a minimum percentage of open area. This open area is necessary to give space to the neccessary movement of air. Especially high frequency sound waves need the open path to travel.
Typical perforated metal sheet types are the straight or staggered round holes, the straight or staggered square holes, round end slots and hexagons.
The open area of the perforated grille material should be at least 60% to 65% with a hole diameter of 3 to 8 mm.
Public Address ceiling speakers work fine with an open area of about 40%.