Reference numeral 8 denotes a second insulating support (hereinafter referred to as ???second support???) on which an approximately circular air hole 9 is formed in a center region, and the second support 8 preferably comprises a plastic film such as polyester, polyolefin, polyamide, polyether, polyamide-imide, polystyrene, polycarbonate, poly-??-phenylene sulfide, polyvinylchloride.