operation the interior area of diaphragm 72 surrounded by rib 73 is uniformly thick in cross section and sized to tautly cover opening 58 when diaphragm 72 is secured to bottom panel 57 when properly secured diaphragm 72 completely covers opening 58 to provide an airtight seal so as to not allow any products of combustion to escape through opening 58 the tautness of the instant diaphragm is believed to provide better sound and vibration transmitting properties than a diaphragm which is loosely disposed or in other words disposed with excess folds or flaps of materials over an opening rib 73 is circular in cross section and centered within the thickness or depth of diaphragm 72 a pair of opposing frustoconical projections 75 which positively locate diaphragm 72 in relation to plate 78 are disposed on the side of diaphragm 72 facing plate 78 referring now to fig5 and 6 pressure port plate 78 is designed to substantially cover and thereby protect diaphragm 72 which vibrates inward and outward during furnace operation plate 78 prevents a person from contacting and accidentally tearing or puncturing diaphragm 72 which may allow combustion gases to escape into the furnace room plate 78 also prevents an object from being placed directly against or too close to diaphragm 72 which could hinder vibratory motion of the diaphragm and thereby frustrate its sound transmitting function plate 78 is.