member comprising a hollow cylindrical member 29 is placed against the lower surface of the plate 22 surrounding the opening 23 and a suction line 31 applies suction until openings aligned with the openings 13 are formed in the film 10 due to the suction the broken edges of the film 10 become attached to the inner surfaces of the holes 13 of the plate 11 because the plate 11 is still heated and a bond will occur in the next step of the process the film 10 is subjected to a voltage so as to form electrets fig6 illustrates one method wherein a d c voltage from a battery e which might be in the range between 7 k 10 k volts is applied between a needle electrode 33 and the backplate 11 so as to cause a corona discharge therebetween the end of the needle electrode 33 might be spaced from the backplate 9 10 mm the resulting electret film 10 has a charge density of 10 4 q m 2 which is satisfactory for use as an electret transducer fig7 and 9 illustrate another method for forming a backplate with a synthetic resin film attached thereto in fig7 a metal plate 36 is heated to.