A moving ribbon microphone is a velocity microphone in which a ribbon-like diaphragm (ribbon foil) having backplates attached to its opposite ends is placed in a magnetic gap provided by a pair of permanent magnets. Before the advent of a moving coil microphone, Siemens & Halska in Germany developed the first moving ribbon microphone in about 1933 (see Non-Patent Document 1; “50 Years of Audio History” issued by Japan Audio Society on Dec. 4, 1986 (pp. 180 to 187).
In the moving ribbon microphone (simply referred to as a ribbon microphone), the diaphragm is generally composed of an aluminum foil. This is because aluminum is more suitable for the ribbon microphone than other metals in terms of conductivity and specific gravity.
Further, in order to reduce the resonance frequency of the diaphragm, conventional techniques use a molding apparatus such as a gear to fold the ribbon foil into a folding screen form (zigzag form). Also in this respect, aluminum, which is ductile, is suitable.
On the other hand, aluminum offers a very low conductor resistance (0.1 to 0.2 Ω). Accordingly, a diaphragm made of an aluminum foil provides a low signal level. Thus, in a ribbon microphone having a diaphragm consisting of an aluminum foil, generally, the diaphragm is connected to a primary side of a booster transformer to obtain a high signal level from a secondary side (see Non-Patent Document 2; JOURNAL OF THE AUDIO ENGINEERING SOCIETY (Vol. 1 to 27) p. 204).
As described above, the aluminum used in the diaphragm is preferably a pure aluminum material, which contains no other metals, in connection with ductility. However, the pure aluminum material is prone to couple to moisture in air to form an aluminum hydroxide film on its surface. The pure aluminum material thus offers only a low corrosion resistance and is disadvantageously susceptible to secular variations.
Further, a backplate supporting the diaphragm is generally composed of a copper alloy such as brass which is very conductive. However, this results in a bimetallic junction with aluminum, which may lead to electrolytic corrosion. That is, since the diaphragm is connected to the primary side of the booster transformer via the backplate, when this part has an increased resistance value, problems may occur such as the occurrence of noise, a decrease in sensitivity, and a rise in impedance.