Patent ID: 6904158

Claim:
A speaker apparatus comprising: a primary coil disposed in the vicinity of a gap of a magnetic circuit and to which a current corresponding to an input audio signal is supplied; a secondary coil, disposed in the gap, for inducing a current corresponding to a current that flows in said primary coil; and a vibrating plate vibrated by said secondary coil with an interaction of the current induced by said secondary coil and a magnetic flux in the gap, wherein the following formula is satisfied N ×( R 1 × R 2 ) 1/2 /{2 Π×L 1 ×(1 −k 2 ) 1/2 }≧20000 Hz where R 1 is the DC resistance of said primary coil; L 1 is the inductance of said primary coil; N is the number of turns of said primary coil; R 2 is the DC resistance of said secondary coil; and k is the coupling coefficient of said primary coil and said secondary coil.