Reinforcing channels (3) of a channel-section shape steel are fixed in the form of a lattice by welding onto a side plate (2) of a transformer tank so as to surround the tank. an elongated thin steel plate (4) is welded to the outer circumferential edge of a sound insulation panel (5) covering each of the windows formed by the channels. The plate has a predetermined spring constant and is welded at its periphery to the reinforcing channels at the inner circumferential edges of THF window. A weighty body (6) in the form of a rectangular frame is fixedly attached onto the panel in the vicinity of the boundary between the plate and the panel. A dynamic damper is provided of which the natural frequency may be adjusted to equal the vibration frequency of the body so as to cancel the latter's movement.
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