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The ferrite cores of transformers and inductors are commonly excited under dc bias conditions in power electronic circuitry. Results have previously been presented showing core losses increase significantly with dc bias, along with various explanations for the cause. This paper presents measurements showing a strong correlation between core losses, and the magnetostrictive vibration caused by the presence of a dc bias. The measurements are made on power grade material ferrite toroidal cores at frequencies equal to, and distant from magnetomechanical resonance. Based on this evidence it is proposed that a mechanism causing core losses to increase with dc bias is the increase in vibration with dc bias, with losses being dependent on the amplitude and nature of the vibration waveform. The means by which core losses increase with vibration are also described.