Transformer audible noise is a mechanical vibration phenomenon with electromagnetic origins. The handbook systematically categorizes the causes:

  1. Core-Related Noise (The Fundamental Source):

    • Magnetostriction: As detailed in Article 9, the alternating magnetization causes the silicon steel laminations to expand and contract at twice the power line frequency (100/120 Hz hum). This is the primary and unavoidable source of noise, its magnitude dependent on material, stress, and flux density.

    • Magnetic Forces on Laminations: The alternating magnetic field also creates attractive and repulsive forces between adjacent laminations. If the core is not tightly clamped, these forces can cause them to vibrate against each other, producing a buzzing sound.

  2. Winding-Related Noise:

    • Electromagnetic Forces: Load currents in the windings, especially under short-circuit conditions, create strong alternating electromagnetic forces between conductors. These forces can cause the windings to vibrate at the power frequency (50/60 Hz) and its harmonics. Poorly secured or impregnated windings amplify this.

    • Loose Windings: Windings that are not wound tightly or have become loose over time can vibrate within the winding window or against the core.

  3. Assembly & Mounting Noise:

    • Loose Core Clamping: For laminated cores, if the clamping bolts, frames, or brackets are not sufficiently tight, the entire core stack can vibrate.

    • Resonant Mounting: The transformer mounted on a chassis or panel can induce vibrations in the mounting surface itself. If the mechanical natural frequency of the transformer-system is close to the excitation frequency (100/120 Hz), mechanical resonance occurs, dramatically amplifying the noise.

    • Loose Components: Any loose part, like a shielding band or a terminal, can be set into vibration.

Handbook’s Analysis: The noise is always electromagnetically excited but mechanically manifested. The core’s magnetostriction provides the base excitation. The severity is then determined by: 1) the amplitude of the excitation (material, $B_m$, stress), and 2) the mechanical response of the structure (tightness, rigidity, damping, resonance).