why do magnets make noise

Q: why do magnets make noise

Magnets make noise primarily due to magnetostriction, where magnetic materials physically vibrate when subjected to a changing magnetic field. This is especially pronounced in devices like transformers, where alternating currents cause rapid, audible vibrations in the iron core.

The Deep Dive

The core phenomenon is magnetostriction, a property of ferromagnetic materials like iron, nickel, and cobalt. When placed in a magnetic field, the microscopic magnetic domains within the material align, causing a minute but instantaneous change in the material's physical dimensions. If the magnetic field is static, this change is a one-time shift. However, in most technological applications, the field is dynamic, typically from an alternating current (AC) in an electromagnet. This causes the domains to rapidly realign and the material to expand and contract at the frequency of the current (often 50 or 60 Hz). These rapid mechanical oscillations create pressure waves in the surrounding air, which we perceive as a low hum or buzz. The effect is most audible in large, laminated iron cores found in electrical transformers, inductors, and some types of speakers, where the entire structure can vibrate sympathetically.

Why It Matters

Understanding magnetically induced noise is crucial for engineering quieter electrical infrastructure, from household appliances to massive power substations. It guides the design of materials and core geometries in transformers and motors to minimize energy loss as sound and vibration, improving efficiency. In medicine, it informs the design of quieter MRI machines, enhancing patient comfort. This knowledge also helps in diagnosing failing electrical equipment, as a change in the characteristic hum can indicate loose windings or degraded core laminations, preventing potential failures.

Common Misconceptions

A common myth is that all magnets inherently hum, but a permanent magnet in a static state is silent; the noise requires a changing field. Another misconception is that the sound comes from the magnet "struggling" or being defective. In reality, the hum is a normal, albeit sometimes inefficient, byproduct of electromagnetic energy conversion in AC systems. The noise level is a design consideration, not necessarily a sign of malfunction, though excessive or unusual noise can indeed indicate a problem like loose components or core saturation.

Fun Facts

The 'mains hum' you hear from power lines and some electronics is literally the sound of the alternating current grid vibrating at either 50 or 60 times per second, depending on your country.
Some researchers have explored using magnetostrictive materials to create silent, solid-state cooling systems by exploiting the heat changes that occur when the material is magnetized and demagnetized.