Buzzing Magnetic Components? That's Magnetostriction

Magnetics are often driven with AC signals as this is how you would access inductance or coupling in these components. Driving with a typical AC sine wave, sawtooth wave, or square wave signal could produce an annoying whining or buzzing sound. You might notice that the pitch and volume change as the frequency and voltage/current change while the component is driven. Eventually, you might notice the buzzing sound cease and the device returns to being quiet.

This electro-acoustic phenomenon is caused by magnetostriction. When ferromagnetic materials are exposed to alternating magnetic fields at certain frequencies, it is possible to hear audible noises due to magnetostriction, something which may not be desirable in many products. This is an innate property of ferromagnetic components, particularly components built with ceramic ferrites and ferrite alloys. Let's look at what causes magnet restriction and what can be done to reduce or eliminate it.

What Causes Magnetostriction?

Magnetostriction is a property inherent to all ferromagnetic materials, and it varies greatly in terms of noticeable frequency range and magnitude. It is related to the crystal structure and allowed electronic states in the material, and it refers to creation of alternating strain when the material is exposed to an alternating magnetic field.

As was discussed above, some components exhibit magnetostriction that can be heard as a buzzing or vibrating sound in the audible frequency range. The vibration excited by an AC signal can have a broad spectrum and is not exactly the same as the power spectrum of the driving signal, primarily due to the nonlinear mechanical properties of the material experiencing magnetostriction.

Certain components are well known producers of magnetostriction, such as:

• Lead-based ceramic ferrites

• Sarah magnetic alloys

• Heavy rare earth metals

Metals which are known to exhibit magnetostriction are iron, nickel, and cobalt. One highly magnetostrictive material is known as Terfanol-D, which is an alloy of iron and some heavy rare earths.

In terms of inductive components in electronics, magnetostriction is often observed in ceramic ferrite cores used in inductors and transformers. This is commonly observed in PWM or square wave driving in the kHz range, where the fast edge rate of a square wave can excite a broad range of frequencies.

These inductors are part of a group of switching regulator circuits, which may produce audible noise due to magnetostriction.

When magnetostriction is acting on a component, it causes the component to expand and contract as the spatial distribution of magnetization changes around the component. This expansion and contraction occurs even along the surface of the magnetic material, which then vibrates the air around the component. When the vibration is at the right frequency, it can be heard as an audible noise. This is the same reason that large transformers in utility stations often emanate a low frequency buzzing sound that can be heard easily.

Transformers in utility installations produce audible humming due to magnetostriction.

At best, magnetostriction is a minor annoyance and will not violate the performance parameters of your design. At worst, it is loud enough that it degrades the user experience and some step needs to be taken to reduce or eliminate it.

Reducing Magnetostriction

Unfortunately, you can never totally eliminate magnetostriction, it will always occur in ferromagnetic materials, even at extremely minor levels that may not be audible to the human ear. However, magnetostriction might be significant in terms of strain, but your system could operate in a frequency range or power handling level such that restriction is no longer audible. Some of the design options available include:

Change the Operating Frequency

This is the simplest approach in power systems with magnetics is to change the operating frequency of the driving signal which excites the magnetic component. For example, the operating frequency could be increased to move the audible noise up to a higher pitch and, hopefully, outside the audible range.

Unfortunately, it is not so simple as just increasing or decreasing the frequency and magnetostriction no longer produces noise. Due to nonlinear structural deformation in different frequency ranges, subharmonic and superharmonic excitation are possible, and magnetostriction could produce audible noise even if driven well above the audible range.

Change the Magnetic Field Strength

Another option is to change the magnetic field strength while keeping the frequency constant. This could change the volume and frequency content of the emitted sound. A change in field strength can translate into a change in frequency due to subharmonic and superharmonic excitation. At some point, you may bear the buzzing sound disappear as the excited frequency content moves outside the audible range.

Sound Dampening Materials

The enclosure can be built or lined with materials that dampen sound, such as cast polyurethane sheets. These materials can help reduce noise, but they might also alter the thermal properties of the enclosure. If heat is a problem, adding an absorbing material might trap heat in the enclosure.

Any method you want to take to reduce noise from magnetostriction requires the best PCB design features in OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset and additional simulation options for systems analysis. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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