How Do Honeycomb Vent Filters Work?
The working principle and selection criteria of the honeycomb (waveguide) filter that lets air into a shielded room while keeping signals out.
3 min readOctober 12, 2026
Why Isn't Normal Shielding Enough?
The galvanized steel or copper panels used in shielded rooms stop radio-frequency signals by reflecting and absorbing them. But low-frequency magnetic fields such as 50 Hz pass through these panels with almost no attenuation.
Substations, high-current busbar systems and lift motors produce such fields. Flickering screens in nearby offices and drift in sensitive measuring instruments are typical effects.
How Does Mu-Metal Work?
Mu-metal is a nickel-iron alloy with very high magnetic permeability. It doesn't stop magnetic field lines; it guides them through itself and around the protected area. As a result, the field strength inside drops markedly.
To reduce cost over larger areas, silicon transformer steel is also used. Most applications combine several layers, and the gap between layers increases performance.
Application Examples
The most common application is lining the walls and floors of offices or control rooms next to a transformer room. Cladding around cable trays is also an effective solution.
In scientific applications, multilayer magnetically shielded rooms are built for extremely sensitive instruments such as MEG and electron microscopes. For the practical side, see our article on offices near transformers.
Measure First
Magnetic shielding done without measurement can be needlessly expensive or insufficient. The right material thickness and number of layers can't be decided without measuring the field's source, direction and strength.
In our solutions for the energy sector we map the field first, then propose the most economical cladding to reach the target value. We report the improvement by measuring after installation.
Frequently Asked Questions
What's the difference between mu-metal and aluminum?
Aluminum is a good shield at high frequency but doesn't attenuate low-frequency magnetic fields. Mu-metal is used specifically for low-frequency magnetic fields.
Can mu-metal be bent?
It can, but heat treatment may be needed after forming to preserve its magnetic properties. That's why parts are usually made specifically for the project.
How is the field target set?
The limit given by the manufacturer of the affected equipment, or the relevant health and safety limits, are used as the target.
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