EMF frequency guide: what's around you, and what our products block

Most of the technical questions we get come down to one thing: "Does it block X?" Sometimes X is 5G, sometimes it's the smart meter on the side of the house, sometimes it's an AirTag. The honest answer depends on the frequency involved, so we put this guide together to cover all of them in one place.

First, what "frequency" actually means

Every electromagnetic field has a frequency, which is how many times per second it vibrates. It's measured in hertz (Hz). A kilohertz (kHz) is a thousand vibrations a second, a megahertz (MHz) is a million and a gigahertz (GHz) is a billion.

Frequency and wavelength go hand in hand. The faster a wave vibrates, the shorter it is. The mains electricity in a US home runs at 60 Hz and has a wavelength of about 5,000 km. A Wi-Fi signal at 2.4 GHz has a wavelength of about 12.5 cm. That difference is exactly why the two behave so differently when they meet a piece of shielding fabric, which we'll get to below.

Everything in this guide is non-ionizing. That means it doesn't carry enough energy to knock electrons off atoms, unlike X-rays or UV light further up the spectrum. When people talk about "EMF" from electronics, this is the part of the spectrum they mean.

The frequency chart

Here's where common devices sit, from lowest frequency to highest. We've included the wavelength because it's the thing that decides how easy a signal is to shield.

Exact bands vary by country and carrier, but these ranges cover what you'll run into day to day in the US and Europe.

Two very different kinds of EMF

That table really contains two different things, and it helps to split them up.

Low-frequency fields from electricity

The first row, power lines and wiring, is extremely low frequency. These fields don't travel as waves the way a phone signal does. They stay close to the wire and fade quickly with distance. They have two parts: an electric field and a magnetic field.

Conductive fabric can reduce the electric part, but mainly when it's grounded. The magnetic part goes straight through thin fabric. Blocking low-frequency magnetic fields takes thick sheets of special alloys, which isn't something you can wear or fold into a pouch. The most effective thing you can do about these fields is distance. Moving your bed a meter away from a wall with a lot of wiring behind it does more than any fabric will.

Radio-frequency signals from wireless devices

Everything from contactless cards up to 5G is radio frequency (RF). These signals do travel as waves, and they're what your phone, router, smart meter and car key use to communicate. Metal-lined fabric is very good at reflecting and absorbing RF, which is why shielding fabric works so well here. This is the range our products are made for.

Shielding fabric is built for radio frequencies. For power-line fields, distance works better than any fabric.

What our products are designed to block

All of our shielding products use conductive fabric, with copper, nickel or silver woven into it or coated onto it. What changes from product to product is how completely that fabric surrounds the thing you want to protect, and that makes a big difference.

Faraday bags and pockets

A sealed bag surrounds the device on every side, so it works as a true Faraday enclosure. Everything in the chart from 13.56 MHz upward is cut off, whether it's coming in or going out. That's why a phone in one of our bags drops off the network entirely, and why a key fob inside can't be relayed. The closure is what makes this work, so it has to be fully rolled or folded shut. We explain why in how Faraday bags work.

Low-frequency 125 kHz access cards are a slightly different case. They work by magnetic coupling over a few centimeters, not by radio waves. A multi-layer bag usually stops a reader from picking them up at the distances those readers work over, but it's worth testing with your own card if that's what you're protecting.

Clothing and blankets

The fabric in our hoodies, beanies, underwear and blankets is the same kind of conductive material, and it reflects and absorbs RF the same way. The difference is that clothing can't be sealed. You need a neck hole, sleeves and a way to get it on and off. So instead of cutting signals off, clothing reduces the RF reaching the parts of your body it covers. Your phone still works when it's in your pocket, and it should.

That's also why coverage matters more than anything else with clothing. A hood that's up shields more than one that's down, and a blanket over you shields the side it covers, not the side facing the mattress. If you want a device completely offline, that's a job for a bag, not a garment. Our Faraday bag vs phone case guide goes into the same idea for phones.

Are higher frequencies harder to block?

Not for the fabric itself. A common worry is that 5G, especially millimeter wave, must be "stronger" or harder to stop. In practice the opposite is closer to the truth. Higher-frequency signals are reflected very well by metal and don't travel far through walls, bodies or even leaves.

Where high frequencies do matter is gaps. A signal can slip through an opening that's large compared with its wavelength. A 1 cm millimeter wave can get through a gap that a 69 cm key fob signal can't. The weave of good shielding fabric is microscopic, so it's not the issue. Seams, zippers and closures are. That's why our bags use fold-over closures and why the seams matter as much as the material.

How to check for yourself

For bags and pockets

The simplest test is also the most reliable. Seal your phone in the bag, wait 30 seconds, and call it from another phone. It should go straight to voicemail. To check Wi-Fi and Bluetooth, stream something to a Bluetooth speaker or over Wi-Fi next to your router, then seal the phone in. The audio or video should stop. For key fobs, seal the key in and try the car door.

For clothing and blankets

You can't do a call test with a garment, because the phone is supposed to keep working. An RF meter is the way to go here. Take a reading with the meter out in the open, then another with it wrapped in or covered by the fabric, at the same spot and facing the same way. Look at the difference between the two readings rather than the exact numbers, because consumer meters vary a lot. Also check which range the meter covers. Many cheaper ones stop at around 6 to 8 GHz, so they won't show millimeter-wave 5G at all.