By Blake Marston, founder, SCRT (New Plymouth)
Someone asked on our Facebook page: "Why not use a biscuit tin?" To be fair, it's a good question. A sealed metal tin is a rough Faraday cage, and the physics behind it is real. So here's the short answer. Shielding works by putting a closed, conductive layer around a device, and a tin can do that up to a point. Where tins usually let you down is the lid, and the fact that you can't carry one in your pocket.
That's really what this post is about. EMF protection gets talked about in all sorts of ways online, and a fair bit of it is overcooked. I'd rather tell you what shielding does, what we measured, how to check it yourself, and where it stops being useful.
How it works
Your phone, car key fob, bank card and laptop all talk to the world using radio waves. Cell towers, Wi-Fi routers, Bluetooth earbuds and contactless readers are all part of the same conversation, just on different frequencies.
A Faraday shield is a continuous layer of conductive material. When a radio wave hits it, the material reflects and absorbs most of the energy, so very little gets through to the other side. That works in both directions. Signals from outside can't reach the device, and the device's own signals can't get out.
The catch is in the word continuous. Any gap, seam or opening is a way in, and in general the bigger the gap compared to the wavelength, the more leaks through. This is why a biscuit tin is hit and miss. The body is fine, but the lid is usually a loose push fit, often with paint or a lacquer coating between the metal surfaces, so the two halves aren't properly joined electrically. A well-made shielding bag has the same problem to solve, which is why the closure matters more than anything else on it.
SCRT bags and sleeves are designed and made in New Zealand, lined with three layers of our EliteBlock fabric, with either a magnetic or a fold-over closure so the opening is sealed as well as the sides.
What we measured
I didn't want people to take our word for it, so in September 2025 we sent EliteBlock fabric to RF Test Solutions in Wellington. They're an independent laboratory accredited by IANZ (to ISO/IEC 17025), and they tested it using the IEEE 299 method for measuring shielding effectiveness.
Here's what they measured, for a single layer of fabric:
| Frequency | Horizontal | Vertical |
|---|---|---|
| 1 GHz | 75.1 dB | 77.9 dB |
| 2.45 GHz (Wi-Fi, Bluetooth) | 64.1 dB | 69.3 dB |
| 5 GHz (Wi-Fi) | 41.9 dB | 48.1 dB |
| 10 GHz | not reported | 55.8 dB |
Decibels are a log scale, so the numbers are bigger than they look. Every 10 dB is a tenfold drop in power.
- 40 dB means the signal getting through has 1/10,000th of the power. That's a 99.99% reduction.
- 41.9 dB, our lowest result, is roughly 1/15,000th of the power, about a 99.994% reduction.
- 77.9 dB, our highest, is roughly 1/60,000,000th, about a 99.999998% reduction.
Three honest notes. First, some overseas brands advertise 80 to 100 dB or more. Our results ranged from 41.9 to 77.9 dB, and I'm not going to round them up to sound better. In practice, 40 dB is generally treated as the point where shielding becomes effective for this kind of use, and every point the lab tested came in above it. Second, the lab tested a single layer of fabric. Our products use three layers, so we'd expect them to do better, but these figures are for one layer. The same report also includes a separate test on the actual three-layer construction, done in June 2025 by the Beijing Institute of Radio Metrology and Measurement (a CNAS-accredited lab, CNAS L1665) using the GJB 8820-2015 shielding room aperture method. Three layers measured between 50.7 dB (at 8 GHz) and 84.1 dB (at 40 GHz) across 13 frequencies from 1 to 40 GHz. That's a different lab, standard and method from the Wellington test, so the two sets of numbers shouldn't be compared directly. Third, a properly closed bag blocks cellular, NFC and key fob signals too, and the quickest way to prove it is the call test, card tap test and door test below, because they check the whole bag against real signals. Real-world results also depend on how well you close the bag.
You can >download the full lab test report and read it for yourself.
Try it yourself
You don't need a lab to check whether a shield is doing its job. The call test takes about a minute.
- Put your phone in the bag (or the biscuit tin, if you want to settle the Facebook argument).
- Close it properly. On a fold-over bag, fold the top right over and fasten it. On a magnetic closure, check it has snapped shut along its full length.
- Get a friend to ring you, or ring yourself from another phone.
- If it goes straight to voicemail, the shield is working. If it rings, something is leaking.
- Now open the closure a few centimetres and ring again. It'll usually connect straight away, which shows you how much the seal matters.
You can do the same thing with a Wi-Fi connection, a Bluetooth speaker, or a car key fob (stand next to the car with the fob inside the pouch and try the door). For a contactless bank card, put it in the bag, close it, and tap the bag on a payment terminal or your phone's card-reading app. A working bag won't let it read. If you do try the tin, I'd be interested to hear how you got on.
Where people actually use it
Shielding is a practical tool for a handful of everyday problems:
- Car theft. Keyless entry cars can be opened with a relay attack, where thieves boost the signal from your key fob inside the house. A fob in a shielded pouch can't be reached. We've written about how NZ car thieves steal keyless cars in 30 seconds, and the Key Fob Faraday Bag is the fix.
- Privacy and location. A phone in a Small Faraday Bag for Phones stops pinging towers or sharing its location until you take it out. The For Phones collection has the other sizes and a waterproof option.
- Focus and family time. A Digital Detox Box holds up to six phones and key fobs, which is handy for dinner, meetings, or getting the kids off their screens at bedtime.
- Clothing. Some people want a layer they can wear. Our EMF Blocking Beanie and the rest of the Personal EMF Protection range use shielding fabric too. The lab figures above are for the EliteBlock bag fabric, not the clothing fabrics, so I won't apply them to clothes. And remember a beanie is open at the bottom, so it isn't a sealed enclosure the way a bag is.
If you want the full lineup, start with our page on EMF protection in NZ.
What it doesn't do
This is the bit a lot of brands leave out, so I'll be plain about it.
- It doesn't stop your microphone or camera. A phone in a bag can't send anything out while it's sealed, but it can still record audio. If it reconnects when you take it out, anything stored can be sent then. If that matters to you, switch the phone off as well.
- It doesn't block magnetic fields or the low-frequency fields from household wiring, power lines and appliances. Those are a different kind of field altogether. Thin fabric shielding is designed for radio frequencies, and it does very little to the extremely low frequency (ELF) fields that wiring produces.
- It only works while it's closed. An open bag is just a bag.
- It isn't a health product. I'm not going to make health claims about it. In New Zealand, Health NZ and the Ministry of Health set and explain the radiofrequency exposure standard, and you can read their information directly in the sources below. What I can tell you is what shielding does to a signal. Whether you want that for privacy, security or a quiet evening is your call.
FAQ
Does a biscuit tin work as a Faraday cage? Sometimes, partly. A sealed metal tin can block or weaken signals, but lids often leave gaps or have coatings that stop the metal touching properly. The only way to know is to do the call test. A tin is also hard to carry around and doesn't fit a key fob in your pocket.
What does 40 dB of shielding mean? It means the signal getting through has 1/10,000th of its original power, a 99.99% reduction. A single layer of our EliteBlock fabric measured between 41.9 dB and 77.9 dB at the frequencies the lab tested (1, 2.45, 5 and 10 GHz). A separate test of the three-layer construction, by the Beijing Institute of Radio Metrology and Measurement, measured 50.7 dB to 84.1 dB from 1 to 40 GHz. A properly closed bag also blocks cellular, NFC and key fob signals, which you can check yourself with a call test, a card tap test or by trying the car door with the fob inside.
Can my phone still record audio inside a Faraday bag? Yes. The bag blocks signals going in and out, but it doesn't turn off the microphone or stop apps running. Switch the phone off if that's a concern.
Does EMF shielding block the fields from power lines or house wiring? No. Fabric shielding is designed for radio frequencies. It doesn't meaningfully block magnetic fields or the extremely low frequency fields from wiring and power lines.
Is it legal to use a Faraday bag in New Zealand? Owning and using a bag that blocks signals to your own devices is a passive product, not a jammer. Signal jammers, which actively transmit to interfere with other people's devices, are a different thing.
Agency or business order?
If you're buying for a team, a fleet or an agency, see our police, defence and government orders page, or get in touch and I'll help you sort the right gear.
Any questions, drop us a line. Everything is designed and made here in New Zealand, and we ship from New Plymouth.
Sources
- RF Test Solutions Ltd, >SCRT EliteBlock Faraday Shielding Test Report, September 2025
- Health New Zealand, >Cellsites and 5G
- Health New Zealand, >Power lines
- Radio Spectrum Management NZ, >Preparing for 5G in New Zealand