Do Faraday bags really block your phone? We had our fabric lab-tested in Wellington - SCRT - It's your choice

Do Faraday bags really block your phone? We had our fabric lab-tested in Wellington

By Blake Marston, founder, SCRT (New Plymouth)

"Do those bags actually work, or is it just a pouch?" It's a fair thing to ask, and I didn't think "trust me" was a good enough answer. So in September 2025 I sent our EliteBlock fabric to RF Test Solutions, an independent lab in Wellington, so you wouldn't have to take my word for it. The short answer is yes: a properly made and properly closed Faraday bag stops a phone reaching the network. Here's what the lab did, what the numbers mean, and how to check any bag yourself.

If you want the bigger picture on what shielding is and isn't for, I covered that in EMF protection in NZ: what shielding actually does, and what it doesn't. This post is the nuts and bolts.

How the lab tested it

RF Test Solutions is accredited by IANZ, New Zealand's accreditation body, to ISO/IEC 17025. That's the international standard for testing and calibration labs. In plain terms, someone independent checks that their equipment is calibrated and that they follow their methods properly.

They used IEEE 299, a standard method for measuring how well a material or enclosure shields against radio signals. The setup goes like this:

  • A transmitting antenna sits on one side of a screened room, and a receiving antenna sits on the other.
  • They measure the signal with nothing in the way, then clamp a sample of fabric into the knife-edge seal of the screened room door and measure again.
  • The difference between the two readings is the shielding effectiveness, in decibels (dB).
  • They measured with the antennas both horizontal and vertical, because radio waves can arrive either way.

One thing I want to be upfront about: the lab tested a single layer of EliteBlock. Every SCRT bag and sleeve uses three layers. We'd expect three layers to do better, but these are single-layer figures and I won't stretch them to cover three. There is a separate three-layer test, which I cover further down.

What we measured

These are the single-layer results from the report:

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

The lowest result was 41.9 dB, so every point they tested came in above 40 dB, which is generally treated as the minimum for effective shielding. The >full lab test report is free to download, with the chart, photos of the setup and the method.

A second test on the three-layer construction

The same report also includes a separate test on the actual three-layer construction we use in every bag. I'm keeping it apart from the Wellington results because it's a different lab, a different standard and a different method.

  • Lab: Beijing Institute of Radio Metrology and Measurement, accredited by CNAS (China's national accreditation body), accreditation number CNAS L1665.
  • When: June 2025.
  • Sample: EliteBlock, three layers.
  • Method: GJB 8820-2015, using a shielding room aperture method.
  • Frequencies: 13 points from 1 GHz to 40 GHz.
Frequency Shielding effectiveness
1 GHz 69.0 dB
3 GHz 68.1 dB
5 GHz 72.3 dB
8 GHz 50.7 dB
10 GHz 56.5 dB
13 GHz 70.1 dB
15 GHz 76.9 dB
18 GHz 83.3 dB
20 GHz 73.0 dB
25 GHz 68.6 dB
30 GHz 60.9 dB
35 GHz 67.1 dB
40 GHz 84.1 dB

The lowest result was 50.7 dB at 8 GHz, which is roughly 1/117,000th of the power getting through. The highest was 84.1 dB at 40 GHz, roughly 1/257,000,000th. Because the method is different, I wouldn't line these up against the Wellington figures one for one. For example, the Wellington single layer measured 75.1 to 77.9 dB at 1 GHz, while the Beijing three-layer test measured 69.0 dB there. Different setups give different numbers, which is exactly why I show both rather than picking the best one.

Cellular, NFC and key fobs

A properly closed SCRT bag blocks cellular signals, contactless (NFC) cards and car key fobs as well. The lab figures above are for the frequencies each lab measured, so I don't attach them to those other signals. Instead, the proof is the practical tests below: a phone call, a card tap and the door test at your car. They check the whole bag against the real thing, which is what counts.

What a dB figure actually means

Decibels are logarithmic. Every 10 dB is a tenfold cut in power, so the numbers climb fast:

  • 40 dB lets through 1/10,000th of the power, a 99.99% reduction.
  • 41.9 dB, our lowest result, is roughly 1/15,000th.
  • 69.3 dB is roughly 1/8,500,000th.
  • 77.9 dB, our highest, is roughly 1/60,000,000th.

Why does a phone go dead at those levels? A phone needs a minimum signal to hold a connection with a tower. If the signal reaching it drops by a factor of 10,000 or more, it usually falls below that floor and the phone shows no service. That's why the call test is such a good check. It tests the whole bag, not just the fabric.

You'll see some overseas brands quote 80 to 100 dB. Those numbers usually come from different methods, different frequencies, or a finished enclosure rather than one layer of fabric, so they're hard to compare directly. I'd rather show you our actual figures and how they were measured than pick a bigger number.

Where cheap pouches fail

The fabric is only half of it. Most failures I'd expect to see come down to general physics rather than the material:

  • The closure. Radio gets in through gaps. A single fold, a loose flap or a strip of Velcro can leave a slot that signals pass straight through. A good closure overlaps the opening and holds it shut. SCRT bags use either a magnetic closure or a fold-over closure, depending on the bag.
  • Seams and stitching. Every stitch is a tiny hole. If the conductive layer isn't continuous across the seam, you get a leak line along it.
  • Coatings that wear. Some cheap pouches use a thin conductive coating or a single light mesh. Folding and rubbing can crack it over time, and you can't see the damage.
  • Size. A pouch that only just fits the phone gets stretched at the opening and may not close fully.

None of that means every cheap pouch fails. It means you can't tell by looking. Our older post on why a $10 bag can let you down goes into this, but the honest answer is to test whatever you've got.

Try it yourself: the call test

  1. Put your phone in the bag. Leave mobile data and Wi-Fi on.
  2. Close it fully. 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.
  3. Ring the phone from another phone, or get a friend to.
  4. If it goes straight to voicemail, the bag is doing its job. If it rings, buzzes or a message arrives, something is leaking.
  5. Open the closure a couple of centimetres and ring again. It'll usually ring, which shows you how much the seal matters.

A few things to know. Give it a few seconds before you ring, as a phone can take a moment to drop the network. A missed call notification arriving when you take it out is normal, because the network tells the phone afterwards. Try it now and then, especially with a bag you use every day.

For key fobs, put the fob in the pouch, stand right next to the car and try the door handle. If it unlocks, the pouch isn't working for your car.

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.

What a Faraday bag doesn't do

  • It doesn't turn your phone off. The microphone, camera and apps can still run inside the bag, and anything stored can sync once it's out.
  • It only works while it's closed.
  • It doesn't block magnetic fields or the low-frequency fields from house wiring.
  • It isn't a health product, and I don't make health claims about it.

Which bag for what

SCRT Small Faraday Bag for Phones, a black shielded pouch with a clear front pocket
Small Faraday Bag for Phones: the everyday option for phones.
Hands holding the grey SCRT Faraday Phone Sleeve with its fold-over flap closed
Faraday Phone Sleeve: a slimmer fit with a fold-over closure.

Everything is designed and made in New Zealand and ships from New Plymouth. You can see the whole range of EMF protection products on the homepage.

Agency or business order?

If you're buying for a team, fleet or agency, see our police, defence and government orders page, or get in touch and I'll help you sort the right gear.

FAQ

Do Faraday bags really work? A well-made bag that's closed properly does stop a phone connecting to the network. Our EliteBlock fabric measured between 41.9 dB and 77.9 dB as a single layer in the Wellington test, and a separate test of the three-layer construction measured 50.7 dB to 84.1 dB from 1 to 40 GHz. The best proof is the call test: seal the phone in and ring it.

How was SCRT's fabric tested? RF Test Solutions in Wellington, an IANZ-accredited lab (ISO/IEC 17025), tested a single layer of EliteBlock using the IEEE 299 method, at 1, 2.45, 5 and 10 GHz, with horizontal and vertical antenna polarisation. Separately, the Beijing Institute of Radio Metrology and Measurement (CNAS-accredited) tested the three-layer construction in June 2025 using GJB 8820-2015, from 1 to 40 GHz.

Is 40 dB enough to block a phone? In most cases, yes. 40 dB cuts the power by 99.99%, which usually takes the signal below what a phone needs to stay connected. The call test will tell you for sure.

Why does my phone still ring in my pouch? Usually the closure isn't fully sealed, or there's wear or damage along a seam or coating. Re-close it carefully and test again. If it still rings, the pouch isn't doing the job.

Where can I buy a Faraday bag in NZ? SCRT bags are designed and made in New Zealand and ship from New Plymouth. Start with the Small Faraday Bag for Phones.

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