What a Faraday Bag Actually Is

A Faraday bag is a pouch or enclosure made from conductive material that blocks radio frequency signals from reaching — or escaping — a device placed inside it. The principle is the Faraday cage, named after Michael Faraday, who demonstrated in 1836 that a conductive enclosure cancels external electromagnetic fields within its interior.

The physics are straightforward. When an electromagnetic wave hits a conductive surface, the free electrons in the material rearrange to create an opposing field that cancels the incoming signal. If the enclosure is complete — no gaps, no holes, no openings larger than a fraction of the wavelength — the interior is effectively shielded from external RF energy. The same principle works in reverse: a transmitting device inside the enclosure cannot get its signal out.

This is established physics. It works in military SCIFs, in MRI rooms, in the RF-shielded chambers where your phone was FCC-tested. It works because it’s physics, not because it’s marketing. The question with consumer Faraday bags is not whether the principle works — it’s whether the product in your hand implements the principle correctly.

Most don’t.

Why Most Faraday Bags Fail

What “Attenuation” Actually Means

Attenuation is measured in decibels (dB). Decibels are logarithmic, which means every 10 dB represents a tenfold reduction in signal power. Understanding the scale is essential for evaluating any shielding product.

When a manufacturer says their bag provides “signal blocking,” ask: how many dB, at which frequencies? If they can’t answer — or if the answer is a single number without frequency specification — the product has not been properly tested.

The minimum viable specification for a Faraday bag that actually isolates a modern smartphone: ≥40 dB attenuation from 600 MHz to 6 GHz, tested at multiple frequency points, with the closure sealed. That covers the cellular, WiFi, and Bluetooth bands your phone uses to transmit. Anything less is a privacy placebo.

The Test You Can Do Yourself

You don’t need a network analyzer to verify basic Faraday bag function. You need your phone and sixty seconds.

What to Look For

If you’re evaluating Faraday bags — whether ours or anyone else’s — here are the engineering specifications that separate functional products from marketing exercises.

What the Market Gets Wrong

The Faraday bag market has a fundamental misalignment: the products are marketed as privacy tools, but they’re evaluated as fashion accessories. Customers buy based on appearance, price, and brand presence. The actual electromagnetic performance — the only thing that matters — is almost never independently verified.

Here is what we see consistently in the market:

Products that block cellular but leak WiFi. This is the most common failure mode. Cellular frequencies (600–2100 MHz) have relatively long wavelengths and are easier to block. WiFi (2.4/5/6 GHz) and Bluetooth (2.4 GHz) have shorter wavelengths that exploit closure gaps and seam weaknesses. A bag that stops your phone from receiving calls may still be leaking your location via WiFi probe requests and Bluetooth beacons. You feel protected. You are not.

Products that test well new and degrade within months. Conductive coatings on nylon or polyester wear off with use. Fold points develop micro-cracks in the conductive layer. The product that tested at 50 dB in the factory tests at 25 dB after three months in your pocket. Nobody re-tests. The manufacturer tested it once. You can’t test it at all without a network analyzer.

Products with no disclaimer about what happens inside the bag. When you seal your phone in a Faraday bag, the phone detects that it’s lost signal and increases transmission power to maximum — trying harder to reach a tower. If the bag leaks even slightly, the phone’s maximum-power transmission may punch through the weak point. Worse: the phone’s battery drains faster inside a mediocre Faraday bag than it does in open air, because the radio is operating at maximum power continuously. A bad Faraday bag is worse than no Faraday bag in terms of both battery life and the intensity of RF energy concentrated near the device.

Use Cases That Actually Make Sense

Faraday bags are not tinfoil hats. They are engineering tools with specific, practical applications. Here’s where they provide real value — and where they don’t.

The Honest Position

We make a Faraday product. We are therefore not a neutral source in this buyer’s guide. We acknowledge this because we think you should evaluate our claims with the same scrutiny you apply to everyone else’s — more, even, because we’ve just spent two thousand words teaching you how to do it.

Here is what we’ll say about our own approach: we designed around the failure modes described above. Multi-layer shielding. Roll-top closure with sufficient overlap. Published attenuation specifications across the frequency bands that matter. A disclaimer printed on the product that tells you what it does and what it does not do — including the fact that it does not make you invisible, it does not protect data already collected, and it does not substitute for comprehensive operational security.

We put “go dark” on the packaging because we think it’s funny and because it describes the literal function — the device goes dark on the network. We put the disclaimer there because we think honesty is a better long-term business strategy than marketing fiction.

Whether our product or anyone else’s is right for you depends on your specific threat model, which is a concept most Faraday bag manufacturers never mention because it requires acknowledging that no single product solves the problem. We just did. You’re welcome.