Faraday bags are rated by how much radio frequency energy they block, but the numbers on the packaging rarely tell the full story. Learning to read attenuation charts lets you verify if a bag matches your specific security needs.
What Is RF Attenuation?
RF attenuation measures the reduction in signal strength as electromagnetic waves pass through a shielding material. It is expressed in decibels (dB), a logarithmic unit where every 10 dB increase represents a tenfold reduction in signal power. A bag rated at 60 dB reduces the incoming signal to one-millionth of its original strength. This metric applies across the spectrum, from low-frequency RFID readers to high-frequency 5G millimeter waves. The higher the dB value, the more effective the shield.
Understanding dB Ratings for Faraday Bags
Manufacturers often advertise a single peak number, but real-world performance varies by frequency. Knowing the tier thresholds helps you filter marketing claims.
Below 40 dB – Weak shielding
Products in this range block only a fraction of signal power. A 30 dB rating reduces signal by roughly 99.9 percent, which sounds impressive until a strong cellular tower or nearby router overwhelms the residual leakage. These bags might stop passive RFID skimming at close range but will fail against active transmissions like phone calls or GPS tracking.
40-60 dB – Moderate shielding
This tier handles everyday RF environments. It blocks standard Wi-Fi, Bluetooth, and cellular signals in typical urban settings. However, proximity to a cell tower or a high-power transmitter can still penetrate the shield. This level suits general privacy needs but not high-threat models.
60-80 dB – Strong shielding
Bags rated here provide reliable isolation for law enforcement, corporate travel, and digital forensics. They attenuate signals enough to prevent remote wipe commands, location pings, and contactless payment skimming even in high-RF environments. Look for independent lab verification at this level.
80-90+ dB – Professional shielding
This tier approaches the physical limits of flexible materials. It is required for military, intelligence, and sensitive compartmented information facility (SCIF) applications. At 90 dB, only one-billionth of the signal passes through. Achieving this across a broad bandwidth requires multi-layer construction and precision sealing.
What Frequencies Matter Most?
A attenuation chart plots dB on the Y-axis against frequency (Hz) on the X-axis. The shape of that curve reveals what the bag actually stops.
Low-Frequency Bands
Covering 125 kHz to 13.56 MHz, this range includes RFID access badges, NFC payment cards, and vehicle key fobs. These long wavelengths penetrate thin materials easily. Effective shielding here demands high-permeability ferrite layers or thick conductive fabrics.
Mid-Frequency Bands
Spanning roughly 400 MHz to 3 GHz, this zone holds legacy cellular (2G/3G/4G), GPS, Wi-Fi 2.4 GHz, and Bluetooth. Most consumer threats live here. A flat response curve across this band indicates consistent protection for daily device isolation.
High-Frequency Bands
From 3 GHz to 6 GHz, you find 5G sub-6 GHz, Wi-Fi 5/6, and satellite communications. Shorter wavelengths exploit smaller gaps in seams and closures. Performance often dips here compared to mid-bands unless the bag uses overlapping flap designs or conductive adhesives.
Ultra-High Bands
Above 6 GHz into millimeter wave (24 GHz to 40 GHz) and Ultra-Wideband (UWB) around 6-10 GHz. These frequencies support high-speed 5G, automotive radar, and precision tracking tags like AirTags. Shielding effectiveness drops sharply at these wavelengths; specialized metallized textiles are required to maintain 60+ dB.
How to Read an RF Attenuation Chart
Charts supplied by reputable vendors show a curve, not a flat line. The X-axis is logarithmic. The Y-axis shows attenuation in dB. Look for the lowest point on the curve within your threat frequencies. That valley is your true rating.
Ideal Curve
An ideal curve stays high and flat across the entire spectrum you care about. A “V” shaped curve that peaks at 2.4 GHz but dives to 30 dB at 800 MHz and 28 GHz leaves gaps for LTE and mmWave 5G. Compare the curve against the frequency list in the previous section to map protection to your use case.
Why Attenuation Ratings Vary Across Frequencies
Physics dictates that no single material blocks all wavelengths equally. Understanding the mechanisms explains the chart shape.
Low frequencies (RFID)
Magnetic field dominance at low frequencies requires magnetic loss materials. Standard copper or nickel fabrics rely on eddy currents, which weaken as frequency drops. Without ferrite loading, the curve plunges below 1 MHz.
Mid frequencies (cellular, Wi-Fi)
This is the sweet spot for conductive fabrics. Skin effect concentrates current on the surface, maximizing reflection loss. Multiple layers add absorption loss. Most bags show peak performance here, often exceeding 80 dB.
High frequencies (5G)
As wavelength approaches the size of fabric weave gaps, diffraction increases. Seam leakage becomes the primary failure mode. Conductive tape seams or double-fold closures are necessary to maintain the mid-band plateau.
Ultra-high frequencies (UWB, mmWave)
At millimeter wavelengths, surface roughness and coating thickness variations cause scattering. Thin metallization on polyester substrates can become transparent. Solid metal foils or dense nanoparticle coatings perform better but reduce flexibility.
How Manufacturers Test Faraday Bags
Credible ratings come from controlled environments, not a phone call test inside a closet. Standard methods follow IEEE 299 or MIL-STD-285 protocols adapted for enclosures.
- Spectrum Analyzers measure the power level of received signals across the frequency range. They provide the raw data for the attenuation curve.
- Signal Generators create precise, stable RF sources at specific frequencies and power levels. They simulate the threat emitters.
- Anechoic Chambers absorb reflections to isolate the bag’s shielding from room multipath. This ensures the measured signal passed through the material, not around it.
- Independent Lab Certification third-party validation removes vendor bias. Look for reports from NVLAP-accredited facilities or similar bodies.
For a deeper technical reference on shielding effectiveness test standards, consult the NIST publications on electromagnetic shielding.
Why Cheap Faraday Bags Often Fail Testing
Budget bags frequently use a single layer of metallized fabric with a simple Velcro or ziplock closure. Three failure modes dominate:
- Insufficient conductivity: Thin vacuum deposition wears off at fold lines, creating pinholes that act as slot antennas.
- Closure gaps: Non-conductive seams leak RF. Velcro gaps of 1 mm can leak 20-30 dB at 5 GHz.
- No low-frequency layer: Missing ferrite composite means RFID and NFC pass through unaffected.
Marketing photos showing a phone losing bars inside the bag prove nothing. Bars indicate received signal strength indicator (RSSI) thresholds, not absolute isolation. A phone shows “no service” at roughly -110 dBm but can still receive silent SMS or command signals well below the noise floor.
Attenuation Testing Example (Conceptual)
Comparing two hypothetical products illustrates how to apply chart data.
Budget Faraday Bag
The chart shows 85 dB at 2.4 GHz but drops to 35 dB at 13.56 MHz and 45 dB at 28 GHz. The closure is a single Velcro strip. In practice, this bag stops Wi-Fi and Bluetooth reliably. An NFC reader at 2 inches reads a credit card through the bag. A 5G mmWave signal from a nearby small cell penetrates enough for a silent ping. The low-frequency gap allows key fob relay attacks.
Premium Bag
The curve stays above 70 dB from 100 kHz to 40 GHz. The closure uses a triple-fold magnetic seal with conductive gasket. Ferrite-loaded inner layer handles RFID. Multi-layer metallized fabric handles mmWave. This bag defeats relay attacks, prevents remote wipe on all current cellular bands, and blocks UWB tracking tags. The chart valley is the spec you trust.
Choosing the right bag starts with defining your threat model. If you only need to stop contactless payment skimming, a verified 40 dB rating at 13.56 MHz suffices. If you carry a device that must remain dark against state-level surveillance, you need the flat 80 dB curve. Always ask for the full attenuation chart, not the peak number. For more on selecting gear for specific scenarios, see our guide on Faraday bag selection guides and our breakdown of testing methodologies.