Faraday bags: what a phone check can tell you

Put a phone in a Faraday bag, call it, and listen. If it stays quiet, what have you learned?

Less than the phrase “signal blocked” suggests. You have an observation about one attempted connection, on one device, in one place. That can be useful. Turning it into a claim that every radio is isolated, everywhere, requires evidence the call did not provide.

For Koocanusa Tech and Tactics, this is a good place to practice useful technical judgment: define the question, control what you can, and keep the conclusion within the evidence. No test results are being reported here. This is a method you can adapt to equipment you already have.

Start with the bag and the connection

A bag's shielding depends on more than its main fabric. Seams, closure gaps, and cables passing through the opening can compromise isolation. NIST's 2014 mobile-device guidance describes these limitations and cautions that attenuating a signal does not necessarily eliminate communication. That is a physical caution, not a current rating for any particular bag. Follow your bag's closure instructions; there is no universal fold count. Inspect its condition and keep charging cables out of the opening during this demonstration. NIST SP 800-101 Rev. 1, §§4.3–4.3.2

Treat cellular, Wi-Fi, and Bluetooth as separate checks. An outcome on one connection does not cover every frequency band or radio in the phone. NIST enclosure research illustrates why geometry, openings, and source position belong in a shielding measurement. Its rigid-enclosure experiments do not establish a fabric bag's performance. NIST enclosure measurement research

Establish the baseline first

Start with a baseline outside the bag. Confirm the exact activity works in the same location, with the same devices and settings. Record the bag model, closure, phone model, software version, connection being checked, and distance to the other device or router. Note the band if your existing equipment identifies it; otherwise write “unknown.” A network name or “5G” icon is not a complete frequency record.

Keep the intended radio on. On an iPhone, tapping Wi-Fi or Bluetooth in Control Center can disconnect an ordinary connection while leaving other functions available. Check the relevant device settings rather than treating a dim icon as a complete radio-state description. Apple's Control Center explanation

Choose a repeatable observation

Choose an observation window before starting. Sixty seconds and three baseline–closed–reopened runs are a manageable example, not a standard or a statistical guarantee. Keep placement and orientation consistent for those runs. Record when you close the bag, what happens while it remains closed, and whether normal operation returns after reopening. If you later change orientation or location, label that as another setup.

• Cellular: If you already have a second phone and a plan that permits the call without an added charge, call the enclosed phone. First verify that the intended path is cellular: Wi-Fi Calling can carry phone calls over Wi-Fi. Count an alert from the target phone during the closed interval, not ringback at the caller or an alert on a linked watch. Silence alone is weak evidence if the ringtone could be muted or muffled. If you cannot establish the path or observe the target reliably, record “inconclusive.” Apple's Wi-Fi Calling documentation

• Wi-Fi: With an existing, working local-network transfer between your own devices, observe fresh data arriving at the device outside the bag while the phone remains inside. Record a changing byte count or timestamp if your existing software provides one. A cached page, old connection badge, or buffered stream does not demonstrate new traffic. For internet-based checks, consider cellular fallback: Apple's Connectivity Assist is one device-specific example. If you cannot establish which path carried the traffic, leave Wi-Fi untested. Apple's Connectivity Assist documentation

• Bluetooth: If you already own a paired speaker or headphones, play a locally saved audio file from the phone inside the bag to the accessory outside. This removes internet streaming from the setup. Observe the whole interval and recovery; a brief continuation can reflect buffered audio. Record interruptions without treating one accessory's behavior as a test of every Bluetooth use. Bluetooth SIG's discussion of receiver buffers

Keep the conclusion within the evidence

Write narrow results: “communication observed,” “communication not observed during this interval,” or “inconclusive.” Include the setup and each run. A working connection shows that particular path remained usable. A failed connection does not measure how much RF reached the phone or prove universal protection. A failed baseline, unclear path, or missing recovery needs investigation before interpreting the bag.

A phone check does not measure attenuation

Laboratory shielding work compares controlled measurements at defined frequencies and configurations. NIST researchers use measured power and repeatable chamber procedures, and separately study measurement uncertainty. Phone bars and missed calls do not provide enough information to assign an attenuation value in decibels. NIST shielding measurements, NIST measurement uncertainty research

A useful field record is modest and specific. It says what you tried, what you could observe, and what remains unknown. That makes the demonstration repeatable without turning it into a security certification.

This educational method is not a KTT equipment test, product endorsement, or security certification. No measured bag-performance results are reported.