A USB sound card costs about $20 and records anything electrical up to half its sampling rate. Put a loop antenna or pickup coil on its input and it becomes a very-low-frequency (VLF) magnetic-field detector. It isn't radio in the usual sense, and Consentiva labels it that way everywhere, but it shows things a radio receiver can't: mains hum, appliances, and lightning thousands of kilometres away.
What you can see
- Mains hum and its harmonics: 60 Hz in the US (50 Hz in much of the world), with peaks at 120, 180, 240 Hz and up. A good first test of the setup.
- Switching power supplies and motors: chargers, LED drivers and variable-speed motors make tones from hundreds of hertz to tens of kilohertz.
- Lightning: crackles and bursts called sferics, spread across the whole range, from storms far away.
- VLF transmitters: powerful navy and time stations in the 15 to 30 kHz range, such as the US Navy's NAA at 24.0 kHz in Cutler, Maine. You need a sound card that samples at 96 kHz or more to reach them.
What you need
- A USB sound card or audio interface with a line or microphone input that supports 96 kHz sampling.
- A loop antenna or pickup coil. A coil of many turns of wire (a large loop of 50 to 100 turns, or a commercial induction pickup) works. Bigger and more turns means a stronger signal.
- A cable from the coil to the sound card input.
- A laptop with Chrome or Edge.
Recording in Consentiva
- In your computer's sound settings, choose the USB sound card as the input and turn off noise suppression, echo cancellation and automatic gain.
- Open the app and choose Lab: sound card + loop antenna under Receiver. The browser will ask for microphone access.
- Start the walk. The spectrum covers 0 up to half the sampling rate. Mains hum shows first. Move the loop toward and away from an appliance and watch its tones rise and fall.
The report and share page for such a walk say "Test data: sound-card input, not radio," and its baselines are kept apart from radio walks. Audio is never saved, only the spectrum.
Limits
A sound card input is not calibrated: levels are relative, not field-strength measurements. Noise from the computer and from mains wiring can swamp weak signals; take the laptop off its charger and move away from wiring for the quietest readings. Support for this receiver is a beta, tested with a simulated input and not yet with a real loop antenna. See what to buy.
Common questions
Is a sound card a radio receiver?
No. A sound card records electrical signals up to half its sampling rate. A loop antenna or coil turns changing magnetic fields into such a signal, which is why it can see very-low-frequency fields. Consentiva labels this mode as a lab test, not radio.
What frequency range can I see?
From a few tens of hertz up to half the sampling rate: 24 kHz for a 48 kHz sound card, 48 kHz for a 96 kHz card. Extremely low frequencies such as the 7.83 Hz Schumann resonance are below what a normal sound card passes.
Does Consentiva save the audio?
No. Only the spectrum, one reading a second, is saved. The audio is never stored.
Why turn off noise suppression?
Noise suppression and automatic gain are built for voices and will erase or distort the steady tones you are trying to measure. Turn them off in your system sound settings.