QIF-T0094

Magnetometer speaker-leakage eavesdropping (magnetic field emanation capture from speaker voice coils)

Speaker voice coils are electromagnets — when driven by audio current, they produce proportional magnetic field emanations. Smartphone magnetometers (used for compass/navigation) are sensitive enough to detect these emanations from nearby speakers, earbuds, or headphones. Zhang et al. (2020) demonstrated that a smartphone's magnetometer placed within 10-20 cm of earbuds can reconstruct the audio being played, including speech. This creates an eavesdropping channel through magnetic emanations rather than acoustic leakage — it works even when the audio is not audible (noise-canceling headphones, low volume). Matyunin et al. (2019) showed that magnetometer data can also fingerprint websites and applications by their characteristic audio/vibration patterns. Magnetometer access requires no permission on most platforms, making this an unrestricted side channel.