contact microphone

A microphone that hears solid objects instead of air. A piezoelectric element is pressed or bonded against a surface and responds to the surface’s vibration, which makes it almost completely insensitive to sound arriving through the room. Guitarists use them on instrument bodies. They cost around ten to fifteen dollars.

Their value here is diagnostic. If a window or wall is being driven as a loudspeaker — see structure-borne audio — a contact microphone on that surface registers it directly, while an ordinary air microphone in the same room hears only room tone. The instrument distinguishes between sound arriving through the air and sound being produced by the structure itself.

Documented real-world example: the counter-surveillance literature treats a windowpane as an acoustic component in both directions. A 2016 paper in the Journal of Vibroengineering, proposing a defence against laser eavesdropping, specifies a piezoceramic transducer bonded to the pane and driven at its first resonant mode — the same physics, used deliberately.

One honest limit: vendor literature on technical surveillance countermeasures lists contact microphones as a THREAT to be defended against, not as standard sweep equipment. No government or standards-body manual was found prescribing them as diagnostic instruments. Useful, cheap and physically correct for the question — but not, on the available evidence, established professional practice.

Related terms: structure-borne audio, piezoelectricity, laser microphone, sampling limit

Sources: Contact microphone — Wikipedia; Protection of speech information against laser eavesdropping — Journal of Vibroengineering (2016); TSCM threats, detection equipment, practices and procedures — Bastille