A change in the dielectric constant indicates a dense object behind the wall, normally a framing stud.

Larger and denser objects vibrate more slowly (and have lower pitch) than smaller and less dense objects.

Millimetre-wave energy detectors can pick up concealed dense objects because these emit heat energy at a different rate from the human body.

Instead, they could be cold, dark, dense objects with hard surfaces.

Because less dense objects rise in relation to denser objects, the area being heated becomes a broad dome (see isostasy).