What are those devices called that detect hollowed out pocket or spaces.?

These devices detect hollowed out pocket and spaces what are they called

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Wall Stud Finder If you have drywall walls, a stud finder locates studs quickly and accurately. It s less effective on lath / plaster walls, but some have a metal-scanning feature that may locate the nails securing the lath to the studs. OR Stud Finder Wall Scanner Walabot DIY Wall Scanner


A calculator.