... black shell 7.Detected object range: minimum object is stud, staple, measured area: 20-50mm 8.Alarm way: acousto-optic at ... time, the sound has high-frequency tone when detects big metal object;The sound has lower frequency tone when detects small ... tool for safty-checking, convenience using. It could be detected metal accrately, using place:airport, court, education department,prison, stadium, ...