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I want to hang a ceiling fan at my apartment, where the ceiling is made mostly of a sandy material. How do I find the concrete beam going through it, assuming I do not have access to the blueprints of the house?

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Have you tried a stud-finder device? They are normally used with thin walls to find the wooden studs behind the walls. I think they are really ultrasound devices and they just look for the return time. They should work on any homogeneous material, as long as it's not too thick.

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Stud finders generally don't directly sense the stud, but the nails or screws that secure the plaster/drywall to the stud. The oldest (and cheapest, and least reliable) kind used a magnet, very much like a compass; newer ones work like metal detectors (an electromagnet induces a small current in nearby metal, which generates a tiny magnetic field of its own, which the device then detects). You often see products called "Ultrasonic Range Finder and Stud Finder", but they're two separate devices - the ultrasound isn't being used to detect studs. –  MT_Head Jun 26 '11 at 19:33
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Most stud finders worth buying these days are electronic, not magnetic, and use an electrostatic field to detect a density change. However, with the high density of these walls, you may not find a stud finder that will work on them. wisegeek.com/how-does-a-stud-finder-find-studs.htm –  BMitch Jun 26 '11 at 21:47
    
@B Mitch - I think they are mostly ultrasound for the density check (really they are looking for return time from the wave bouncing off the far side of the object). In theory it can work, but I don't know if it will without trying it. –  Michael Kohne Jun 26 '11 at 22:50
    
Seems like a good option, thanks! –  Shwouchk Jun 27 '11 at 10:16
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Have you tried a simple druminess test. It doesn't work in every case but simply tapping the ceiling with a lump hammer and listening to the sound generated can give you a good idea about the make up of the wall.

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Unfortunately, in my case, the whole ceiling gave the same sound - as far as I could hear... –  Shwouchk Jun 28 '11 at 14:22
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