Clamp two hacksaw blades to a table with a C-clamp so that about 4 inches of each blade extends off of the end and so that the blades form a V. One way to adjust them to the same frequency is to shorten the protruding part of one blade until it vibrates at a recognizable pitch and then tune the other to sound the same pitch. Another way is to strobe each of them, using the light from a TV as a convenient stroboscope. When the two blades are in reasonably close tune, couple them with a rubber band. Strum one of them and watch the beats between the modes. Vary the coupling by moving the rubber band in or out along the blades. If the two blades are not in tune, do you get beats?
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