Symptoms of Problem: In a small Midwestern town, the residents complained that they felt movement of their houses, particularly at night. One of the residents stated that her sister would no longer come and visit her because she thought the house was haunted with ghosts. She felt this way because lampshades would move, pictures would rattle and rocking chairs would rock without anyone being in them.
Test Procedure: In an attempt to identify the cause of the above problem, vibration measurements were made at several of the houses in the community, along the sidewalks and at a factory located near the houses. The testing was performed with an SD 380 analyzer utilizing a 1000 mv/g low frequency seismic accelerometer to convert the mechanical motion into an electronic signal.
Test Results: The vibration signature taken at one of the houses where complaints had been registered showed a level of 1.22 mils at 300 cpm. It was observed that the amplitude of the vibration would oscillate, indicating that more than one frequency might be present. (Beat frequencies are generally produced by two or more closely spaced frequencies adding and subtracting as they go in and out of phase.) To determine if there was more than one frequency present, the zoom feature of the SD 380 analyzer was utilized. A 16:1 zoom plot of the vibration at one of the residences clearly showed the presence of several frequencies, all close to 300 CPM. The next step in the investigation was to make a survey of the vibration present at the nearby factory. The factory was a foundry, which contained several vibratory conveyors that moved parts from one area to another. Zoom plots were taken at each conveyor to determine their frequencies as close as possible. Exact matches were found between the frequencies present at the houses and several of the vibratory conveyors in the factory. Additional testing confirmed the correlation between the operation of the conveyors and the vibration at the houses.
Conclusions: Rather than having ghosts, the residents of the small town were experiencing low frequency vibrations from the vibratory conveyors in a nearby factory. The 5 hz (300 cpm) frequency is easily perceived by the human body, particularly at night when other motion and noise is at a minimum. It also can excite low stiffness structures ( i.e. lampshades and rocking chairs). The factory eventually installed balancing devices to reduce the amount of force entering the ground from the conveyors.