John Huntington and his friends decided to test the reported phenomenon of infrasound (very low-frequency sounds) causing people to feel spooky chills and thrills, a phenomenon blamed for ghost sightings and reports of hauntings. They created a spook-house with a double-blind randomized infrasound generator and used surveys to check for a correlation between infrasound and creepy feelings. John exhaustively documented the experimental setup (and the setbacks encountered in getting things up and running), and the results. Spoiler alert: they didn't find a correlation.
We picked a 19 Hertz (Hz, or cycles per second) sine wave as our infrasound source, since that's the frequency Tandy and Lawrence had found in their initial investigation of a "haunted" space (see Part I of this series for details). For our initial tests last spring, we used an Audio Toolbox as our signal generator, and connected it up to our ancient Apogee P-10 subwoofer processor and a Crown K2 amp, which drove an Apogee AE-10 double 18" subwoofer (these units work just fine, but haven't been made in years.) We got the sound going, but after a minute or so, it would cut out. I figured out that the processor was apparently protecting the subs against the "bad" infrasound, and was cutting out. So, I bypassed the processor and its protection circuits, and (carefully) drove the subwoofers directly from the amp. This worked just fine, but I was a bit concerned about damaging the speakers themselves. Fortuitously, over the summer I managed to get for our department a massive, modern Meyer Sound 650-P powered subwoofer. And after some tests in late summer we determined that the unit could generate quite a bit of 19Hz without clipping, and we figured the level was pretty good because by the time we pushed the 650-P up to its limits, the effects of the infrasound would be obvious to anyone in the room, which wouldn't have been acceptable for our purposes.