The Guinness World Record for the longest echo in a man-made structure was recently awarded to the World War II-era underground oil tanks in Inchindown, Scotland. In January, acoustics explorer Trevor Cox rode a backboard through a 24-inch cleanout pipe into the tanks to make the winning measurements. Each tank is approximately 240 meters long, 9 meters wide and 13 meters high, and is dug out of solid rock and lined with concrete. With the tank surfaces still seeping fuel oil, Cox fired his starter pistol (Did you really think this through, Doc?) and recorded the echo, which lasts almost a full two minutes! The recordings stop at 80 seconds, but the standard measure of reverberation, RT60 (the time to drop 60dB), is extrapolated from them to be 112 seconds at 125 Hz.

Those who would like to hear this phenomenon without being shoved into an oily old tank can listen to Cox’s shared recordings at www.freesound.org/people/acs272/packs/13598. His measurements demonstrate a reverberation which is near the theoretical limit of what is possible. I wouldn’t have believed it, but you really can hear the shot for a full 80 seconds (listen for the “heartbeat” of the sound produced by the 0.7-second longitudinal mode travel time). It’s amazing to realize that at the end of the recording you’re listening to sound that has traveled over 28 kilometers (17 miles).