On our day trip to North Adams last summer, when we cranked some cranks to spin some tubes, we also checked out a sound installation under the nearby Route 2 overpass. There’s a speaker under the bridge with some pretty neat sounds coming out of it:
To produce these rolling tones, artists Bruce Odland and Sam Auinger affixed two 16-foot tuning tubes to the guardrail on the north side of the bridge on either side of the overpass. The length of the tubes determines the fundamental tone: a sound wave at such a low pitch is 16 feet long and must be generated (whether for sound art or a pipe organ) with a 16-foot tube. Inside each tube, a microphone is placed at a certain harmonic interval (the 5th in one tube, the 4th in the other). These locations accentuate the harmonic and give a slightly different timbre to the two Cs. (source)
It was ominous.
Here is a picture I took of the underbelly of the underpass.