More things to do on my inevitable trip out to western Massachusetts:

As traffic passes by, its noise generates a sympathetic resonance in the columns of air inside the tubes. High-pitched sirens and even voices generate higher harmonics, while the low rumble of trucks creates low ones. The sound is carried from the microphones in the tubes to a control room, where the sound signal is then amplified and transmitted to the concrete cube speakers under the bridge. There are no electronic effects added. The sounds have been simply extracted from the traffic noise above, as one might extract precious metal from a baser substance.

You can hear a sound clip here, or see a short video at the “harmonic bridge” link at the bottom of this page.

(via MASS MoCA)