Constant‐frequency infrasound (about 5 to 10 Hz for different sources) has been detected at a number of locations from different directions. In an intensive study of 8.5‐Hz infrasound detected regularly at Lamont‐Doherty Geological Observatory of Columbia University in Palisades, N. Y., positive sound fixes have been obtained at the Tappan Zee Bridge. Geophones placed at several locations on the bridge recorded vibrations of the same frequency. We conclude that the vibrational motions of bridges radiate infrasound into the atmosphere and that the infrasound is most readily detected during times when atmospheric wind and temperature structure favor acoustic channeling near the ground. The acoustic signal occurs in pulses having a duration which, although fairly constant on a given occurrence, may vary from a fraction of a minute to several minutes.