Infrasound of about 5 Hz was measured near ahighway bridge in Kashilba-cho, Nara, Japan. The generationmechanism of the sound is studied. A similarity of measuredwaveforms of the sound and the vibration of the bridge beam aswell as the distance attenuation of the measured sound at about11dB per double distance, i.e. inversely proportional soundpressure to the squared distance in the range of 25 to 50 meterssuggest a possibility that the bridge beam constituted a dipolesource. Assuming an array of dipoles with intensitiescorresponding to the amplitudes of each point of the bridge beam,sound-pressure field near the bridge was estimated. The pressurewas almost uniform beneath the bridge and exhibited 11.5 to11.6dB at tenuation per double distance in the range of 25 to 100meters in consistent with the measured results.
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