Acoustic and vibration background noise in the collapsed structure of the World Trade Center (L)

Measurements of acoustic and vibration background noise were made on 18 September 2001 at the southern edge of the World Trade Center collapse. Sensors were deployed in a configuration reasonable for survivor search and near an on-going recovery operation and heavy debris removal. Geophones were placed on steel beams that extended into a deep void and a microphone was lowered below ground level into a pocket in the rubble. Even in what appeared at ground level to be a high-noise environment, weak or distant taps and bangs generated either by the authors or by recovery personnel produced distinct signals from the microphone and geophones.