Acoustic Study Could Turn Tiny Sound Into a Big Clue

Although infinitesimally tiny, the mysterious bit of sound at the end of the tape on TWA Flight 800’s cockpit-voice recorder could provide key clues to what caused the jetliner to crash.

The National Transportation Safety Board will now turn to acousticians who specialize in trying to determine from the “signature” of a sound what caused the noise. By displaying the sound visually at a slow rate on oscilloscopes, these scientists may be able to infer whether the noise came from a blast or from a metal-to-metal hit, or a combination of both.

Researchers said that theoretically, it might be possible to determine what part of the plane was first involved in the catastrophe.

“Scientists have a vast vocabulary of experimental data, including the sounds produced by the dynamic testing of plane structures, blast acoustics and acoustic interaction with mechanical structures,” said Virenera Sarohia, director of the sensor research technology section at the Jet Propulsion Laboratory in Pasadena.

“Investigators will look for correlation of the noise with what witnesses saw on the ground,” Sarohia said.

Robert Francis, vice chairman of the NTSB, on Thursday described the noise as a “fraction-of-a-second sound” 11 1/2 minutes into the flight. Francis said the tape then went silent.

“They can put the sound on an oscilloscope and expand the scale and they can look at the signature, the lifetime,” Sarohia said. “Even though it may be for the ear a short time, instruments respond a lot faster than the ear.

“Instruments can play the sound and stretch the time scale of the sound, and they can look at the wave shape of the sound. . . . The sound of a missile is a well-documented sound.”

FBI investigators are working on three prevailing theories of what happened to the TWA jet: a massive mechanical failure, a bomb or a heat-seeking missile.

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Presumably available to government scientists studying the tape is the sound from the voice recorder recovered from an Air India jet that crashed in the Atlantic in 1985 off Ireland, killing all 329 people on board. British authorities concluded that a bomb blew up that plane.

Sarohia said government scientists would certainly have available the sounds of the “metal-to-metal hit” of a missile and the noises and oscilloscope images that various explosive warheads make.

“An external hit can be distinguished from an on-board explosion,” he said. “That should not be too difficult to distinguish from the signature of the sound.”

Sarohia said different parts of a plane if hit can send out distinctive sounds which, when matched with recovered wreckage, can greatly aid in determining the cause of a crash.