The National Archives said audio experts were unable to recapture unintelligible words from test tapes designed to simulate the recording made famous in the Watergate scandal.
"I am fully satisfied that we have explored all of the avenues to attempt to recover the sound on this tape," U.S. archivist John Carlin said in a statement. "We will continue to preserve the tape in the hopes that later generations can try again to recover this vital piece of our history."
In 2001, the archives created a panel of experts to determine whether advances in the field of forensic audio technology could recover what was on the tape recorded three days after the break-in. Based on the results of two tests, Carlin decided not to continue the effort to turn the gap — a series of clicks, hisses and buzzes — into intelligible speech.
The gap is part of a recording made June 20, 1972, in the old Executive Office Building as Nixon chatted with his chief of staff, H.R. Haldeman. News of the erasure, late the following year, eroded Nixon's credibility at a time when his presidency was unraveling over the June 17, 1972, break-in at the Democratic National Committee headquarters in the Watergate.
Nixon's secretary, Rose Mary Woods, testified that she was transcribing the tape when a phone rang. She said she must have pushed the wrong button and left her foot on a pedal, accidentally recording over part of the original conversation.
A panel of experts set up in the 1970s by federal judge John Sirica, who presided over the Watergate criminal trials, concluded that the erasures were done in at least five — and perhaps as many as nine — separate and contiguous segments. The panel never figured out what was erased.
Audio experts trying to recover what was said were not allowed to do tests on the original tape, which is stored at 65 degrees in a vault at the archives in College Park, Md. They had to prove they could retrieve voices from test recordings that have been erased without damaging the test tapes, said Susan Cooper, a spokeswoman for the archives.
The first test tape, distributed to experts in February 2002, consisted of erased test tones and spoken words. Results included partial identification of test tones, but none of the spoken words were recovered.
A second test tape was sent to experts in August 2002. This tape was recorded using blank 0.5-millimeter tape confiscated from the Nixon White House. It contained speech similar in quality to speech heard before and after the gap on the tape. None of the experts recovered any intelligible audio from the second test tape.
"We were really hopeful that there would be some technology out there to recapture the sound, but we'll have to put the tape away and possibly re-examine newer advances at a later date," Cooper said.