A modern look at the buzzes and clicks on the Watergate tape reveals new information on the perpetrators of the tape erasure and their motives.

A closer look at the gap in the Watergate tapes revels information about who may have erased the tape and why.Watergate, the greatest political scandal of the 20th century, remains shrouded in mystery, in part, due to an 18½-minute gap in the tape recording of a Watergate discussion between President Richard Nixon and his chief of staff Bob Haldeman. Despite many attempts to recover its contents, unlocking the secrets of the gap has, until now, remained beyond the realm of possibility. This article documents forensic breakthroughs in understanding the “18½-minute gap.”

Watergate Conversation on June 20, 1972
Early in the morning of June 17, 1972, five men were arrested at the Watergate headquarters of the Democratic National Committee. The men were eventually linked to Nixon’s re-election campaign.At the time of the arrests,Nixon and his chief of staff were enjoying a weekend getaway at Key Biscayne, Florida, before returning to the White House on Tuesday, June 20, to assess the damage from the arrests. Meeting that morning at 11:30, Nixon and Haldeman discussed the Watergate situation. A secret White House taping system installed at Nixon’s orders recorded the entire 69-minute conversation. Some 17 months later, as investigators searched for Watergate–White House connections, the 18½ minute Watergate portion of the Nixon–Haldeman conversation was found to be completely obliterated.

Nixon’s loyal secretary, Rose Mary Woods, admitted to mistakenly making a single erasure of 4½ to 5minutes on the tape while making a phone call but vehemently denied making the entire 18½-minute erasure. Woods later conceded that she may have erased between 5½ and 6minutes of the tape, but nomore. All that remained of the 18½-minute discussion was a sequence of clicks, pops, and buzzing sounds. Little of the situation made any sense. Why would Woods admit to making a portion of the erasures but deny making the remainder? Why not claim the entire erasure was an accident? And what caused all of the strange sounds remaining on the tape?

1974 Advisory Panel Report on the Tape Gap
Upon discovery of the tape gap in November 1973, an advisory panel of scientists convened to assess the condition of the tape. After months of analysis, the panel issued a report1 that concluded that the 18½-minute gap was composed of five to nine erasures made with Woods’Uher-5000 tape recorder. Woods’ tape recorder was found to have a faulty bridge rectifier that served to convert the AC power to DC, when this component failed during testing it was replaced and the faulty component was tossed in the trash—a critical error. The report left many questions unanswered. The panel was unable to explain or reproduce the erasures’ buzz sounds, was unable to explain the loud-soft-loud buzz sequence across the erasures, and was unable to correlate the magnetic marks left on the tape to Woods’ admitted erasure accident.

Figure 1 is the summary diagram excerpted from the 1974 Advisory Panel Report. As shown, the erasure consists of an initial loud-buzz segment, then a longer soft-buzz segment, and finally another loud-buzz segment. Interspersed between some of the erasures are short un-erased segments lasting only seconds each. In the middle of the third erasure the buzz transitions from a loud-buzz to a soft-buzz. In a technical note to the report, the decrease in the buzz level is attributed to unexplained “transient noise.”Curiously, the length of the initial loud-buzz segment precisely matched Woods’ admission to erasing 4½t o 5 minutes of the tape. However, according to the panel scientists, the third erasure does not end with the transient noise but instead continues on for more than five minutes of soft buzz.

The scientists knew of no way to recover the erased conversation. Unable to clarify the gap’s mysteries since several persons had access to the tape but each denied responsibility, no charges were ever filed. With Nixon’s resignation, the tapes and recorder were moved into the National Archive and Records Agency (NARA) climate-controlled vaults for safekeeping.NARA hoped new technologies might later enable recovery of the erased conversation.One NARA effort begun in 2001 ended when participants failed to recover any intelligible audio from a simulated erasure. The head of NARA expressed a hope that, “…later generations can try again to recover this vital piece of our history.”

Figure 1: Summary of findings from the Advisory Panel Report.

More Clues Surface
Despite the continuing release of documents and tapes from the Nixon administration, no theories to date have offered a comprehensive or coherent explanation for the sequence of clicks, pops, and buzzes across the gap. While some speculate as to the content of the erased conversation, many assume that Nixon and/or Woods were responsible for the entirety of the 18½-minute gap.

One record from the Nixon White House stands out among all others as a possible clue to the sequence of tape erasures. In 1991, NARA released documents from Rose Mary Woods’ files that claimed White House lawyers had “secretly entered her office in her absence searching for proof to support their felony charges.” The document, apparently prepared by Woods and her lawyers, was dated January 10, 1974. In response to the release of Woods’ document, one White House lawyer stated that he had no recollection of entering Woods’ office, but stated that another White House lawyer might have. Another former White House lawyer confirmed that White House lawyers had indeed entered Woods’ office on more than one occasion without Woods’ knowledge or consent. According to the Woods document, the subpoenaed tapes (including the “gap” tape) and tape recorder were stored in Woods’ office at the time of the alleged White House lawyer entries.

Could a White House lawyer have erased a portion of the tape? For this to be plausible, the theorized White House lawyer would likely have to meet the following criteria:

  • The lawyer would have had some known Watergate involvement prior to the Nixon–Haldeman meeting on June 20, 1972.
  • The lawyer would have had access to the White House and Woods’ office between the date of Woods’ admitted erasure (October 1, 1973) and the date the tape was duplicated (November 13, 1973).

Could this theorized access to the tape following Woods’ 4½ to 5 minute erasure help make sense of Woods’ admissions and the discovered state of the tape?

NARA’s Quest to Recover Haldeman’s Meeting Notes
After watching a television documentary on Watergate in 2004, I became intrigued with the method a college professor used in an attempt to discover the identity of Washington Post reporter Bob Woodward’s mysterious source, “Deep Throat.” Professor William Gaines and his students traced the information that Deep Throat provided Woodward and systematically eliminated Deep Throat candidates that would not have possessed the required information at the time it was provided.

As it turns out, Deep Throat and Woodward had their own tie to the 18½-minute gap. In the first week of November 1973, just before the Nixon White House publicly admitted to knowledge of the erased tape, Deep Throat advised Woodward that the White House tapes contained deliberate erasures. I used Internet sources to create a timeline around the Gaines’ Deep Throat nominee that, I reasoned, would help me confirm Gaines’ conclusion. Hours turned into days and days into weeks as my timeline became more and more detailed without any clues or confirmation of the professor’s Deep Throat candidate.

Abruptly, after approximately three weeks of timeline research, I realized that I had stumbled upon the sequence of events surrounding the tape erasures. Further, I believed that I now knew who, in addition to Woods, had erased part of the conversation. Looking for further evidence, in 2008 I visited NARA at College Park, Maryland, and explained my predicament to an archivist. He immediately went scrambling to a vault and took out Haldeman’s original notes from the meeting.

Donning white cotton gloves, the archivist and I studied Haldeman’s original notes from the erased meeting. I quickly surmised that the notes did not match the scant information available on the erased conversation. Though Haldeman normally took prodigious notes during his meetings with Nixon, in this case, all that remained was half a page of notes for the erased 18½-minute conversation. It appeared to me that some of Haldeman’s notes might be missing. Four sets of staple holes in the two pages of notes hinted that the notes had been stapled and unstapled multiple times. Was it possible that some of Haldeman’s notes had also gone missing?

Within days I submitted a 9-page document to NARA requesting that Electro Static Detection Analysis (ESDA) be conducted on the notes in an attempt to recover writing from the potentially missing pages. ESDA would recover any remnant handwriting impressions left on the remaining pages.

On November 18, 2009, the National Archives issued a press release, an excerpt of which follows, announcing plans to test Haldeman’s meeting notes for writing impressions:

The National Archives has assembled an examination team in an attempt to clarify some mysteries surrounding the June 20 meeting, of which Mr. Haldeman’s notes are the only extant account. Historians and scholars have long speculated on the subject of that meeting. The team will attempt to determine whether there is any evidence that additional notes were taken at the meeting that are no longer part of the original file.

Instrumental examinations of the documents will include Hyperspectral Imaging at the Library of Congress to study the ink and to possibly reveal latent or indented images on the paper; Video Spectral Comparison (VSC) of the ink entries and paper substrates; and Electrostatic Detection Analysis (ESDA) to reveal indented images that could correspond to original handwriting on these or other pages—present or no longer present—among documents from the Haldeman files.

Team members include experts from the:

  • Library of Congress Preservation Research and Testing Division
  • Treasury Inspector General for Tax Administration Forensic Science Laboratory
  • Bureau of Alcohol, Tobacco, Firearms, and Explosives Forensic Science Laboratory.

NARA anticipates making the results of this testing available late this year. My hope is that recovered notes will help resolve the reason for the tape erasure.

Figure 2: Halderman's handwritten notes from the erased June 20, 1972, conversation.
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Figure 3: Comparison of the magnetic markes from the reasures in the 18.5 minute gap. Figure 4: Chronological sequence of erasures accross the tape gap.
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Breakthrough in Decoding the 18½-minute Gap
The inexplicable nature of the gap’s third erasure was troubling to me. Why, in the middle of the erasure was there a transient noise and why did the buzz level change after it? The Advisory Panel attributed the drop in buzz level to the transient noise without further explanation. I suspected that the Advisory Panel had misidentified the magnetic mark at 275 seconds and that the mark was actually a begin-erasure event marking the beginning of a new erasure.

To gain further insight into the 18½-minute gap, I requested a high-quality digital copy of the tape erasures from NARA. To my astonishment, I was told that I was the first to request a digital copy of the erased tape—others had made do with the copies of the tape available on cassettes at NARA. NARA mailed me a CD containing a high-quality digital copy of the tape erasure.2

Using commercially available audio analyzers, I closely studied the magnetic marks in an effort to identify a “signature” for begin erasure events. From this scrutiny and the images of the magnetic marks scattered throughout the Advisory Panel Report, I discovered what I believed to be a signature marking the start of each erasure. I compared the signature images at the ten begin-erasure points on the tape with the magnetic mark 275 seconds into the tape gap identified as “transient noise.” I also contrasted the images of the five “end erasure” events from the gap. As shown in Figure 3, the similarity of the magnetic mark at 275 seconds to the other begin-erasure events, especially those at 0 and 49 seconds, convinced me that the mark identified as transient noise was actually a begin-erasure event.

Based on these findings, I restructured the sequence of tape events into ten erasures, dividing what was previously the third erasure (beginning at 155 seconds and ending at 611 seconds) into two separate erasures (the first from 155 to 275 seconds, and the second from 275 to 611 seconds).

The initial set of loud buzz erasures lasting 4 minutes, 35 seconds was now consistent with Woods’ admission to erasing between 4½ and 5 minutes of the tape.With this seemingly minor adjustment, I could now explain, at least theoretically, the structure of the clicks, pops, and buzzes that occur during the 18½-minute gap.

On October 1, 1973, Rose Mary Woods was reviewing the tape containing President Nixon’s June 20, 1972, 11:26 A.M. meeting with chief of staff Bob Haldeman when she made a 4½ to 5- minute erasure. Woods claimed the erasure was an accident.

Analysis of Initial 4 Minute, 35 Second Loud-buzz Erasure:
Unbeknownst to anyone at the time,Woods’ tape recorder was faulty, inadvertently storing the AC hum from the connected power line onto the tapes it erased. In addition, the AC hum on the power line in Woods’ office was louder than normal, therefore, erasures made in Woods’ office using the faulty tape recorder caused a loud buzz to be stored on the tape. Consistent with Woods’ admission to erasing about 4½ to 5 minutes of the tape in her office using the faulty tape recorder, the 4 minute, 35 second segment exhibits this characteristic loud buzz. However, Woods’ tape recorder required manual intervention to both start and stop erasing. Woods claimed the erasure was an accident, but an examination of the tape shows that this segment is composed of three separate erasures, indicating Woods’ erasure was deliberate.

Following Woods’ 4 minute, 35 second erasure on the tape is a 12 minute, 46 second soft-buzz erasure. On November 6, 1973,Woods told Robert Bennett, the White House tapes custodian, that she found a gap where she expected a recorded conversation. I suspected that this was the day that she found the unexpected erasure segment. I assumed Woods was alerted to check the tapes that day as a result of Deep Throat’s “deliberate erasures” tip. Just the day before,Carl Bernstein, Woodward’s reporting partner, had begun asking White House press secretary Ron Ziegler and others at the White House questions about possible erasures as a result of Deep Throat’s tip to Woodward.

Analysis of 12 Minute, 46 Second Soft-buzz Erasure:
This soft-buzz segment of tape consists of four separate erasures. Therefore, this erasure segment was also deliberate. While the AC hum on the tape during this erasure segment is consistent with Woods’ faulty tape recorder, the lower level of AC hum stored on the tape (soft buzz) during this erasure is inconsistent with the louder AC hum found on the power line in Woods’ office. Thus, sometime after Woods’ initial 4½-to-5minute erasure, an unidentified person deliberately erased 12minutes, 46 seconds of the tape using Woods’ faulty tape recorder. However, this erasure appears to have been made somewhere other than in Woods’ office.

After the 12 minute, 46 second erasure on the tape is a 1minute, 9 second loud-buzz erasure. This last erasure occurred no later than November 13, 1973, when technicians made copies of the tapes.

Analysis of 1 Minute, 9 Second Loud-buzz Erasure:
This 1 minute, 9 second segment consists of three separate erasures. Therefore, this set of erasures was also deliberate. As with Woods’ initial erasures, the level of AC hum on the tape during this segment is consistent with both the use of the faulty tape recorder and the high-level of AC hum found on the power line in Woods’ office. Woods later denied erasing more than 5½ to 6minutes of the tape, a change in her initial admission consistent with Woods subsequently adding a 1 minute, 9 second erasure to her previous 4minute, 35 second erasure for a combined total erase time of 5 minutes, 44 seconds.

In summary, as illustrated in Figure 5,Woods’ faulty tape recorder made the entire 18½-minute gap. Woods deliberately erased 4minutes, 35 seconds of the tape in her office, on October 1, 1973, though Woods claimed it was an accident. Someone later deliberately erased the following 12minutes, 46 seconds of the gap, but not in Woods’ office. After that, someone deliberately erased the final 1minute, 9 seconds while again in Woods’ office.

Figure 5: Explanation of loud-soft-loud buzz sequence across the tape gap.
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A Piece of Watergate’s Deep Throat Puzzle
On May 31, 2005, W.Mark Felt, the deputy director of the Federal Bureau of Investigation from the first year of the Watergate investigation until his retirement on June 22, 1973, admitted publicly that he was Bob Woodward’s source known as Deep Throat and Woodward immediately confirmed the assertion. However, Felt’s admission did not resolve the question of how Felt knew about the deliberate erasures on the tape. According to my timeline, White House lawyers first gained access to the tapes stored in Woods’ office on November 1, 1973, the day Nixon’s chief of staff Alexander Haig ordered White House lawyers to get the tapes ready to turn over to the court. Felt told Woodward of “deliberate erasures” only days later.How did Mark Felt know about the deliberate erasures on the tape when Rose Mary Woods herself did not discover the added soft-buzz erasures until November 6, 1973? Did the theorized White House lawyer somehow tip off Felt after making the soft-buzz erasures?

The Missing Pieces
With my minor updates to the 1974 Advisory Panel’s report on the tape gap, I now had a coherent explanation for the sequence of clicks, pops, and buzzes across the gap. One question remained: if Rose Mary Woods had erased the incriminating portions of the Nixon-Haldeman conversation on October 1, 1973, why would her discovery of the additional erasures on November 6, 1973, lead her to erase even more of the conversation? This seemed inexplicable. I found the answer, however, in Nixon’s own Memoirs. When Nixon was notified of the 18½-minute gap, he “…asked if we could still argue that it wasn’t covered by the subpoena.” There was the answer to the puzzle. Woods erased the last 1 minute, 9 seconds of the Watergate conversation in an attempt to avoid the subpoena. By erasing the last of the Watergate conversation, Nixon thought the subpoena might no longer apply since the tape no longer mentioned Watergate.


  1. The 1974 Advisory Panel Report and its associated Technical Notes, long out of print, are available on the Audio Engineering Society’s website:
  2. A copy of the audio file NARA provided me with is available online:

Phil Mellinger is the Chief Scientist at Trusted Knight Corporation, focusing on the reverse engineering and defeating of computer crime ware. Mellinger has worked in high-tech security for 30 years. He has served as the chief information security officer at First Data Corporation, as a systems analyst at the National Security Agency, as an Air Force Technical Special Agent (bugging/debugging), as NATO’s chief crypto engineer, and headed Federal efforts in digital signatures. Mellinger holds a number of patents and has graduate degrees in computer science from Johns Hopkins and George Washington University.