The HSCA based their conclusion on a Dictabelt recording from a Dallas motorcycle officers radio channel. The claim that there is a sound of a 4th gunshot on the tape has been repeatedly and conclusively discredited by numerous forensic audio examiners. Using an amateur film shot of the motorcade, the HSCA concluded that the recording originated from the motorcycle of police officer H. B. McLain, who later testified before the committee that his microphone was usually stuck in the open position. However, McLain did not hear the actual recording until after his testimony, and upon hearing it, he adamantly denied that the recording came from his motorcycle. He said that the other sounds on the recording did not match any of his movements. Sirens are not heard on the recording until over two minutes after what is supposed to be the sound of the shooting; even though, McLain accompanied the motorcade to Parkland Hospital immediately after the shooting, with sirens blaring the whole time. When the sirens are heard on the Dictabelt recording, they rise and recede in pitch (i.e., the Doppler effect) and volume, as if they are passing by. McLain also said that the engine sound was clearly from a three-wheeled motorcycle, not the two-wheeler that he drove. "There's no comparison to the two sounds," he said.
The only evidence that HSCA had for a second shooter was the Dictabelt sound recording. Four of the twelve HSCA members dissented to the HSCA's conclusion of conspiracy based on the acoustic findings, and a fifth thought a further study of the acoustic evidence was "necessary". Dissenting members of the committee included Congressmen Samuel L. Devine, Robert W. Edgar, and Harold S. Sawyer. Responding to a question asking how he would handle the Committee's report if he were at the Justice Department, Sawyer replied: "I'd file it in a circular file."
The FBI's Technical Services Division studied the acoustical data and issued a report on December 1, 1980 (dated November 19, 1980). The FBI report concluded that the HSCA failed to prove that there were gunshots on the recording and also failed to prove that the recording was made in Dealey Plaza. In fact, using the techniques of the previous investigators, the FBI matched a gunshot recorded in Greensboro, NC in 1979 with the sound that was supposedly a shot from the grassy knoll – purportedly suggesting that the initial investigation's methods were invalid.
The United States Department of Justice reviewed the HSCA report and the National Academy of Science's study of the sound evidence. It reported to the Judiciary Committee on March 28, 1988, and rebuked the HSCA's conclusion of a probable conspiracy.
In 2003, an independent researcher named Michael O'Dell reported that both the National Academy and Dr. Thomas had used incorrect timelines because they assumed the Dictabelt ran continuously. When corrected, these showed the impulses happened too late to be the real shots even with Thomas's alternative synchronization. In addition, he pointed out that the claim of a 95% or higher probability of a shot from the grassy knoll was logically incorrect. It was a 5% chance of finding a match in random noise and did not consider other possible causes.
A November 2003 analysis paid for by the cable television channel Court TV concluded that the putative gunshot impulses did not match test gunshot recordings fired in Dealey Plaza any better than random noise. In December 2003, Thomas responded by pointing out what he claimed were errors in the November 2003 Court TV analysis. However, in 2005, five acoustic experts (Linsker, Garwin, Chernoff, Horowitz, and Ramsey) reinvestigated the acoustic recordings and noted several errors made by Thomas.
In 2013, Professor Larry J. Sabato, Ph.D. commissioned a study on the Dictabelt recording using more modern analytical techniques. The report concluded that the recording did not contain sounds of the assassination gunfire and that it would be of "doubtful utility" as evidence to prove or disprove a conspiracy.
90% of Dealey Plaza spectators reported to have heard 3 shots or less]
