The House Select Committee on Assassinations stunned many people with its finding that Kennedy was "probably assassinated as a result of a conspiracy." The HSCA’s finding was based in large part, though not solely, on its analysis of acoustics evidence.
Mary Ferrell and others brought to the attention of the HSCA the existence of a police dictabelt which might contain sounds of the shooting in Dealey Plaza. One channel of police transmissions had been open during this time due to a stuck microphone switch. The HSCA hired two outside laboratories to analyze the dictabelt. While the roar of motorcycle engine noise drowned out much of the audio from a human listener’s perspective, distinct spikes in volume could be analyzed statistically, based on comparison between the pattern of such spikes and the echo patterns which the buildings in the Plaza would create. The experts found 6 impulses on the dictabelt whose echo patterns matched what one would expect from gunfire in the plaza. The HSCA then conducted a field study, placing microphones in Dealey Plaza and firing rifles fired from the Book Depository’s "sniper's nest" and from spots behind the fence on the grassy knoll.
In the end, the scientists found a solid match for a shot from the grassy knoll. Due to its medical conclusions that all shots which struck the motorcade came from the rear, a finding which has long been questioned, the Committee determined that the grassy knoll shot missed. This strange juxtaposition of the existence a shot from the knoll, but calling it a missed shot, opened the Committee to disbelief and ridicule from all sides.
The HSCA’s analysis was later called into question by a panel of scientists headed by Norman Ramsey. But that “debunking” has itself been called into serious question by the re-analysis of scientist D.B. Thomas, described most thoroughly in his book Hear No Evil.
Echo Correlation Analysis and the Acoustic Evidence in the Kennedy Assassination Revisited, by D.B. Thomas (PDF).
Hear no Evil: The Acoustical Evidence in the Kennedy Assassination, by Dr. Donald B. Thomas.
The Acoustic Evidence in the Kennedy Assassination, by Michael O'Dell.
Double Decker, by Stephan N. Barber.
The HSCA's Acoustical Evidence: Proof of a Second Gunman?, by Michael T. Griffith.
Acoustic Evidence Revisited, by Greg Jaynes.
Crosstalk: Synchronization of Putative Gunshots with Events in Dealey Plaza, by Dr. Donald B. Thomas.
The Bike with the Mike, by Don Thomas.
Silencers, Sniper Rifles & the CIA, by Carol Hewitt.
HSCA Hearings of 29 Dec 1978 on acoustics evidence. HSCA Report, Volume V, p.499.