None of the acoustic studies actually place the shots at specific frames. They came up with a timing and decided how that best fit with the Z-film. There have been differences of opinion on that even amongst people who believe the acoustics. Thomas believes that the alleged knoll shot hit at Z313 while that HSCA thought it missed.
There is nothing that can place the impulses at the right time. On the contrary, there are a lot of indicators (sirens, engine speed, background) that indicate the impulses don't happen at the right time or that the mic wasn't in Dealey Plaza at all.
The National Academy of Sciences panel that looked at this after the HSCA found proof that the impulses had to have happened after the assassination took place. They did this by using crosstalk, background voices from the other radio channel, that got picked up faintly by the open mic.
Thomas presented arguments to try to explain the timing problem, but his arguments were based on faulty data and have been refuted.
What were the impulses? I am working on that. It would be premature to get into it now. I believe there is a simple explanation for them. There are other similar patterns where nobody claims there are shots, and there are other causes that make patterns like the ones they were examining for echoes.