Basu if you look at the right side of the selected area in the picture above you see a down spike at 4.6365 seconds and an up spike at 4.638 seconds.
The difference between those two spikes is 0.0015 seconds.
I have some observations to make about such short time sequences
1. Even at 1,100 feet per second, sound could have travelled only 1.6 feet (half meter) in that span of time. An air rifle pellet would hardly have gone much further. So the actual difference in fps is too small to be significant. It really does not matter whether you choose down spike or up spike.
2. When you record the sound - a sound wave hits the diaphragm of the microphone initially as a compression or rarefaction. This causes a mechanical movement of the diaphragm. There will be some energy loss and time lag betwen sound molecules hitting the diaphragm and the movement of diaphragm caused by that. This in turn sets up an electric current and that current is sampled by the computer and then represented as a sound wave graph. This change of sound energy to mechanical to electrical to a graph is roughly accurate but cannot be considered a fail proof representation of sound of very very short duration - such as 0.015 seconds. There are too many sources of inaccuracy.
The only way anyone can achieve any degree of accuracy is to simply take the first and tallest (up) or deepest (down) spike - whichever comes first and take several (3 to 10) measurements and find the average, The result may not be exact but it will be a good approximation.
