For those who are uninitiated "Audacity" is a free sound editor. The sound of an air rifle shooting and hitting a target at a known distance is recorded and the recording is opened using Audacity. This shows a graphic display of the sound wave. Usually the sound of rifle and sound from target appear as two separate triangular shapes (the triangles are lying on their sides with base vertical on the left side).
The image below is of a sound recording I made yesterday of an IHP 35 hitting a metal target at 10 meters as seen on Audacity.
The recording is a stereo recording and that is why you see two blue triangles on two lines one above the other. The triangle on the left represents the sound made by the rifle and I have marked its length (duration) with a fat red line. The triangle on the right represents the sound from the metal sheet target. I have marked that with a fat green line.
Note that both sounds start very suddenly - "BANG!" and then the vibrations continue for some time. The left/rifle sound probably starts with the sound of the trigger sear being released, continuing as the scraping sound of the spring and piston, ending with the sound of pellet leaving super-imposed on sound of piston hitting the end of the receiver, followed by vibrations sounds of spring, stock and barrel. Because of all these combined sounds, the left/rifle sound is not a good triangle. It is like a half-egg shape and only after a short time gap the sound begins to fade down towards zero.
The right target sound starts with pellet hitting target followed by the gradually fading vibration of the target. This is a single "Tannnn" sound that creates a nice triangle shape as it fades.
The problem is as follows. We must know exactly the point at which teh pellet leaves the barrel and the point at which the pellet hits the target. Pellt hitting target is easy - the exact point when the tagert sound starts is the time when pellet hits target. But when exactly did the pellet leave the muzzle?.
If you measure the tme from start of rifle sound to start of target sound you get a muzzle velocity of about 90 m/sec. but the actual measured velocity by Chrony when I fired this particular shot was about 150 m/sec. So you cannot take the start of rifle sound as the point of measurement. What I did was to calculate the pellet travel time using the Chrony measurement. The value is 0.67 sec. I then drew a fat magenta coloured line from the point where the target "tannn" sound starts - to some point in the middle of the rifle sound - and that magenta line represents 0.67 seconds. Now how the hell can anyone look at the rifle sound and say "This is the point where the pellet left the muzzle and this is the point where I wil start my time measurement"?
Here is my suggestion:
The left side/rifle sound remains at the same high volume for a short distance after it starts. Then it just begins to fade down towards zero. When you want measure muzzle velocity using Audacity, start measuring time from the point where the rifle sound volume starts coming down and measure up to the start of the target "tannn" sound. In fact the magenta line coincides with this suggesting that the "true secret" of measuring pellet travel time on Audacity lies in measuring from the point at which the rifle sound begins to fade in volume.
What do you think Basuda?
