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Most users ever online was 101 on Fri Nov 22, 2013 4:13 pm
Marble, 20ft, 400 psi
What did you need analysing exactly?
If you need velocity calculated, you need to shoot at a hard target (like a sheet of metal) that will make a distinct noise, a considerable distance away from the launcher, and the distance between the launcher, microphone and target needs to be known.
Muzzle to target 20 ft.
Mic at the center.
I can hear the 2 distinct sounds but I cannot differentiate the 2 pips.
There doesn't appear to be a distinct peak for projectile impact, going from mylimited experience with this method it's probably going too fast for the distance in question. Can you try a shot with say 50 psi and see what happens? What are you shooting at?
There doesn't appear to be much extractable in the file since it was grossly over-recorded.
Even looking at a pitch or spectrum display show very little.
You need to;
1. Drop the gain on the recording down so it doesn't go off scale. I would drop it 5x to 10x.
2. Hit something that is hard enough to make a distinct sound, like a hunk of wood. To avoid a direct ricochet put the target at an angle.
3. Put the MIC at the target. Firing sounds confuse target sounds but the reverse isn't true. It's easy to id the muzzle sound, it helps to make the target sound so loud there is no question of confusing target with muzzle or echo sounds. Measure the distance so you can correct for the SOS.
I slowed the recording down 10X and there might be a pair of sounds ~0.06 seconds apart in the original recording. For 20' that would be 320 FPS. The second sound could be an echo or something else.
In Audacity, Select all, effect menu, change speed selection, Percent change -90.000. Takes the recording from 0.44 to 4.4 seconds total length.
This is a video of the same shot.
The camera mic is much closer to the plywood.
You can see the indent made from the marble impact and also you can see the wadding falling to the right.
A wooden board is probably not the best target to generate a clear impact peak, some thin metal sheet would be much better.
Using Audacity as already shown the details are in the shot. When the time is expanded the reverberation in the room blends the noises together, but the change in sound is very clear. I took the liberty of expanding the area of interest. There is a hiss prior to the bang so this appears to be a traditional piston cannon with a pilot valve. The pilot is relatively slow opening as evidenced by the rate of sound increase. At just after 0.04 seconds the main valve opens. It is unclear at which point after that the projectile exits the barrel. It didn't take long. While the noise of discharge is still happening the impact is very sudden with a high frequency component that is seen at about 0.1 seconds. I think this is the time you are looking for. The time between the shot at about 0.042 (Note the step in the waveform as the initial muzzle pressure wave hits the mic before the blowing hiss of discharge) and impact at 0.102 seconds. This gives a flight time of 0.06 seconds for an average speed of 333 ft/sec.
The change in volume would have been more evident if the record level was set lower so the waveform was not clipped.
333 fps is about half the speed that GGDT predicts.
If I had to guess, the reason for the poor performance is my slow opening of the 3/8" ball pilot valve.
What do you think might be the reason?
Need to analyze the cannon and not just the sound. Care to post the cannon construction? Piston weight, piston OD/Valve seat ratio, flow path, etc all contribute to the overall performance. A very small chamber and a relatively large piston EQ can quickly bleed down the chamber pressure.
That's exactly what I got slowing down the audio 10X. There are two fairly clear "thumps". I like your analysis better though since it is pretty clear from your analysis that the second thump isn't an echo.
I was giving the waveform more study. The waveform I posted yesterday in a couple posts ago shows the hiss boom of the pilot and main valve opening and the projectile exit with the resulting muzzle pop. I am concerned about the amplitude of the pop of the projectile exiting the barrel right after the hiss. It did not clip the recording. This low amplitude may indicate the valve is restrictive somehow. This can be either the piston ratio is poor so the valve re-closed early, didn't open all the way or bounced off the bumper and re-closed. The valve performance is most likely affecting the performance from what GGDT predicts.
For comparison here is a hiss boom from one of my launchers. This is a dry shot so there is no second bang against a target. On this shot the pressure wave from the muzzle clearly clipped in the recording and flat topped. Edit, I added the sound so you can hear this shot.
At 400 PSI you should have double the speed and 4X the energy, enough to punch a hole in the plywood. I've been punching holes in plywood with 1 inch gumballs on 100 PSI and Splitting 2 X 4's with jawbreakers.
Again, it may be time to analyze the launcher and the valve for performance issues.
As for slow opening of the ball valve, that should not be an issue unless the piston has excessive leakage from an oversize EQ port. In the wave below, the valve was opened slowly also. A close ratio (seat to piston OD) valve will pop open even with a slow pilot valve.
Perhaps this photo will help.
The piston will retract and almost disappear.
The fact that there's very little difference between the piston and barrel diameter means that good piloting becomes a bit more critical.
Is that me, or does that piston shove up the T piece so far that it has a direct gap into the pilot volume? Or does the piston have a second piece behind it?
Any chance of a pic of the piston? (Always take pictures from parts before making them unreachable)
Also the piston diameter seems very very close to the porting. This is only recommended when your piston is very tight and your pilot valve is near overkill, because this way the piston opens very late, and more air may leak out of the pilot volume before it snaps open. However, if your piston is very tight and your pilot valve is good, a "snapping" piston gives a faster opening and possibly less bounce due to the pilot volume being at a lower pressure when the valve starts opening.
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