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Audacity audio from slingshot

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Unread postAuthor: Technician1002 » Mon Aug 31, 2009 8:17 pm

Thanks for the file. It was most helpful. Now my take on it as far as I can tell.

There is some noise prior to the shot, but that's just people moving. Take a listen. Then there is the release of the band. This can be seen as a sudden noise prior to the first impact noise which is the sling reaching the end of travel. See first photo to see the launch.

The second impact is very abrupt. It's definitely the sudden noise of impact.

I expanded this relevant parts of the to events so the timeline is clearly visible. The band contraction time is consistent with a flight distance of 20 feet to the target.

The launch time can be seen from 0.045 to .060 concluding with the release of the ball. The impact is at .315 for a 20 foot flight time of .225 seconds. This is relatively slow. The time from release of the pouch to release of the ball is 15 ms. Not bad.

Click on photos for full size to read the time line.
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Sling1.jpg
The launch sequence. Release and noise from end of travel.
Sling2.jpg
The very abrupt start of the impact noise.
Last edited by Technician1002 on Mon Aug 31, 2009 11:28 pm, edited 1 time in total.
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Unread postAuthor: boyntonstu » Mon Aug 31, 2009 8:44 pm

Technician1002 wrote:Thanks for the file. It was most helpful. Now my take on it as far as I can tell.

There is some noise prior to the shot, but that's just people moving. Take a listen. Then there is the release of the band. This can be seen as a sudden noise prior to the first impact noise which is the sling reaching the end of travel. See first photo to see the launch.

The second impact is very abrupt. It's definitely the sudden noise of impact.

I expanded this relevant parts of the to events so the timeline is clearly visible. The band contraction time is consistent with a flight distance of 20 feet to the target.

The launch time can be seen from 0.045 to .060 concluding with the release of the ball. The impact is at .315 for a 20 foot flight time of .225 seconds. This is relatively slow. The time from release of the pouch to release of the ball is 15 ms. Not bad.


2.66 ft/0.015 = 177 fps pouch speed.

The 20 feet/.25 or 88 fps is half the pouch speed.


I wonder what slowed the speed of the marble?


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Unread postAuthor: Technician1002 » Mon Aug 31, 2009 9:32 pm

boyntonstu wrote:
Technician1002 wrote:Thanks for the file. It was most helpful. Now my take on it as far as I can tell.

There is some noise prior to the shot, but that's just people moving. Take a listen. Then there is the release of the band. This can be seen as a sudden noise prior to the first impact noise which is the sling reaching the end of travel. See first photo to see the launch.

The second impact is very abrupt. It's definitely the sudden noise of impact.

I expanded this relevant parts of the to events so the timeline is clearly visible. The band contraction time is consistent with a flight distance of 20 feet to the target.

The launch time can be seen from 0.045 to .060 concluding with the release of the ball. The impact is at .315 for a 20 foot flight time of .225 seconds. This is relatively slow. The time from release of the pouch to release of the ball is 15 ms. Not bad.


2.66 ft/0.015 = 177 fps pouch speed.

The 20 feet/.25 or 88 fps is half the pouch speed.


I wonder what slowed the speed of the marble?


BoyntonStu


From the sound, it may have struck part of the slingshot and lost energy at the end of the stroke. Maybe the pouch bunched ahead of the marble. The launch does sound a lot like an impact. Maybe the pulleys need further separation for a proper release.
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Unread postAuthor: POLAND_SPUD » Mon Aug 31, 2009 9:46 pm

marbles are really poor projectiles -> that's a fact :wink:

a month ago or so I modelled them in chairgun (an external ballistics program for airgunners ) and it showed that they retain 50% of their muzzle energy at ~30 meters clicky clicky

I hope you realize that recording of shot&impact sound doesn't give you muzzle velocity... it isn't as serious on 20ft as it is on greater ranges but still it has an effect



it's a great programme really.. but rag is right that it has some restrictions... Nevertheless, I think that you can learn a lot with it
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Unread postAuthor: Technician1002 » Mon Aug 31, 2009 11:38 pm

POLAND_SPUD wrote:marbles are really poor projectiles -> that's a fact :wink:

a month ago or so I modelled them in chairgun (an external ballistics program for airgunners ) and it showed that they retain 50% of their muzzle energy at ~30 meters clicky clicky

I hope you realize that recording of shot&impact sound doesn't give you muzzle velocity... it isn't as serious on 20ft as it is on greater ranges but still it has an effect


it's a great programme really.. but rag is right that it has some restrictions... Nevertheless, I think that you can learn a lot with it


The other important thing to consider is 1/2 speed does not = 1/2 energy. Energy goes up as a square of the speed. 1/2 speed is 1/4 the energy.

Loss of half the energy doesn't mean loss of 1/2 the speed. it is the loss of 1/3 the speed.
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Unread postAuthor: POLAND_SPUD » Tue Sep 01, 2009 3:11 am

yeah but it's just a fact that probably 95% of members know

however, it's difficult to realize all real life implications when all you know are just a few equations...

being able to put some data in and get something like this ->Image

helps a lot... even if everything you put in is not 100% precise you do get a lot of info from that
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Unread postAuthor: Technician1002 » Tue Sep 01, 2009 3:27 am

POLAND_SPUD wrote:yeah but it's just a fact that probably 95% of members know

however, it's difficult to realize all real life implications when all you know are just a few equations...


IMO it helps a lot... even if everything you put in is not 100% precise you do get a lot of info from that


The models are very beneficial in getting expectations for performance. Often the performance deviates from the model in unexpected ways. Measuring actual performance can reveal design or construction flaws.

I do use the models and then test and tweak. Modeling the t shirt launcher indicated a longer barrel would be most helpful, but reality showed the last 3 feet contributed little to the acceleration on the 10 foot barrel.

Some things are hard to model. On my 2 inch valve, only testing revealed that the 3 inch barrel outperformed the 2.5 inch barrel with shirt launches. The smaller barrel was predicted to be a better performer.

Shirt tumble with a long skinny roll really cut the range. The shorter fat roll for the 3 inch barrel worked much better. It is difficult to model the flight pattern of a rolled up t shirt. Breaking 300 feet was much harder than I anticipated. Hitting 400 feet is very difficult.

I did get lucky once with a backwind and got a 500 foot shot. :D That was an exception, not the rule. It's amazing what a 20 mile an hour wind will do.

What was unexpected is the Marshmallow Cannon with the very small tank and only a 1 inch valve can chuck t shirts 200 feet with the 36 inch long barrel. This was a huge suprise to me. :shock:
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Unread postAuthor: Ragnarok » Tue Sep 01, 2009 5:54 am

@Poland_Spud: Talking about Chairgun, where are you getting the airgun ballistic coefficients for marbles and steel/lead balls?

You're using the right ones (give or take a smidge), but I'm interested to find out where you found them. Are you using a chrony and calculating them yourself, or are you just using an equation like this one:
Mass in grams/(3.5 * Cd * Diameter in mm<sup>2</sup>)

boyntonstu wrote:The time delays are equal and are ignored.

Sometimes this isn't the best idea. At short distances with noisy cannons, the muzzle blast (or echoes of it) can make it hard to make out the exact start of the impact sound.

By keeping the recorder near (or even on a line behind) the cannon, then that delays the impact sound by Distance/SOS, which can prove useful in differentiating the muzzle blast and impact sound.
Sure, it's an extra couple of numbers in the equation, but it can actually get you better results.
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Unread postAuthor: POLAND_SPUD » Tue Sep 01, 2009 11:51 am

@rag
when you move the cursor over the BC the programme displays its cd value...

so I simply used that and manually entered different BCs to get a cd of 0.48 or so (I don't really remember now if it was 0.46 or 0.48 )

yeah it's not the most accurate way of doing it - you don't have to tell me that.. but since no one is sure what cd value spheres have it's good for getting a general idea of how different projectiles perform

edit
Pic added..
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Unread postAuthor: jimmy101 » Wed Sep 02, 2009 11:51 am

Tech's analysis is probably correct but it points out that there is a need for a nice clean signal when the ball is released from the pouch. Since that doesn't make much noise then something else is needed to get decent, and believable, results.

Need something similar to the bang you get when the ammo "uncorks" the barrel in a pneumatic or combustion gun.

Perhaps hang a piece of paper a couple feet in front of the slingshot, far enough out that the pouch can't reach it. The pop when the ammo tears through the paper should be a much cleaner and stronger signal than trying to tease the correct signal out of the slingshot noise. If the paper is light enough it should'nt have much of an affect on the ammo's velocity.
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Unread postAuthor: Technician1002 » Wed Sep 02, 2009 12:07 pm

jimmy101 wrote:Tech's analysis is probably correct but it points out that there is a need for a nice clean signal when the ball is released from the pouch. Since that doesn't make much noise then something else is needed to get decent, and believable, results.

Need something similar to the bang you get when the ammo "uncorks" the barrel in a pneumatic or combustion gun.

Perhaps hang a piece of paper a couple feet in front of the slingshot, far enough out that the pouch can't reach it. The pop when the ammo tears through the paper should be a much cleaner and stronger signal than trying to tease the correct signal out of the slingshot noise. If the paper is light enough it should'nt have much of an affect on the ammo's velocity.


I have actually been considering a variant of this. Instead of a microphone, a coil would be wound around a small magnet and the magnet would be suspended very close to a sheet of tin foil. When the foil moves (from impact) the magnetic field will induce a current in the foil. The current in the foil creates an opposing magnetic field. The opposing magnetic field is picked up by the coil on the magnet as a change in the field and creates a current. This is fed to the capture card in place of the microphone. A pair of foils on a pipe 1 foot long finishes the chronograph.

This non contact magnetic pickup is common in industry to for example to measure the crank angle in a car by counting teeth on a gear as it rotates. The pickup is a magnet with a coil of wire on it. It is immune to heat, grease, dust, light, etc.

Link for more on these sensors is below. They are easy to build. Wire, magnet, conductor of iron, brass, copper, or aluminum, that can move.
http://www.sensorsmag.com/articles/0298/mag0298/main.shtml
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Unread postAuthor: jimmy101 » Wed Sep 02, 2009 2:18 pm

Technician1002 wrote:I have actually been considering a variant of this. Instead of a microphone, a coil would be wound around a small magnet and the magnet would be suspended very close to a sheet of tin foil. When the foil moves (from impact) the magnetic field will induce a current in the foil. The current in the foil creates an opposing magnetic field. The opposing magnetic field is picked up by the coil on the magnet as a change in the field and creates a current. This is fed to the capture card in place of the microphone. A pair of foils on a pipe 1 foot long finishes the chronograph.

This non contact magnetic pickup is common in industry to for example to measure the crank angle in a car by counting teeth on a gear as it rotates. The pickup is a magnet with a coil of wire on it. It is immune to heat, grease, dust, light, etc.

Link for more on these sensors is below. They are easy to build. Wire, magnet, conductor of iron, brass, copper, or aluminum, that can move.
http://www.sensorsmag.com/articles/0298/mag0298/main.shtml

Yep, just use a Hall Effect detector (~$0.50) mounted near the sheet and glue a magnet to the sheet.

Or, glue a piezo transducer or a mic to the sheet.

No real advantage to making your own detector since a magnet in a coil is a common enough device, wether it works via the method you describe or it is just a microphone (which works about the same way). Indeed since what you are trying to record is in the audio domain you might as well use a device specifically designed for audio. One that interfaces easily with the sound card data acquisition system. So there are a number of cheaper and simpler ways to get the data.
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Unread postAuthor: Technician1002 » Wed Sep 02, 2009 5:18 pm

Or, glue a piezo transducer or a mic to the sheet.


Using an unattached detector is useful when you wish to change the foil often. Wire is cheap and an old pair of broken headphones provides free small magnets. The amplitude of the signal is well within the levels provided by low impedance microphones so interfacing is not a problem to the sound card.

I do a lot of stuff with a "free" cost and often refer to many of the projects at Junkyard Wars. Free and low cost is important to many spudders.

Using a hall sensor requires a bias supply and the proper selection of the sensor. Some have a built in schmitt trigger and do not output an analog signal. Ones cannibalized from a computer fan are often of this variety and useless for a detector as the magnetic signal change is too low to trigger it.

Microphones work, but add more ambient noise from the air while using the aluminum sheet as the diaphragm provides much better signal to noise ratio for more accurate detection of the collision event. :!:

So what's easier and cheaper to obtain? Microphones, analog hall sensors with battery boxes and batteries, or broken earbud headphones and a 1/8th inch stereo headphone cord and surplus cat 5, telephone cord, or other cable for the wire.
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Unread postAuthor: boyntonstu » Wed Sep 02, 2009 6:00 pm

A few tambourine disks at each end should make enough noise to be seen in Audacity.

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Unread postAuthor: POLAND_SPUD » Wed Sep 02, 2009 6:16 pm

that slowmo casio cam that everyone is talking about would make a pretty good chronograph, don't you think ?
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