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    Measuring arrow speed without a chrony?

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    rolynd
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    Post by rolynd on Tue Apr 24, 2012 7:41 am

    What alternatives do I have measuring arrow speed without a chrony?
    I have heard about two alternative methods
    1. sound based
    2.ballistic pendulum
    1. shoot through 2 hanging paper targets, record the impact sound with a microphone in the middle between the two targets, with the program audacity you can measure the exact time bewteen impacts and with that and the distance between targets you can calculate arrow speed. But somehow the speed of sound changes with ambient air temp. afaik? I thin this has to be included in the calculation?
    How exact is this method and what do have I to watch for in making the least measuring error?
    2. shoot an arrow of known mass into a pendulum of known mass , measure dislocation of pendulum and calculate arrow speed from there.
    Which method is preferrable and has anyone used one of these and can give me some tips?
    Are there other low-tech methods avaliable?
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    Post by stoneagebowyer on Tue Apr 24, 2012 8:41 am

    I have thought about this myself, but I think it would be easier to find an inexpensive chrony on ebay, I think you can find them for less than $100, or contact a local archery club about borrowing a unit. Another route is finding a 3D or other archery competition and seeing if you get an archer with one to meet up with you or at the event.
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    Post by Basilisk120 on Tue Apr 24, 2012 8:47 am

    I think that the preferable method would be the one you are set up to deal with.

    Do you have a preference as to which method you want to do?



    I think if you have the equipment the first method might be easier and more accurate. don't have to worry about friction in the hinge.



    With the first setup the challenge can be to hear the paper being torn with clearity can be a challenge. Something like tinfoil or a noisier medium might work better. I wonder if tensioning the paper would make it "Snap" louder/cleaner when hit. Or using blunts instead of field tips would help.

    the other trick would be to place the microphone in the middle between the paper. That way the speed of sound and time lag to the microphone cancels out. The time lag cancels out because your interested in the time difference not the actual start and stop times. It should be pretty exact assuming you know exactly when the arrow hits each piece of paper.



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    Post by rolynd on Tue Apr 24, 2012 9:30 am

    Yes, I could just buy a chrony and be done with it, but here a shooting chrony would cost me ~120€
    If I can do this with a simple setup and my laptop it would cost me nothing .
    the laptop records sound with 44,1khz (cd-quality) so resolution is not the problem.
    As I have not tried this before i was wondering about how exact you could distinguish the paper being hit, I guess the fletching passing through the paper does make a sound also. Maybe tin foil makes a better sound.
    Aa said by basilisk one mic in the exact center canceles out sound/time lag but this may be tricky to place exactly right?.
    Maybe it is better to use two microphones each set up directly at a foil/paper and record the channels individually?
    Just dunno, I guess I have to try something...
    What kind of distance between the frames would you suggest?
    If this method can give me +/- 10fps it would be good enough for me but I have no idea about how big are the measurement errors and how to avoid.
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    Post by mac on Tue Apr 24, 2012 9:36 am

    Rolynd,

    For 70 bucks, you can have one from Wallmart. http://www.walmart.com/ip/F1-Chronograph/4382668?sourceid=1500000000000003260410&ci_src=14110944&ci_sku=4382668 and it will tell you your velocity more accurately than anything you are likely to rig up.

    If you want to tinker with other methods, you should do so, but if all you want to know is velocity, and your time is worth anything, I think you would do better to just buy a chronograph.

    In my opinion, you should buy the chrony and spend your time building the ballistic pendulum. When you were done, you would have a very powerful set of analytical tools.

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    Post by mac on Tue Apr 24, 2012 9:43 am

    Opps! I see that Wallmart is not an option for you.

    If your tinkering time is your pleasure, the paper and microphone method will certainly work. I guess I would see if one mic would serve before trying two mics. With one mic, the speed of sound would have to be figured in. (but you knew that).

    edit. Doh! my bad, I see that you and Basilisk are already talking about the speed of sound thing. I'll leave you guys to it then.



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    Post by Basilisk120 on Tue Apr 24, 2012 2:33 pm

    Since work is being boring today I have spent a bit of time thinking about this.

    Running some numbers to try to figure out errors and what are important values. It looks like if your close to being centered you should be fine. Assuming my math isn't terrible, being 6 inches off center will give you an error around 2% (Actually don't take the 2% as gospel it would depend on how far apart the plates were and that was made assuming 3 feet apart). The faster the bolt the greater the discrepency between calculated and actual speeds. You should be able to get it centered to less than an inch so the error from that should be negligable.



    Using two mikes could work. But a possible issue would be having a reference start time. I suppose if both tracks are being recorded by the same program and both start when the record button is pressed then its likely a non-issue.

    Anouther solution, which could help if you only have one input on the laptop, could be to wire the mikes togather so the both feed into the same channel.

    Obviously place both mikes the same distance from the plate. i.e. centered on the bottom looking up.



    if your wondering how far apar to set the plates? The farther apart the less of an effect an off center mike will have but too far apart and the bolt could slow down between the plate. i think 2 to 4 feet could be a good range. You would want the plates, at minimum, an arrow/bolt length apart to help desinquish between the hits so it not all one sound.



    Let see what else. If your really into tinkering with electronics There might be some benefit of thinking of microphones (and speakers can be used as mikes) as linear accelerometers and Audacity as a vibration recorder. Thinking you could take the mike apart and mount the correct parts to the paper and just record the vibration that way. Thinking the inital hit would really vibrate the paper expecially if its tight.

    Just thinking of different ways to use the same parts.



    That should be enough rambling for one day. hopefully you can gleam something usefull form that.



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    Post by stoneagebowyer on Tue Apr 24, 2012 3:06 pm

    That is a bit expensive. I am not sure, but I think I have heard of smart phone apps that are chronographs. That may be an option.

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    Post by Gnome on Wed Apr 25, 2012 11:38 am

    Thanks, Mac. I'm not usually a fan of Walmart, having grown up in a small town and watched nearly all the businesses on the square board up their windows after the big box moved in on the edge of town, but I just ordered one of those chronys. The price is great, but I was really sold when I found out that they ship for free to Fedex Office locations in certain areas, including mine, and there's a Fedex Office a block from my house so I don't even need to visit the despised mega-super-small town business killer in person! Convenience, that's how you seduce folks to the dark side. Convenience!
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    Post by hullutiedemies on Fri Jul 27, 2012 9:54 am

    Quick and cheap and a bit unreliable:

    A freefalling object drops 120 cm during first half second.

    Just launch bolt from waist height flat forward.
    And then pace out the distance it flies.

    I use this all the time. Gives at least an idea.
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    Post by Phoghat on Sat Jul 28, 2012 2:32 am

    In dry air at 20 °C (68 °F), the speed of sound is 343.2 metres per second (1,126 ft/s). This is 1,236 kilometres per hour (768 mph).
    If you wire both mikes into the same recording apparatus ( computer ) and using audacity, it will give a very rough approximation of the speed.
    A long time ago, when I coached Little League Baseball, I used something similar to find out how fast a kid could pitch. Cheap, and rough, but adequate.

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