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[derangedhermit]

I've read that the best location for a depinger is halfway down the pressure tube. That means that the tube is operating as a free-free standing wave resonating tube. This is kind of surprising, since one end is fixed to a fair bit of mass (the rear end) and the front has little mass attached to it. I would expect it to vibrate more like a free-fixed tube, in which case the depinger could go at the end, or about 1/3 of the way down towards the end. (You want to put the depinger where the antinodes are.)

Of course it seems to start out as a traveling wave, from the end where the valve is, but my understanding is that a traveling wave in a closed tube is quickly converted to a standing wave.

Anyway, if anyone knows more about pressure tube vibration modes, I'd like to hear about it. And if the free-free case is somehow true, then further ping reduction could be had by damping the second harmonic at 1/3 of the distance from either end (by adding a second depinger). Usually harmonics beyond that are so small in energy they can be ignored (1/4, 1/5,etc).

There used to be a freeware audio program where you could provide sound input and look at the waveform. Audacity looks like it would work.

[RayK]

Why don't you try it both ways and compare?

Let us know what you find.

Ray

[ward360]

So my marauder was sitting next to me and my Netbook in front of me (Audictiy installed already) and you got me thinking

I took a sample of the ping (no Depinger installed)

What I ended up with was from 2800 psi to 2100 psi 41 cycles in .081 sec
1\.081=12.34567901234568
12.34567901234568*41=506.1728395061728
So we are looking at a 506hz tone
It seemed to stay a consistent frequency throughout the ping cycle. Which is amazingly long, .56 seconds at 2100 psi (longer with more Pressure)

Humm is that right? I found this page. http://www.phys.unsw.edu.au/jw/hearing.html
500hz sounds just about perfect!
The 1/2 wave of 500 hz is 250hz, go to this page and bounce back and forth between the two, my ears say that -3 to -6 db down on 500 hz is the same loudness as 250hz

Our hearing sensitivity is peak in the 2000hz to 5000hz range, 500 is way closer then 250hz to our peak sensitivity range. And the 500hz tone is very harsh on my ears! The 250hz range is a much softer tone.

Instantly by installing a Depinger we are dropping the frequency of the ping out of our hearing range by a lot! not counting the Vibration absorbing quality of the vinyl tubing against the steel!

This Weekend I will probably Deping my Marauder. I think I will do it in thirds! I will sample it and my friends 1/2 Depinged Marauder and repost.

I suspect that between the Dampening effect of the Vinyl and the Wave dropping away from our peak sensitivity we will not notice a difference between 1/3 and 1/2. I will sample anyways!

I will repost when I know more!

[cappy227]

Very interesting, and a good topic for my recent purchase: a Talon Tuned resevoir extension for my .25 M-rod, which at this point is not de-pinged.

With the resevoir extended, where would be he best place to slide the vinyl tubing?

[Motorhead]

I happened to have some silicone tubing of @ 1/4" wall thickness a tad larger in diameter than air tubes I.D.
Cut a 1 1/2" long section and pressed it down the tube lubricated with a light film of silicone grease.
Fit was quite snug and the ping is 90% gone. Best part a MUCH cleaner install than rolled up plastic tubing

Currently chasing down a 500# per 12 hour leak someplace within rifle ... so about to go all the way into the guts, deburr the parts with o-rings and surfaces they slide over getting assembled. New o-rings etc ...

Then I am going to do a 2 piece de-pinger using 2 each 3/4" pieces splitting the air chamber into thirds. I too will report my findings in a week or so.

[WarBIrdII]

I used a 4" piece of vinyl tube. The ends touching from end to end (O) inside the tube. I didnt do the spiral (@) method that also seems to work. I have no "Ping" any more in either the .177 and the .22 Mrod. It is dead center in the tube on both. I could of gone smaller, but another member here suggested usng 4". I am happy with my results. A dead quiet .177 with baffle mods, and a quieter .22 with baffle mods.

[Motorhead]

I happened to have some silicone tubing of @ 1/4" wall thickness a tad larger in diameter than air tubes I.D.
Cut a 1 1/2" long section and pressed it down the tube lubricated with a light film of silicone grease.
Fit was quite snug and the ping is 90% gone. Best part a MUCH cleaner install than rolled up plastic tubing

Currently chasing down a 500# per 12 hour leak someplace within rifle ... so about to go all the way into the guts, deburr the parts with o-rings and surfaces they slide over getting assembled. New o-rings etc ...

Then I am going to do a 2 piece de-pinger using 2 each 3/4" pieces splitting the air chamber into thirds. I too will report my findings in a week or so.

Stall out on the 2 piece de-pinger. Found my leak at gauge threads and rifle is now Air Tight, so going into rifles guts put on hold for now.

[ward360]

So I have done my research and taken images I added text to the images so they should be Pretty self explanatory.
I really started thinking about sound waves and reflections of sound waves and quiet rooms..

This is an Images of a quiet room the do sound testing why not make a depinger like a quiet room?



For those of us with a mill and some Delrin It wont be very hard. It would be better to use a soft material, like foam but I am not putting any of that in my gun!

Anyways here is the sample of the full shot and ping cycle of our Marauder's! it is 1.3 seconds of time!



This one is where I figured out the frequency. The temperature is different the day I took this sample so the frequency has changed slightly. At about 70F I calculated 506hz. At 50F I calculate 465.9hz. It will change because the speed of sound changes with temperature.



The next one is .5 seconds after the shot



here is a zoom shot of .1 to .125 seconds after the shot



and a larger zoom of .1 to .111 seconds after the shot



I am still going to do a double Depinger but I am going to do them out of delrin with a hole drilled diagonally through them and a faceted surface.

food for thought... at 70F the wavelength of a 500hz sound wave is 2.26 feet. By guesstimating with a tape measure externally our pressure tube is 1.125 feet long. So it appears we are working on a half wave and the tube resonates there nicely.

What I get out of this is the primary goal of a depinger is to break up the sound waves and redirect them into different waves by breaking up their energy into smaller forces that can't get out of the metal pressure tube as a audible sound.



Does that make sense? In short, The more irregular shapes and surfaces in your pressure tube the better! And a breakup of the main tube length!

[Motorhead]

Good stuff there .... sure beats seat of your pants trial and error !

[pb177]

Gentlemen,
For what it is worth, I have been installing 2 vinyl depingers in my air tubes at approximately 1/3 internal tube length intervals.
I have had great results. I don’t have diagnostic equipment, just my ears, however there is a marked difference in what I can hear.
Previously, I had used 1 ½”-2” long vinyl tubing about half way down the tube as many others have.
It worked well, but I thought about the wasted space within the airtube and shot count. I remembered some lessons learned in my Physics classes about pressure waves and breaking them up, so I took 2 thin pieces of tubing and placed the first ahead of the valve about 1 ½” less than 1/3 the inside length of the airtube and the other piece halfway down the remainder of the tube. Leaving 3 unequal lengths of space to break the wave’s frequency. I reduced the amount of vinyl in the airtube by about 75%.
I picked shot count back up and the rifle was noticeably quieter than with the single vinyl tube.
I would encourage you to give this a try and record and report your results. I think I have keen hearing but it would be nice to see quantifiable results.

Thanks for sharing your results so far.
Best of luck and success with your project.
Paul in N.TX