Interesting results comparing 2 different length LDC's.

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NeilClague

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Apr 1, 2015
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I do know from experience that when you dry fire an air gun with an LDC on it, it is normally not as quiet as when you fire a pellet. I recently built 2 identical LDC's, the only difference was the length, one was 7 inches long, the other was slightly over 8 inches long. When I dry fired my BSA with the shorter LDC on it, it was quieter than the longer LDC, however when I went to my bench and shot pellets through both of them the longer unit was quieter. Now I have no idea why this would be, why would the shorter one be quieter dry fired than the longer one? This has me slightly baffled, I do find it interesting though and would love to get opinions on why it might be so? Neil.
 
Are you using a material or baffles in yours/?. With nothing directing the air into the baffles, or material (depending on your design) the LDC would turn into a resonating chamber instead of a sound suppressor. With the longer one, it (sound) would resonate more throughout the chamber. 

I know the science, have seen the videos, pics and other math behind it. Unfortunately it isn't an exact science. 

How are yours built?
 
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Its the baffles, they are making it louder for sure. each one adding to the sound since there is nothing to channel the sound/ air into the respective baffles. Pellets of by one baffle, the sound/air is channeled into that baffle as it expands. Next baffle, same thing, on an on and on. With MORE baffles and no pellet channeling the sound/ air - It makes it louder by expanding throughout the whole area/ chamber. 

Out of curiosity, What was the DB level of each of them before and after?
 
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Skygear it is not the baffles, they both have the same number of baffles. I have been making these for years and this is the first time I have ever come across a shorter one being quieter when dry fired than a longer one of the same design. I will not go into the internals but there is no way with the sound deadening material used that the tube would resonate, there is too much material between the air and the walls of the tube. 
 
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"NeilClague"I do know from experience that when you dry fire an air gun with an LDC on it, it is normally not as quiet as when you fire a pellet. I recently built 2 identical LDC's, the only difference was the length, one was 7 inches long, the other was slightly over 8 inches long. When I dry fired my BSA with the shorter LDC on it, it was quieter than the longer LDC, however when I went to my bench and shot pellets through both of them the longer unit was quieter. Now I have no idea why this would be, why would the shorter one be quieter dry fired than the longer one? This has me slightly baffled, I do find it interesting though and would love to get opinions on why it might be so? Neil.
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Hi Neil. 
I had a good chuckle at "had me baffled"...Don't know if that was intentional or not! 
That is rather strange - Are the internal designs the same (apart from length)?

Matt
 
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In trying to protect your dedicated and persistent adventure in making LDCs you have also isolated yourself from answers about your design. I can certainly understand after many trails of what doesn’t work and what does work that you have built up a knowledge base that you would rather not just hand out. So please be understanding that your question without the specifics of design or plan well……… It is like asking what pellet shoots best in my rifle and withholding the make/model.

I do not expect you to get any useful answers. But as you are always in an ongoing process of perfection I suspect you will be able to answer this question in the near future. Just as you have taught yourself from the beginning. Good Luck.
 
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Matt the pun was not intended, I laughed myself when I read your response and realized what I had put. The internals are identical, the only thing that changed is the size of the first and last chambers, maybe I just hit on a size that for some reason reacts strangely without pellets and 17 bullet it is not that I am trying to hide my internals from anyone, it is because I think being more descriptive of them might be against the rules of the forum and get me a wrist slap! It is not really like asking what pellet shoots best out of each gun, I have mentioned that there are the same number of baffles in each LDC and that they both had identical internals, so comparing them to what pellet woks best out of a gun is nothing like the situation here, the details have been given on their lengths, the internals and the baffles. The only reason I can think of this happening is because I have hit on an unusual combination of chamber sizes, just like with some suspensions on cars on some roads might hit a harmonic frequency and get vibration, maybe the way the chamber sizes and the way the thick felt pads react to the air being expelled is different.
I felt for sure some tech expert would have had some explanation using physics to this anomaly, But as long as it is quieter using pellets I do not have an issue with it, I just wondered why the difference without, I know I am going to be thinking about this for some time myself, Neil.
 
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"CptMoonlight"Have you built the very same design in the past with different results?



I have but they have all been made of aluminum, maybe your question has unintentionally hit on a possible answer, these were both made of carbon fiber, maybe it has something to do with the carbon fiber tube. If one of the tubes had some pockets of air in the weave that could make a difference. I know I have discarded some carbon fiber tube in the past because it sounded like the weave was separating, Possibly just a quality issue with the carbon fiber build up of the tube. Now that would make sense, since tapping on a carbon fiber tube with loose fibers it does have more of a dull thud, I think we might have a winner here? Thanks for the question, now this will have me looking more closely at the tubes before I use them, Neil.
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Something that might help, also might be a humbling moment. http://www.silencertalk.com/forum/

It is mostly firearm suppressors. However, most of the science should transfer over and might help with some future designs. The guys over there are great, like any board though, there are some trolls. Just brush them off and stay focused. 

Resonance frequency ~ Harmonic frequency. Yes, Physics. 
 
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As for tapping on the tubes, I had a sneaky suspicion it might have been CF ;) I have made things with it in the past and also translates to bike frames. We would test them by sound to test if the frame was 'toast' for lack of a better term. 

As the other member stated though. Without specifics and actual data to compare. It is like shining a flashlight in a corn field in the middle of the night with no moon trying to find the rat running around. Without specific data it is hard to get it. 
 
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Skygear I am not sure what other data I could give that would help, I have a sneaky feeling that it has something to do with the tube, not the internals. I had not thought about the CF tube being the problem. I could post DB readings sizes of felt and baffles etc, but none of it would point towards what the problem is.That is why I posted on here, I knew someone would mention something that would wave a red flag. Just his mentioning different materials made me think about what was different, since this has never happened with aluminum tube or Titanium tube, it made sense that the problem was in the tube itself not the internals. The physics of fire arm silencers and air gun silencers is totally different, fire arms silencers do not work very well at all on air guns and I would hate to think what would happen if someone tried an air gun silencer on a fire arm?
I am afraid sometimes people want more data than is needed, just by questioning what was different from what I had made in the past pointed me in the right direction to find out the problem. I have inspected the 2 tubes very closely and the larger one does have a slight bulge on one side, this would tend to indicate either an air pocket, or some other problem with the weave, this would explain everything. I had not noticed it previously since it is on the underside when on the gun and not visible. Thanks for all the responses, I feel we have now determined what the problem is, Neil.
 
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I am doing it daily too, but this is the first time I have ever come across this, it had not even occurred to me that it could have been a problem with the tube itself. Only when it got mentioned did it occur to me that it had not happened with aluminum and it reminded me of the one batch of tube I received that had a faulty weave on it. My memory is not as good as it could be and a little reminder sometimes helps. The suggestions made on here helped me figure out where to look and eventually came up with the cause, many thanks to all those who responded. In future if I notice a similar thing I will know that I need to take a close look at the tube for faults, sometimes they are not visible and this would be a good indicator not to ship the product out, thanks guys, Neil.
 
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