Noise - when 90 fpe doesn't sound like 90 fpe

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MrP

MrP

Member
Apr 29, 2020
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New Lebanon, NY, USA
Ok I have a formerly LOUD .25 airforce with custom valve. At optimal pressure, typical shots are around 110 FPE and can be pushed to at least 131 fpe maybe higher (haven't tested highest hammer spring settings yet).

I built a reflex ldc and it silences it considerably. But here is something it THINK I've observed: the same fpe shots don't all sound the same... let me explain

At 4000 psi power is lower at any hammer setting, and if I don't touch the hammer spring the power climbs until about 2800/2900 psi and gradually dropping after that. Peak power at low to moderate hammer load is around 3000 PSI or a hair under.

What's interesting is that I think that when I shoot at a 80-90 FPE and the tank is at around 4000 psi it is much quieter than 80-90 fpe shots when the tank is down around 2000-2500 psi. Same slugs same velocity just much louder in the tank has lower pressure.

I'm guessing this has something to do with a slightly different way that the power is being generated? Here is my best guess at why, assuming in am not imagining the noise change: When the tank is topped off, the 80 fpe is the result of a short burst of high pressure air - a little valve lock slams the valve shut before air for 110+ fpe of power is released. It seems this short burst of higher pressure air is easily handled by the baffles and rearward venting. 


Then when I am on the down side of the curve at 2500 psi The air is coming out at lower pressure so maybe the way 80 FPE is being achieved at low tank pressure is by a longer burst of lower pressure air... more valve dwell since it is easy to knock open. It seems this longer burst of lower pressure air is harder to silence and overwhelms the baffles and venting system?


I keep experiencing nice quiet moderately (for this gun) powerful shots of at high pressure and really noisy shots at equivalent power at lower pressure. I would've thought either 80 fpe always sounded the same in the same gun OR maybe the opposite trend would be the case, that somehow it would be quieter at lower tank pressure

I know high-pressure can be hard on the parts but this has me thinking that if I were to design a low powered air gun it would be best to achieve that with a higher pressure tank tapped open in a little bursts as opposed to a lower pressure tank with more air release.


Can someone who studied more physics than me can explain why in more detail or verify this I would appreciate it. 
 
pressure is the ENERGY that drives the projectile with volume being second. When pressure is high the volume is low and why it gets quieter. As pressure gets lower the Volume must be greater to get near equal speed and you see a shift in noise simply because of a greater Blow gun effect with air still expanding out the barrel after Elvis already has left the building. Lol
 
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Wonder how much of that lower pressure air is wasted and coming out of the tank After the pellet is at its top speed versus how much is actually driving it. Pointless to speculate but a lighter spring or hammer would let me explore that question I guess. Or a return spring that actually added resistance to the valve getting opened far/long. Something to snap it back.
 
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Wonder how much of that lower pressure air is wasted and coming out of the tank After the pellet is at its top speed versus how much is actually driving it. Pointless to speculate but a lighter spring or hammer would let me explore that question I guess. Or a return spring that actually added resistance to the valve getting opened far/long. Something to snap it back.
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Lots with an unregulated gun and a wildly unbalanced valve. Airforce does the whole top hat sizes as the basic tune option, but sometimes a hammer is better left a hammer. The great thing about that hammer is it will smash hard. The bad thing about that hammer is that it smashes hard (Being figurative here). Airforce airguns are incredibly reliable\dependable, but they are still a blunt force tool. The sounds will definitely vary greatly on both stock and moded AF guns, unless you place an aftermarket regulator on it (which defeats the purpose of the rifle to begin with). 
 
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The tank is aluminum but rated for 4500 psi. Got it from eBay. The valve is made by Doug Noble, comes with a very heavy spring.
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Aluminum tank rates to 4500psi? From Ebay? What mil thickness is the wall? Careful! Working pressure and test pressure are too different things, and an ebay account in China could get that very wrong with translation. Do you have a link? Most aluminum reservoirs don't go beyond 3300psi for working pressure. 
 
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I'd be more concerned about the structural integrity of the gun itself at 4000+psi instead of a 5mm - 6mm walled aluminum tank.

HPDAVV Dive Tank,Scuba Tanks,Thickened Explosion-Proof co2 High Pressure Gas Cylinder,Light-Weight Aluminum Bottle https://www.amazon.com/dp/B07DS9P6V5/ref=cm_sw_r_cp_apa_i_G5ZRDPVSF8WND4FMH0YH

Don't try to power a light bulb using a potato.
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Good point. Direct flow valve with an AF gun, but I honestly had not thought of that at all. Smart
 
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Im fairly sure that Doug knows better than Odoyle or prettymuch anyone what comes to AF guns and how to run them on high pressure.

Interesting fact btw is that my Gunpower texan is rated at 200bar fill pressure but the stock bottle on it actually says WP 250bar never exceed manufacturers fill pressures tho no matter if your reservour can take it your gun might not. 
 
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Hi

Let me explain it in this way:

Cut the barrel in 3 part like if you have 600mm barrel cut it to 3X200mm

1: You pull the trigger and your air pressure is 4000PSI:

pellet through barrel start accelerating by 4000PSI air pressure

By first 200mm of the barrel pellet achieve up to 60FPE

By second 200mm it gain 20FPE more and now it has 80FPE

By last 200mm it gain final 10FPE and now exit the barrel with 90FPE.

2: You pull the trigger with 2500PSI left in your tank.

By first 200mm of the barrel pellet achieve up to 40FPE

By second 200mm it gain 30FPE more and now it has 80FPE

By last 200mm it gain final 20FPE and now exit the barrel with 90FPE.

-------------------------------------------------------------------------------------------------------------

So here is the clue:

When you have 4000PSI in the first part you accelerate so quickly and for second and third part you accelerate less which mean less air pressure in that area. I made a sample table:(for each 1000PSI you would achieve 15FPE)&(Volume for each 200mm barrel is: ((6.35/2)^2)*3.14*200=20099=20,000)



Air pressure * Volume= (4000*20,000)+(1333*40,000)+(667*60,000)=80,000,000+53,320,000+40,200,000=173,520,000

if you want that 10 extra FPE in second part you have to fill all the first part and second part here is the table:



Air pressure * Volume= (2667*20,000)+(2000*40,000)+(1333*60,000)=53,340,000+80,000,000+79,980,000=213,320,000

Now for same energy you need 173,520,000 at high pressure and 213,320,000 at low pressure, because when pellet go further through the barrel how ever it need same amount of air pressure but more volume should be fill with that pressure.
 
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Im fairly sure that Doug knows better than Odoyle or prettymuch anyone what comes to AF guns and how to run them on high pressure.

Interesting fact btw is that my Gunpower texan is rated at 200bar fill pressure but the stock bottle on it actually says WP 250bar never exceed manufacturers fill pressures tho no matter if your reservour can take it your gun might not.
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https://www.airgunnation.com/topic/catastrophic-failure-of-airforce-condor-breech/
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And that have something to do with Doug and his work?

Also makes me wonder how come there is no more breech explosions seeing any Condor and Escape can exceed 100fpe at 25cal even with stock configuration. 
 
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Im fairly sure that Doug knows better than Odoyle or prettymuch anyone what comes to AF guns and how to run them on high pressure.

Interesting fact btw is that my Gunpower texan is rated at 200bar fill pressure but the stock bottle on it actually says WP 250bar never exceed manufacturers fill pressures tho no matter if your reservour can take it your gun might not.
Click to expand...

https://www.airgunnation.com/topic/catastrophic-failure-of-airforce-condor-breech/
Click to expand...

Interesting
 
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Thanks for the details arash. Wow - I will study that. 


odoyle - replaced the plastic breech a while ago.


I built my own airforce gun in a TalonP "chassis" hpa man. 


Aluminum breech and heavy hammer from talontunes, Noble valve and spring, 18" Airforce heavy barrel, an aluminum thick-walled actually 4500 psi tank stamped as tested at higher pressure, that I have never fillEd past 4000. Thick walls decrease the volume a bit so it's not a full 0.5 L more 0.45 I think. 

Aluminum tube is a reflex ldc I made - slides back over the barrel (which sticks 5" Out of the end of the shroud) and has 3d printed baffles and some other stuff inside

So, this sound ok or risky?

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1abaf8ac-5e3b-44f4-99fc-0a339411bcbd

 
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