Suppressor Foam

[davidf]

Looking for foam rings that fit inside one of these. 

 

Any idea where I can get some? or if its easier, make some,

 

Thanks,

David

 

Edited May 22 by davidf

[The_Lord_Poncho]

Here you go: https://www.fire-support.co.uk/product/silverback-suppressor-foam-set-10x-small-10x-large

 

I've not bought that set myself, instead buying suppressors that come with foam, removing approx half the rings, and interspersing the remaining rings with strips cut from one of those green scourer pads, and then rolled up. I also believe that another DIY option is to buy  mini foam rollers (the type used for painting), ease off the plastic central tube, and then cut it up. e.g. https://www.amazon.co.uk/Microcrater-Sleeves-Varnishes-Undercoats-Eggshell/dp/B0797D5PWK?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&psc=1&smid=A2AFHQIP450V7L

[davidf]

THANKS !

[Sewdhull]

Get the rubberised foam used on Hvac stuff, I got mine on amazon, also I have use drubber washers to provide the separation between elements. Or yoga mat material.

Edited May 22 by Sewdhull

[Pseudotectonic]

I wonder if you roll up enough kitchen foil so it becomes a semi-solid aluminium block (with just the middle part hollow where the BB passes through) would it be effective because of the mass

[The_Lord_Poncho]

I imagine that it's chiefly the dispersion, slowing, and trapping of the air that provides the noise suppression factor- reducing the 'crack' of excess air at the muzzle. Mass in the suppressor wall would probably help, but if it ends up being effectively a barrel extension transfering the same volume of air at roughly the same velocity to a slightly extended muzzle, I wonder that it might not have a dramatic effect on sound reduction.

[Sewdhull]

We only have some low pressure air and the sound of piston slap to get rid of.

The air needs a place to go and the sound needs holes to go through so it bends into the absorbent material of your choice.

You do need to deaden the metal tube.

I keepeaning to get some butyl in for speriments 

Edited May 22 by Sewdhull

[Pseudotectonic]

Just theorycrafting here... just a sketch to note down what we have to deal with...

 

1. Vibration damping: We know the case will resonate and vibrate from any sound from inside (air expansion) and from the barrel (piston impulse) so the case need to be treated to become a composite material so any vibration is deadened rather than resonate, can be achieved with a lining of vibration damping tape or with similar principle

 

2. Air gap: To limit energy transfer from inside to the case, maybe we can leave an air gap so all the air expansion is contained and processed inside, rather than transferred outside to the case

 

3. Absorbent material: We need to convert as much sound into kinetic energy as possible at this layer, maybe a heavy and absorbent material, maybe some sort of rubber or silicone or something along those lines

 

4. Air expansion chamber to match the cylinder volume ~1:1 so the space can be filled with as much absorbent material as possible (for the highest conversion ratio possible), geometry to match the air expansion pattern, and/or optimised for absorption (depending on how the absorbent layer is made). This area probably needs to be towards the back.

 

5. The "exit tunnel" of the BB should be towards the front so any leftover sound from the air expansion zone can be limited, we just want the BB going out but with minimal air pressure i.e. sound.

 

TLDR we need to convert the vibrations into heat as much as possible, and we try to do this inside while limiting any transfer to the outside

[Sewdhull]

My thoughts and musings.

Red is Butyl, grey is hard foam or washer, green is sound. I haven't drawn in the barrel, but there should be a taper ( in butyl maybe) from it to the butyl in the first stage.

 

Send sound through a hole and it propagates outwards in an arc originating at the hole so long as the hole is small enough ( which it is in this case). The sound level reduces with each hole it passes through if steps are taken to absorb the sound after each hole.

 

An empty silencer can rings because of resonance, alter the resonance and can will absorb the sound instead.

 

Absorbent material like butyl is soft and dense, most metals are too hard to use, it will alter the resonance and absorb sound. Foams will only absorb sound. Rubber washers make good separators as does the rubberised foam in HVAC stuff.

 

The volume of air being moved is the volume of the cylinder, you need to reduce the speed of this to reduce the noise and/or absorb that noise. The diameter of the silencer will do this not the volume as the air will slow to fill a wider space but should be gradual to prevent a sudden release of pressure.

That amount of air will still be pushed out the end of the silencer so it is always important to not be over volumed or the exit hole of the silencer will behave just like the barrel end but with a larger, quieter exit.

The holes in the end of the silencer system will get larger further from the barrel.

 

[Pseudotectonic]

Good point about gradual reduce of the pop, so perhaps the whole thing should be filled with some sort of open cell foam that is high enough porosity for maximum diffusion

 

So perhaps there is some merit about the common paint roller insert

 

Butyl I guess is nice as a middle damping layer between materials, or a lining layer, but not sure if it is the best interface for the air borne noise, because maybe the smooth surface isn't as good as an open cell structure for diffusion of the pop

 

Maybe if you roll up some thin gauge wire mesh to simulate an open cell structure it would be very effective at diffusion and absorption?

[Sewdhull]

What is important is where the noise originates and how loud it actually is. The piston hitting the the cylinder head creates a sound travelling right down the  barrel along with the excess air popping out the barrel.

 

Well volumed guns may have very little pop, but still have the piston slap noise.

 

You need multiple holes or the sound continues forwards, but honestly the sound levels aren't high to start with so the silencer effects might be lees than expectation and benefit from fewer holes.

 

 The holes change the direction not the volume.

 

Interesting huh

[Shamal]

Nope 😂😂

[Pseudotectonic]

Let's do some maths

 

Let's say we want to completely contain the pressure wave within the can so by the time the BB exits there is zero difference in air pressure inside and outside the can, meaning the ideal air volume in the can plus the volume of the barrel should roughly equal cylinder volume:

 

V_can + V_barrel = V_cylinder

 

The typical cylinder volume is what, ⌀ 24 mm x 50 mm? or ~22.62 cm³, if this is 3/4 ported it becomes ~17 cm³

 

If my barrel is a 270 mm barrel, ⌀ 6 mm x 270 mm is about 7.63 cm³ (so the ratio is about 2.23 it seems but this is irrelevant)

 

That gives me about say 9.4 cm³ of air that needs to be diffused inside the can

 

Assume we are using a replica of a 556 can with an external size of say ⌀ 38 mm x 157 mm, subtracting the body thickness and a layer of butyl tape lining of say 2 mm and fittings etc we have an internal volume of say ⌀ 32 mm x 110 mm or thereabout or 88.47 cm³ of total space inside

 

Of course we need to leave a clear corridor for the BB to go though and it cannot be so tight, if it is ⌀ 10 mm x 110 mm this is 8.64 cm³ out of the 88.47 cm³ that needs to be kept clear, giving us say 80 cm³ of diffusion zone available

 

But this BB clear corridor should also be included in the V_can so this ⌀ 10 mm x 157 mm or 12.33 cm³ is already bigger than the 9.4 cm³ volume we need to normalise

 

That means, the foam fill material within the 80 cm³ diffusion zone is really just handling impulse pressures

 

And if we sketch out the wavefronts we can see most pressure is probably in the beginning 1/3 portion of the can, accordingly we can use a dense mesh here, and use a more open mesh in the later 2/3

 

[DanBow]

I must align with Sledghammer, I've got a loudener on mine. 😁

[Sewdhull]

You can't diffuse the air in an open container, it just comes out the other end. I don't see how it's related to the volume of air. The cylinder air will just push the air ahead of it out. You can mitigate the pressure with squidgy things with a view to slowing the build up of pressure in the can like a spring absorbing energy. Air on its own will absorb this energy.

The pressure wave or sound wave is different and that's what we are looking to get rid of. The air isn't moving in that case, but a wave moving through the air, which you can deflect, absorb or reflect.

Holes deflect the sound direction or surfaces will.

Foams absorb the sound wave or filter it so what you hear is less noticeable.

The air pressure ahead of the BB is way lower than that behind it, when the BB exits the barrel it is unplugged and then the air pops out which whatever pressure is remaining immediately so I guess you want a larger void, a foamy one behind the BB.

A larger cross section of the silencer will slow the expansion more and the idea would be to have a gradual reduction in pressure so there is no pop, you don't need to absorb the pressure.

The amount of air (the mass and pressure) will be that same so the hole out of the silencer can't be too small or you'll get a similar pop when the the air compresses again to leave. That's taken care of by the BB needing an exit , around 10 or 11mm on mine.

 

I'm unsure I'm clear, it's late, apologies 

 

 

 

[Lozart]

On 25/05/2024 at 02:13, Sewdhull said:

You can't diffuse the air in an open container, it just comes out the other end. I don't see how it's related to the volume of air. The cylinder air will just push the air ahead of it out. You can mitigate the pressure with squidgy things with a view to slowing the build up of pressure in the can like a spring absorbing energy. Air on its own will absorb this energy.

The pressure wave or sound wave is different and that's what we are looking to get rid of. The air isn't moving in that case, but a wave moving through the air, which you can deflect, absorb or reflect.

Holes deflect the sound direction or surfaces will.

Foams absorb the sound wave or filter it so what you hear is less noticeable.

The air pressure ahead of the BB is way lower than that behind it, when the BB exits the barrel it is unplugged and then the air pops out which whatever pressure is remaining immediately so I guess you want a larger void, a foamy one behind the BB.

A larger cross section of the silencer will slow the expansion more and the idea would be to have a gradual reduction in pressure so there is no pop, you don't need to absorb the pressure.

The amount of air (the mass and pressure) will be that same so the hole out of the silencer can't be too small or you'll get a similar pop when the the air compresses again to leave. That's taken care of by the BB needing an exit , around 10 or 11mm on mine.

 

I'm unsure I'm clear, it's late, apologies 

 

 

 

 

If you pump air from one tube into a wider tube, the pressure will drop when it enters the larger chamber. Simple physics. By adding wadding the pressure waves will be diffused far better than adding rubber or foam though. Something similar to acoustic wadding or felt would work well, closed cell foam or rubber doesn't actually give the pressure wave far to go.

 

Because of the differences in the way airsoft works compared to real firearms you need to stop thinking about how something like a real suppressor works and more like how you dampen a speaker enclosure or an exhaust muffler.

 

More to the point, as mentioned above - if we're talking about a can on an AEG, what comes out the front is way less of a consideration than what comes from the gearbox and piston assembly.

 

[Sewdhull]

5 hours ago, Lozart said:

 

If you pump air from one tube into a wider tube, the pressure will drop when it enters the larger chamber. Simple physics. By adding wadding the pressure waves will be diffused far better than adding rubber or foam though. Something similar to acoustic wadding or felt would work well, closed cell foam or rubber doesn't actually give the pressure wave far to go.

 

Because of the differences in the way airsoft works compared to real firearms you need to stop thinking about how something like a real suppressor works and more like how you dampen a speaker enclosure or an exhaust muffler.

 

More to the point, as mentioned above - if we're talking about a can on an AEG, what comes out the front is way less of a consideration than what comes from the gearbox and piston assembly.

 

The can has a similar restriction on the front meaning the air will speed up again through the exit hole, I agree this is airsoft and the pressure is pretty low so the pop from the air isn't much and ultimately is probably better managed by voluming. 

 

A firearms silencer is all about the combustion gasses and giving them a place to go depressurise and importantly cool down, decreasing in volume. Silencers cool to reduce the pressure...

 

In airsoft all we are doing in allowing the pressure to release slowly, however the gas doesn't cool and contract so playing with volume and baffles doesn't have the same effect.

 

More importantly, absorbing etc, the sound from the piston hitting the cylinder head is the goal, remove the resonance from the can and force the sound through holes which diffract the sound away from the path out the end of the silencer towards the  sound absorbent stuff, what ever you use. Simple physics. There's not a lot of room in a can to fit the sort of foam which is effective on walls etc, and foam in speakers isn't about silencing like we are looking at, it is removing resonance from the cabinets so they sound better.

 

The holes are for diffraction and the spaces between the holes filled with something absorbent,  there are many to choose from but space is limited so a denser foam, even closed cell types should be tried, but soft may work better, I don't have a range of foams to try. Rockwool is good, but may not have enough volume.

 

2mm butyl sheet inside the receiver is an idea I wanted to try, but never got round to it.

[Pseudotectonic]

The more I think about it the more I feel air diffusion does not really matter, because the air pocket will seek the path of least resistance and this will be around the BB and in front of it. When the BB is just entering the suppressor, the clearance will allow the air to go around and this inrush pressure will hammer on the first few mm of material around the barrel exit, but will quickly diminish as the air go around and in front of the BB, rather than hanging around looking for openings in the foam/mesh/wadding. This will have already finished happening before the BB is even a few mm into the suppressor.

 

Therefore the fill material does not really need to diffuse any air, its only job would be simply to absorb the shockwave, but not necessarily absorb any air volumetrically. Therefore it can be a solid material.

 

Now in order to absorb the shockwave and not echo or transmit or reverberate it, it needs to be somewhat flexible I suppose.

 

If one were to isolate this shockwave in audio it will probably sound like a high pitched crack, because it is essentially a sharp hiss of air that is compressed into a very short time, it will be in the upper frequencies because dimensionally and energetically it should not produce any component in the lower frequencies.

 

What fill material (at 10-12 mm thickness) is best for muffling high pitched noise (say 2 kHz to 20 kHz)?

 

If we go by this chart we may be looking at some sort of "fiberglass board" or "sprayed cellulose fiber" (look for a high Noise Reduction Coefficient at high frequency ranges)

 

Now we don't want to be blowing fibreglass particles around so we can rule that out

 

Cellulose on the other hand seems to be the best material for the job but might be impossible to put into a can and keep it there (not to be confused with spray foam)

 

So the key might be instead finding something that has the next highest NRC, perhaps carpet tiles

 

[Sewdhull]

for the thicknesses we can play with, its foams and butyl. Most systems need 25mm or more as the wavelength of the sound reduces ( frequency rises)

 

https://iopscience.iop.org/article/10.1088/1742-6596/1719/1/012062/pdf#:~:text=Commercially available acoustic foam made,coefficient is higher than 0.6.

 

That's for polyurethane foam, I'm sure you cold find others.

 

The goal should be the least noise forward out of the silencer, so we should use diffraction and mass to achieve that.

 

 

 

 

[Pseudotectonic]

Melamine foam (aka magic erasers) might be a good alternate

 

It has rather high NRC and is also in some brands of acoustic panels

 

With PU foam there are many grades and weights and whatnot and not every type is good for absorbing sound (some are), whereas melamine foam seems to outperform even the acoustic grade PU foams especially towards the higher frequencies, which is what we want

 

Magic erasers are rather cheap and easily available, and it is safe to handle and very easy to cut to shape, just get a pack and cut out some round sections and stack them

 

You can even scrub dishes with it