Understanding how suppressors work...

That's a lot of theoretical questions, but there has to be a law of diminishing returns because there is only so much gas that is produced from the amount of powder in a .45 ACP case. To exaggerate the case, a 55 gallon drum would be overkill, even if it were lined with a sound dampening material and "wet". That much gas isn't produced by a .45 round. You've got to consider the volume of gas actually produced by the round.

There can be up to 3 producers of sound in a gunshot. The exploding gases (the biggest sound producer), the supersonic crack of the bullet (IF the projectile is supersonic rather than subsonic) and the sound of the action of the gun cycling (if it's a semiautomatic type action). I guess the ideal suppressor would be one that contains the entire volume of gas produced by the round that deadens harmonics in the metal of the suppressor itself and cools (or supercools) the gases produced before escaping the suppressor. I'm sure it would be completely impractical in size and weight.

There's definitely diminishing returns on can size. About 10 years ago DeGroat made a 12" 9mm suppressor. If memory serves, it was quiet, but didn't have enough benefit over a standard 8" ish can to deal with the extra length.

Sound obsorbing material has been tried for years and was very common in older suppressor designs. The downside is that due to the heat/pressure, most of it eventually degrades making it a pain to replace. User replaceable suppressor parts have an inconsistent regulatory history, so most cans moved away from using them. Wipes are one example that has seen some resurgence in the past few years, and they work well within their limited lifespan.

Another option is making the surface as irregular as possible (getting into sound engineering) and some pistol cans have tried using coatings like linex to make a less uniform surface, but I don't think they helped much (I understand that they did make the baffles easier to clean though). I'm not an expert but my expectation is that dealing with the pressure is significantly more important than dealing with sound waves.

Another limiting factor is that at least in semi-auto actions you're always going to have the weapon cycling which is 110-115dB right there, plus any port pop, or sound that leaves via the ejection port that is not caught by the suppressor.

Your description of bouncing the sound around is only part of how or why that works. "Bouncing" or reflecting the sound wave is effective when the reflected wave cancels an oncoming wave.

"Mufflers" work either by sound wave reflection and cancelation, by directing the sound waves into absorbing material (that converts the waves into heat), or by both methods. "Suppressors" are unique in that they're generally tasked with suppressing a single burst or eruption rather than a nearly constant flow. A V8 engine at 3000 rpm has 200 exhaust strokes per second -- that's a lot faster than a Phalanx CIWS. Because of this, one of the methods a suppressor can use effectively is to simply slow the eruption of the burst by breaking the pressure front up and releasing the pressure from the muzzle (of the can) over a longer period of time.

A suppressor is dealing with pressure levels more similar to those at the exhaust valve on an engine. A muffler is farther down a long pipe where it deals with much lower pressures and more sound than pressure. As a comparison of pressure levels, I can cup my hand over the tailpipe on a car. I would not cup my hand over the exhaust port on the cylinder head or a gun barrel, even with blanks.

The can has a similar effect to making the barrel much longer. This would also work for an engine exhaust but it would be prohibitively large for automotive applications. Imagine a 55-gallon drum size expansion chamber into which exhaust gases would slow, cool, and slow further and exit with a great deal less temperature, speed and pressure than they erupted with at the exhaust valve. We could similarly create this space for expansion and cooling and depressurization with a really long exhaust pipe - or a really long gun barrel, but the suppressor achieves the goal with less length by providing a greater volume for expansion into a larger diameter bore, while also arresting the pressure with a series of baffles to slow down its exit and therefore spread the release of the pressure over a longer period of time, which results in a wave with less sound pressure erupting from the muzzle of the can.

Here's a demonstration of breaking up and slowing the pressure wave. The pressure has to escape through the pillow, which results in much smaller wave fronts erupting out of the pillow into freespace.

I'm gonna quote what I said in the wet can thread, wince the threads are converging to the same discussion...

pdsmith505 said:

I would reason that the phase change involved going from liquid-gas is where running a can wet gets it's performance from. Ultimately, all the can is doing is taking a rapidly expanding quantity of gas and slowing down that expansion (you are slowing the propagation of the pressure wave at each baffle by choking the flow of gas) while providing a path for heat to flow which reduces the pressure of the hot gas in the first place. In each subsequent volume you are also dropping the pressure.

If a liquid is present, espescialy one that gets dispersed by the passing shock wave in the can, it will soak up the energy in the gasses. You can't really stuff a lot of heat into a liquid, but changing phases will do the trick... for water you are looking at around 2,200 J/g to change from liquid to gas, while going from freezing to boiling is only about 400 J/g (for reference, gunpowder contains 5,000-11,000 J/g of stored energy). Taking that heat out of the gas to vaporize the liquid drops the pressure of the gas which reduces the speed that the pressure wave is moving at as well as reducing the magnitude of that wave.

Granted, this a a very back-of-the-envelope analysis of the process that doesn't go into the details of how much time it takes for that energy to flow... but overall slower, smaller pressure waves make less noise.

Click to expand...

The Smarter every day YouTube channel has a few good videos on suppressors complete with high speed video that adds an interesting visual aspect to the explanations.

Texasgrillchef said:

All metals bounce sound about. Fire a gun in shipping container is not a good thing, lots of sound bouncing around. We add foam and other sound absorbing material to make rooms sound proof.
So wouldn’t it make a silencer even more quit if somehow it were possible to line the baffles with sound absorbing material? (Hypothetically speaking, not referring to current capabilities or practicality)

Click to expand...

What goes on inside a suppressor really has nothing to do with acoustics. What makes a gunshot loud is the sudden release of very high pressure; uncorking. A suppressor works by disrupting the flow of the gasses escaping the muzzle, both allowing time for them to cool and slowing their release into atmosphere. The better a suppressor does that, the quieter it will be. At least on a manual action gun...

Texasgrillchef said:

Would keeping the “can” super cooled somehow have any effect on making it a quieter can? Since we want to cool the gases? If the baffles and can were super cooled wouldn’t that cool the gases quicker and more thus making it even quieter?

Click to expand...

Yes, at least in theory, but that's a tall order. There is a tremendous amount of thermal energy in burning gunpowder. Suppressors get hot quick, much faster than barrels-especially on big magnum rifles. My Accipiter is a 1.8x10" all 422 stainless steel can weighing 23 oz, and it's not comfortable to hold onto after a single round of .375 RUM. 2 rounds, it'll burn you. It really isn't possible to have enough conduction with the surface area to effectively cool a suppressor beyond very slow rates of fire, and even then, it wouldn't do enough to be worth the trouble.

Texasgrillchef said:

Would insulating the outside of the can with soundproofing insulation make it quieter as well? Or no effect because we still have a big gaping hole for the bullet to escape?

Click to expand...

No. Again, acoustics comes into play after the bullet leaves the can. That said, materials and geometry can affect tone, and our ears perceive lower frequencies as quieter than higher ones at the same sound pressure level. Suppressors with square profiles or odd angles inside that create corners parallel to the bore tend to be snappy. That's why round is the predominant shape.

Texasgrillchef said:

Outside of practicality of course... Is there a size vs diminishing returns for silencers? To exaggerate to make an example.... if one made a 55 gallon barrel (turned sideways) into a silencer with huge baffles inside it, made it wet. Say for a 9mm or even 45acp. How quiet would it be? Maybe even a can inside a can approach since it’s so big? Obviously not practical. But wondering if their is a law on diminishing returns based on diameter and length. Maybe inside the barrel fill it with super cold nitrogen gases, line the baffles with ceramic sound dampining material, with all that space to abosrb and cool off the gases?

Click to expand...

As mentioned above, yes, there is a point of diminishing returns. And depending on the design, big suppressors can perform worse than smaller ones. Suppressors are optimized for a certain type of cartridge, and if you get too far one way or the other from the performance envelope, they suffer. Generally speaking, it's because the features of the internals & overall volume become inefficient. .22 LR, for example, tends to be louder through full size pistol and rifle cans than quality rimfire cans. There's just not enough gas volume for the guts to do what they're designed to do with the larger cartridges. Conversely, it should be obvious that a rimfire can won't perform very well with something like 5.56 (assuming it holds together) because the baffle spacing, baffle profile and internal volume are geared toward lower velocity cartridges with only about 5-10% the gas volume of 5.56.

However, there are a number of public and private ranges our there that do have stationary silencers that aren't legally silencers because they don't attach to the gun; they're typically along the lines of a 55 gallon drum with some kind of energy absorbing material, holes on both ends big enough to put the muzzle in and see through it optics, often an oblong cutout about 2x5". Of course, you have lots of sound pressure escaping through that hole back toward the shooter, so I imagine most aren't hearing safe, but they'll make for a much more pleasant range day for all patrons.