Definitions, Descriptions and Diagrams.
Last post, I briefly covered my first short term goal – to determine a way of accurately measuring sound as it moves through the environment. My long term goal is to design a barrel or moderator that is lightweight, compact, and reduces the noise produced by a firearm to below damage causing levels.
Today is a history lesson day – For the most part, I have to assume anyone reading this has no idea what a moderator is, or even how sound or a firearm works.
A moderator, as the name implies, moderates something. Thank you thefreedictionary.com for the following definition(3).
A moderator is designed to reduce the severity of the noise produced when shooting a firearm, also known as the report. Moderators are also known as suppressors or silencers, and can date their origins back to a gentleman by the name of Hiram Percy Maxim(4), who developed a device that reduced the sound pressure generated by a firearm. To understand how it works, firstly one must understand how a firearm works.
At the most basic function, a firearm is nothing more than a long pipe capped at one end, and a means to generate a spark. Gunpowder -now known as ‘Black Powder’, as there are now many other forms of powdered explosive, is made up of potassium nitrate, sulfur, and charcoal. To achieve a flame, there must be ignition, fuel and oxygen. The ignition is generated by a spark, the fuel is the sulfur and charcoal, and potassium nitrate is the oxidiser. When placed on a tabletop and burnt, the powder deflagrates – the rate in which it burns is less than the speed of sound. However, when put under pressure and ignited using a very high ignition temperature, it is quite capable of detonation – in which it burns faster than the speed of sound. Early cannons would use this rapid expansion to push a projectile out the end of the barrel – the pressure would build up behind the projectile and expand, and the weakest point (the projectile) would be pushed out and along until the gas expansion is complete, thereby throwing the projectile out of the barrel. If the barrel is too long, the pressure subsides, and friction from the barrel slows the projectile before it can leave. Too short, and not all of the gunpowder can be burnt before the projectile leaves, meaning it cannot obtain maximum velocity. All of this has been demonstrated many times by many famous scientists, and I’ll throw a link down below (5)
When an object moves through the speed of sound, it creates a supersonic boom. These shockwaves are quite audible – and I’ll go into more depth on the science behind them (and sound in general) in the following part. But this shockwave is what a moderator is designed to reduce. On a conventional firearm, there are either one or two shockwaves – first, there will always be a shockwave from the detonation from the gunpowder (as it moves faster than the speed of sound). The second one, may occur as the projectile itself breaks the speed of sound. As this projectile then moves away from the firearm, the supersonic ‘crack’ moves with it – once it has left the barrel, there is no way to contain this crack. The gasses however, quickly lose their speed once the pressure decreases. If they’ve returned to subsonic speeds, the sound also quickly drops off.
The goal then, of a moderator, is to ensure that these gases are moving at subsonic speeds by the time they leave the end of the barrel. This is obtained by using a series of chambers, seperated by baffles – SilencerCo(6) have been very kind to create a gif that shows this in action.
As seen, the top, un-moderated firearm’s gasses are still expanding at a supersonic rate, creating a sonicboom at the point of the muzzle. The second firearm, uses the moderator to allow these gasses to expand out slowly, all the whilst having the moderator contain the supersonic crack.
So, that’s how a firearm, and how a moderator work – the next part will be in relation to sound, and how a supersonic boom is created.
3- Dictionary.com (http://www.dictionary.com/browse/moderate)
4- Forgotten Weapons* (https://www.forgottenweapons.com/accessories/maxim-silencer/)
5- Smarter Every Day** (https://www.youtube.com/watch?v=_TNSUIsjdpY)
*Ian at Forgotten Weapons holds an incredible quantity of knowledge on firearms and their history. I will be using his work a lot through this journal.
*This demonstration uses a fuel/air mix (instead of a powder with oxidising properties) to generate the propulsion – but is one of the best cutaway slow-motion examples of a cannon that I have seen.