Part 2. 26JUL16

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).

Moderate

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. landscape_1425420483-silencer

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)
6-SilencerCo (www.silencerco.com)

*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.

Advertisements

Part 1. 25JUL16

Goals.

To create a firearm barrel/moderator that is compact, lightweight, and ultimately, capable of reducing the noise of a shot to within non-damaging levels. 

Right now, that’s a long way off. Firstly, I’ll need something relative to measure against.

Beginning with ‘Non-damaging levels’
Decibel, Sound Pressure Level, {dB(spl)} is the standard for sound pressure. This is a logarithmic scale which keeps the units relatively close, the other being Pascals, which is a direct measurement of pressure. Either will work for this writing, but dB(spl) will be used. Eberhard Sengpiel has an amazing resource online for this(1), which collates information taken from the Centre of Disease Control & Prevention, National Institute for Occupational Safety and Health, and the Occupational Health and Safety Administration, which outline a fairly standard way of determining damage to ears. This type of damage is called Noise Induced Hearing Loss – (NIHL). It generally occurs over a long period of time. For example, in an environment with a sustained dB(spl) of eighty-five, after eight hours, a person will start taking damage. For every three dB(spl) increment, that number will halve – eighty-eight dB(spl) is four hours, all the way up to 115 dB(spl) which is roughly thirty seconds.

Sengpeil goes on state that sound pressure cannot accurately be measured as it will always change, even just minutely. The moment of pressure as it hits a persons ears may be vastly different to when it originated from the source – the pressure will dissipate energy as it moves through the environment. The first real challenge then, is to create a way of measuring sound pressure accurately at the source, as it moves through the environment, and ultimately, as best as we can at the recipient.

For this, I have a rough idea of what I want to try. In 2013, a YouTuber named Destin, from the channel SmarterEveryDay conducted an experiment by firing a rifle underwater(2). The aim of that experiment was to demonstrate bolt travel in an AK platform, and then cavitation of a projectile underwater. However, it also demostrated the pressure from the gas as it left the muzzle. The resulting gas bubble blossomed enough to get a reading of the pressure underwater – obviously, with a moderator (designed solely to reduce this expulsion of pressure),  the blossom would be smaller. This would be an easy way to create a measurement – however, a frame would have to be bolted underwater with the firearm to remove measurement errors (such as the muzzle moving around). Also, the experimenter would have to be moving in and out of the pool to clear, charge and operate the firearm during this experiment. The other issue would be that this only gives a clear indication of the pressure underwater – In Destin’s video, the cavitation doesn’t move back to the operator – it wants to move in a clear line perpendicular to path of the projectile. My hypothesis is that the gas is following the path of least resistance – in that the projectile is causing the water to part, the gas is following this path, and then pushing out against the water as it falls back in. More research is required here.

Regardless, it doesn’t demonstrate the pressure levels as it moves from the muzzle to the shooter. For this, I have another idea, of which I’ll draw out and upload. Rough concept – the firearm is suspended over a grid of small water tanks, each roughly 100-200mm deep, and 20mm x 20mm wide and long. If a firearm is discharged over these water tanks, the resulting sound pressure should be picked up by the water moving (without the firearm being in it). A high-speed camera should, in theory, pick up the movement, and as the pressure bleeds off before it gets to the recipient.

If this works, then I have a relatively easy, non-complex way of measuring any changes in pressure from the muzzle/recipient.

1- Eberhard Sengpiel “http://www.sengpielaudio.com/PermissibleExposureTime.htm’
2- SmarterEveryDay “https://www.youtube.com/watch?v=cp5gdUHFGIQ”

*I know that this referencing is pretty shoddy. This is merely a backstop for notation at the present moment, and the references are there if people are interested in my inspiration or wish to read further.