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.

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