The Sounds of Silence, Part 3
The story goes that a little over 25 years ago at a show, one of the prominent silencer dealers in the United States was approached by a curiosity seeker. After looking around furtively, the seeker of knowledge asked the manufacturer, “Just how do these silencers work?” After a moment’s hesitation, the answer came back in a whisper to suggest conspiracy: “Sound Catchers. Like in a car muffler – you can get them at any auto supply store.”
There is a certain mystique as to how a structure attached to the end of a barrel can reduce the sound. Hollywood has certainly added to this with their tiny appendages that change the sound of a firearm to a muted cough. In the real world, about the only suppressors that can produce Hollywood results in Hollywood sizes are in weapons chambered for .22 LR and when using standard velocity (or target) subsonic ammunition. The fundamental principles of suppression are not terribly complex, although the implementation may be. For the purposes of basic understanding, this will look at only muzzle suppressors and the issue of the ballistic crack of supersonic bullets will be saved for a later time.
Any sudden release of pressure produces noise, and the higher the pressure (and shorter time interval for its release), the louder will be the sound generated. This is particularly true in a firearm. The pressures propelling the projectile are generated by the explosive burning of the powder, producing large volumes of gas. The gas under pressure has to not only overcome the friction of shoving a bullet through a too-small tube, but to accurately accelerate the bullet to a point where it can impart energy to its target. The actual pressure generated depends on the caliber, the amount of powder, the burn rate, and the barrel length. At the instant the bullet exits the rifling, which we refer to as “uncorking,” there is a very sudden release of this high temperature, high pressure propelling gas making a significant sound.
The object of a suppressor is to reduce the pressure of the propelling gas while it is still within the weapon system and to delay its exit. Assuming complete combustion of the powder, the propelling gases will have a known mass of the gas (or known number of molecules). Just before the bullet uncorks from the barrel, the pressure in the bore will vary from 4-5,000 psi up to 30,000 psi or so, depending on the caliber, amount of propelling powder, and barrel length (volume). To reduce the sound, one has to reduce the pressure. This gets into basic high-school physics.
One way to reduce the pressure is to increase the volume, with more volume being better. The disadvantage is simply one of practicality. If it were possible to reduce the bore pressure to the normal atmospheric pressure of 14.7 psi, there would be no noise. The necessary suppressor size is inconceivable. Instead, if the suppressor’s free internal volume is about 20 times the internal volume of the bore, the pressure reduction will result in sound level reduction in the vicinity of 13 or 14 dB assuming no reduction in temperature of the gases. Each halving of the actual pressure reduces the sound level by 3 dB.
The second method of reducing pressure is to reduce the temperature of the propelling gases. This is one of the two functions of the baffles, which present a large surface area to absorb heat. Metals with a high specific heat will absorb heat faster (and conduct it faster). The heat then dissipates from the suppressor by conduction to the outside shell and from there by convection and radiation.
Suppressors for handguns present a unique problem, depending on caliber. There are definite size limits due to cycling, sighting, and concealability issues as well as having a physically large suppressor making the weapon unwieldy. Introduction of a liquid or gel (an ablative agent) will absorb heat rapidly as the liquid or gel changes phase (boils). Converting water to steam absorbs far more heat than simply heating the water (it requires 1 calorie to raise 1 gram of water 1°C, but 540 calories to convert 1 gram of water at 100°C to steam). Ablative agents lend themselves well to pistol caliber suppressors, but should never be intentionally used in centerfire rifle suppressors.
In addition to reducing pressure, the sound level can be reduced by spreading out the timeline of gas exit from the suppressor itself through generation of internal turbulence, disrupting the gas flow. As an example, a balloon makes a lot more noise if the pressure is suddenly released by rupturing the balloon rather than simple opening the valve and allowing the pressure to dissipate slowly.
Baffles, even the simplistic ones, do trap some of the gas exiting behind the bullet, delaying exit and allowing slightly more time for heat absorption. Considering that the gas following the bullet is traveling forward and expanding, almost any partition with a small hole for bullet passage will delay the gas exit, disrupt flow, and cause turbulence.
Simple conical baffles work amazingly well in moderate volume suppressors. Various surface irregularities and jetting cuts will increase turbulence and their effectiveness. With simple baffles, proper design of the spacer can contribute to gas trapping. There are more modern baffle designs that are engineered to create maximum turbulence and trapping, but many of these have limitations. Complicating the design process is the user’s requirement for smaller, lighter, more efficient units.
There is no single baffle type that is ideal for every possible weapon/cartridge combination, and there is no magic formula to determine the best number, spacing, and design of baffles. The general principles are fairly basic, and most are implementations of the General Gas Law. Hiram Percy Maxim patented his first suppressor 100 years ago, and continued fascination with silencers has inspired numerous and often innovative designs ever since.