pressure wave forward and away from the shooter and bystanders. Recoil-reducing devices such as muzzle brakes however worsen potential hearing damage, as these modulate the muzzle blast by increasing the lateral vectors nearer to the shooter. Compression wave The overpressure wave from a firearm's muzzle blast are infrasonic and thus inaudible to human ears, but it still can be highly energy-intense due to the gases expanding at an extremely high velocity. Residual pressures at the muzzle can be a significant fraction of the peak bore pressure, especially when short barrels are used. This energy can also be regulated by a muzzle brake to reduce the recoil of the firearm, or harnessed by a muzzle booster to provide energy to cycle the action of self-loading firearms. The force of the muzzle blast can cause shock damage to nearby items around the muzzle, and with artillery, the energy is sufficiently large to cause significant damage to surrounding structures and vehicles. It is thus important for the gun crew and any nearby friendly troops to stay clear of the potential directions of blast vectors, in order to avoid unnecessary collateral damages. Recoil Typically the majority of the blast impulse are vectored to a forward direction, creating a jet propulsion effect that exerts force back upon the barrel, resulting in an additional rearward momentum on top of the reactional momentum generated by the projectile before it exits the gun. The overall recoil applied to the firearm is thus equal and opposite to the total forward momentum of not only the projectile, but also the ejected gas. Likewise, the recoil energy given to the firearm is affected by the ejected gas. By conservation of mass, the mass of the gas ejectae will be equal to the original mass of the propellant (assuming complete burning). As a rough approximation, the ejected gas can be considered to have an effective exit velocity of where is the muzzle velocity of the projectile and is approximately constant. The total momentum of the propellant and projectile will then be: where: is the mass of the propellant charge, equal to the mass of the ejected gas. This expression should be substituted into the expression for projectile momentum in order to obtain a more accurate description of the recoil process. The effective velocity may be used in the energy equation as well, but since the value of α used is generally specified