A Quick Course in Shooting Range Noise
By Stephen Katz, President, Stephen Katz & Associates
What Is Noise?
The classic definition of noise is, “sound or a sound that is loud, unpleasant, unexpected, or undesired.” Or, as a jaded friend puts it, “Anything, Springsteen is sound, anything Britney Spears is noise.”
When it comes to shooting ranges, noise is something that can hurt you. The overblast pressure (aka muzzle blast pressure) from small arms, especially larger calibers (>.22) can affect the whole body, not just hearing, especially in the presence of multiple shooters.
Ever come home from an indoor or outdoor shooting range and feel a little tired, a little stressed, got a headache? It wasn’t your shooting; it was that you were exposed to unsafe muzzle overblast pressures. Noise!
The effects are exacerbated for those who train or work on a shooting range. Reported conditions include; hypertension, heart disease, anxiety, headaches, judgment impairment, and sleep disturbance. Then, there is always the possibility of disability claims and lawsuits.
There are solutions to shooting range noise utilizing acoustic engineering noise controls. Done right, shooting range acoustic treatment can significantly reduce the overblast pressure on indoor and outdoor shooting ranges leading to a healthier, safer environment for all.
There are very positive byproducts to acoustically treating shooting ranges. Reduced shooting range noise; encourages more, new shooters, existing shooters stay longer and buy more ammunition; it makes recreational shooting a family-friendly sport.
Other benefits of acoustically treating indoor and outdoor shooting ranges include; keeps gunfire sound out of adjacent areas & leaving the premises, it can negates potential lawsuits, disability claims, and can eliminate neighbor’s complaints and most importantly, it keeps workers and those who train on the range productive & safe.
What Is The Anatomy Gunshot Noise?
The sound of a gunshot isn’t really a sound but a shock wave also-known-as an air blast peak overpressure.
All types of small arms rounds except for the 38 and 45 caliber break the sound barrier (supersonic), with the Ruger 204 hitting a blistering Mach 3.5.
It stands to reason that the faster the round the louder the sound. A typical 5.56x45mm NATO 55 gr FMJ has a velocity of 3240 f/s equal to Mach 2.88. A 7.62x51mm NATO147 gr goes at 2800 f/s equal to Mach 2.49.
Figure 1 below shows the acoustic anatomy of a single gunshot. The supersonic shock wave measures at 142 dB SPL. The muzzle blast rise time is an ultrafast ten-microseconds (10µs). A microsecond is a unit of time equal to one millionth of a second (10-6). The peak overpressure is a very loud 168.4 dB SPL. This gunshot was recorded outdoors in an open field. Image how loud this firearm would be on a not acoustically treated indoor or outdoor shooting range.
What Is Peak Level of Small Arms Fire?
As we saw in Figure 1 the peak level is a loud 168.4 dB SPL. The peak level in dB SPL, of a certain firearm given: the same firearm, same ammunition, same microphone, same microphone placement, same shooter, etc., should record the same peak level, albeit in an anechoic room or in Yankee Stadium. Unless!…
Figure 2 shows the peak reflection off of an untreated steel shooting range baffle. It can be seen that the muzzle blast of the first arrival was 163.9 dB SPL, the first reflection was 170.1 dB SPL, a gain of 7.2 dB. When the same firearm, using the same ammunition, was fired by the same shooter, in the same area after acoustic treatment, the first arrival muzzle blast was 162.7 dB SPL and the first reflection was 161.6 dB, a drop of 1.1 dB. The firearm fired was a 5.56 with a 16 inch barrel.
Figure 3 shows a similar acoustic gain as Figure 2, the difference being that Figure 3 compares a 5.56 with a 16 inch barrel being fired under an outdoor shooting range canopy and the same shooter stepping in front of the canopy and shooting in open space.
While, this author has not come up with a peer substantiated cause of the acoustical gain, we believe it is due to the angular pressure coefficients effect.
What Are the Pressures Generated by Small Arms Fire?
To better understand the impact on the whole body the metrics Pound-Force per Square Inch (psi) and Pound-Force per Square Inch Time (psi(t)) was devised by this author to evaluate shooting ranges and gauge the amount of improvement from sound absorption/noise abatement treatment. Pressure Time measurement advantages include:
• Psi(t) gives an actual representation of the total energy generated within the range
• Psi(t) gives an accurate representation of the effect of multiple shooters firing multiple rounds
• Psi(t) gives an accurate representation of automatic and semiautomatic fire, single or multiple shooters
• Psi(t) can be used to compare any range whether acoustically treated or not
• Psi(t) is an excellent way to compare an indoor or outdoor acoustically untreated range to an acoustically treated range to gauge performance and improvement
• Psi(t) clearly expresses the effect on the total human physiology not just hearing
Figure 4 below is a time series of one-shooter (1) firing one-round on an M4 5.56 on an untreated indoor shooting range versus one-shooter on the same range after acoustic treatment. It can be seen that the pressure on the shooter was reduced significantly.
What Is Reverberation?
Reverberation is the persistence of sound in a particular space after the original sound is produced. Ever drop you car keys outdoors; they fall without making a sound. Ever drop you keys in an underground parking garage, it is so loud you need hearing protection.
Figure 5 shows what reverberation of a single gunshot in an enclosed space could look like. It’s like a boomerang that keeps coming back, and back, and back…
Reverberation on indoor and outdoor shooting ranges is measured by Reverberation Time (RT60). RT60 is a metric used to evaluate shooting range noise but not an absolute as diffraction and diffusion will reduce the RT60 without decreasing the pressure on the shooters. Psi and Psi(t) are a better indicator of the pressure felt by the shooters, and the improvements from acoustic treatment and engineering noise control.
Figure 6 shows the Reverberation Time (RT60) a typical 50 yard range with no acoustic treatment versus the same range fully acoustically treated.
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