Tag Archives: suppressors

Suppressed Subsonic Sound Levels

This post follows the introduction to shots, pops, and sound pressure levels. Virtually all firearms create pressure levels above 140dB, which is the limit established in MIL-STD-1474D to avoid unacceptable hearing damage. Hence, we put suppressors on our guns to bring their peak noise down to “hearing-safe” levels.

We may further reduce the nuisance and noise associated with gunfire by shooting subsonic loads to avoid the loud and unmistakable sonic crack created by supersonic bullets in flight.

Small subsonic loads, like a 40gr .22″ bullet leaving a suppressed rifle muzzle at just 1000fps, make peak sound levels that are roughly the same as manually cycling the bolt of the gun shooting them: about 120dB. (Without a suppressor the same loads meter about 148dB.)

Is Barrel Length Still a Factor with Suppressors?

Yes. Even with low-pressure .22LR ammunition we can see something interesting: Barrel length has a significant effect on muzzle report. Shooting the same loads through a rifle and a pistol (barrel length with AAC Element II suppressor 9″ vs. 25″ for the rifle) the muzzle report is about 6dB higher from the pistol.*

I also ran a variety of subsonic 300BLK loads through two different AR-15s using the same suppressor (an AAC Cyclone): one gun with an 8″ barrel, the other with a 16″ barrel. The shorter barrel produced peak sound pressures 3-7dB higher than the longer barrel (depending on the powder load, as we will see below).

Are Suppressors Effective on Autoloading Actions?

Yes. A common question is whether a suppressed autoloading (i.e., semi-automatic) gun is louder than one with a locked action. Modern autoloaders use gas pressure and/or momentum from the discharged round to eject the empty case and load a new one. This usually occurs while the bore still contains a significant amount of propellant pressure. I.e., some of the same propellant that produces the muzzle report comes out of the breech.

Once we add a suppressor can the breech report exceed the muzzle report? It turns out that it can if a gun is poorly tuned, but that with typical guns and loads designed for them it does not. For example, I tested both a .22LR pistol (the Buckmark) and 10/22 rifle (Feddersen-barreled Ruger). Whether I let the actions cycle normally or held the bolts closed, the peak sound levels were the same, with one exception: 60gr Aguila ammo – which is very exceptional ammunition: Its bullet is 50% heavier than almost any other .22LR, and it is loaded in a .22 Short case. As a result, the case unplugs the breech before the bullet even leaves the muzzle! Since there is no suppressor on the breech, on the rifle this releases pressure of 128dB (vs 121dB from the muzzle with the breech locked) and on the pistol it produces 130dB (vs 127dB from the muzzle with the breech locked). However, it is possible to tune these guns to this unusual round by using heavier bolts and/or springs to prevent the action from unlocking before the bullet has left the barrel.

Similar mismatches can be produced with other actions. In fact, just attaching a suppressor to a centerfire autoloader that wasn’t designed for one can be such a nuisance that many designs and components now allow for the gas system to be adjusted. But as another test: I ran a wide range of subsonic loads through my 300BLK AR-15 with its standard gas system in place (i.e., autoloading), and then with its gas port completely blocked (i.e., locked breech). The peak sound pressure levels were identical in each scenario.

Does Powder Load Make a Difference?

Yes. The standard subsonic 300BLK load uses a 10.4gr charge of a relatively slow powder in order to provide enough gas volume to cycle a wide range of guns. I have experimented with other loads to see how light a powder charge I can use while still cycling my guns and producing the same muzzle velocity (about 1000fps) with the same 220gr bullet. (The only way to do this is to use faster burning powders.) I hypothesized that lighter charges would also reduce noise. Testing with the LxT1 sound pressure meter confirms this:

Powder Charge 8″ barrel dB 16″ barrel dB
A1680 10.4gr   140 137
IMR4227 9.0gr   138 132
Steel 7.7gr   137 130

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Introduction to Shots, Pops, and Sound Pressure Levels

This is a Larson Davis Class 1 sound level meter. With a response time of under 30 microseconds and sensitivity up to 177dB, this is the sort of instrument needed to accurately measure the sound levels produced by transient events like gunshots.
Larson-Davis LXT1-QPR sound level meter
Since I didn’t want to sink over $3,000 into a sound meter, I was able to rent one from Aimed Research, which has become my go-to company for ballistic research equipment and expertise.

I’ll be posting the results of my research shortly. This post explains the basic science needed to fully understand the methods and results.

We use a decibel amplitude scale to describe sound pressure using the formula dBSPL = 20 log10(peak pressure/ambient pressure). Since we will only be talking about sound pressure levels going forward we will assume that all dB values refer to dBSPL.

We will be looking at explosive noise events that we will call “pops:” sounds dominated by a single, rapid peak in air pressure. The human ear is not very good at assessing peak “loudness” of short pops.* But the magnitude of that peak can predict both the audible distance of the pop and its potential to damage hearing of nearby listeners.

The distance at which we measure a sound is as important as the dB value, because sound pressure decays linearly with distance. On the decibel scale, this means that the same sound measured from twice the distance will be 6dB lower. As is customary, unless otherwise stated, all dB measurements will be given for a distance of one meter from the source of the sound.

For reference, here are some typical pops and pressure effects.

Category Event dB
Bad Stuff 99% lethal overpressure   205dB
1 pound TNT   203dB
Tissue damage observed   185dB
Gunshots .338LM 25″ bbl w brake   177dB
.338LM 25″ bbl suppressed   145dB
.223 16″ bbl   166dB
.223 16″ bbl suppressed   132dB
.22LR 16″ bbl   148dB
.22LR 16″ bbl suppressed   126dB
Sonic Crack .308   152dB
.223   148dB
.22LR HV   144dB
Gun Actions AR-15 bolt release 123dB
10/22 bolt release   119dB
AR-15 dry fire   113dB
10/22 dry fire   105dB
Steel Hammer Nail into wood 133dB
Iron anvil   133dB

*For example, I’ve never been able to hear the “first-round pop” that tends to occur with cold suppressors. But in my test data I did find many cases where the first round peak was 3-4dB higher than subsequent shots.