Let's make some noise.
"If a tree falls in the forest, and there is no one there, does it make a sound?" From the first time I heard that, it struck me as the worst kind of fuzzy-headed, indulgent nonsense devised. Of course it makes a sound. Aha! Apparently, the fuzzy-headed make a distinction between "sound" which is what we hear, and "noise" which is the same thing without a human ear to receive it.
No wonder my Philosophy prof threw her hands up every time I asked a question, I was too attuned to Newtonian Physics to appreciate the subtleties of age-old questions.
But, what is a sound? Or noise? Simply put, what we hear is a compression wave traveling through the air. (Or water, or other sound-conducting substance.) Sound has a number of attributes, only one of which is volume, or apparent loudness. The compressed air, when it strikes your eardrum, causes the eardrum to flex away from the pressure. This movement is translated by the internal parts of the ear (medical students quick; name the three bones in the ear) and finally turned into electrical impulses to the brain.
Our ears provide the sound origin in much the same way our eyes do: differences in arrival time background sound level (the day I measured it) of 65 dB. My shop, a soundproofed concrete bunker, with a dehumidifier running, is 65.2 dB. "Noisy" and "quiet" can be relative terms.
In the world of physics, you can start arguments over frequency weighting for decibel measurements. The "A" scale (shown as dBA) is the industrial scale. It is perhaps not the best for impulse sounds, such as gunshots, where the "C" scale or, no weighting, is probably better. Due to the limitations of the equipment available to me, we'll go with C weighting for this, and future features, unless otherwise noted.
Firearms, even a lowly .22 Long Rifle, exceed that by a high margin. A "quiet" .22 can be over 130 dBC, and the higher the operating pressure (and thus muzzle blast pressure) the higher the dB, running up near 170 dBC. So, what keeps us from being deafened, even with muffs on? Simple; sound is much like light in that the distance traveled decreases power, and quickly. In light, it is the "inverse square" law, where you double the distance and decrease the illumination by four times. So it is with sound.
With sound, doubling the distance decreases the noise by 6 dB. (Remember; the scale is logarithmic) So, while your 7mm Mag may be generating 160 dBC at the muzzle, by the time the blast gets to your ears, it can be down to 120 dBC. Alas, your left ear, being closer, gets less of a distance benefit.
Which is why a lot of old shooters and combat vets turn their right ear towards you to hear better. I've had occasion to use a decibel meter while coaching a shooter, and found I was getting subjected to 105 dBC on each shot, standing to the right and just behind him.
Since it is a logarithmic scale, you can't simply add up the total dBs incoming, and get a total. For instance, if I'm standing between two shooters, and each is generating (to my position) 105 decibels, I'm being pummeled by repeated 105 dBC exposures. If they happen to shoot simultaneously, I'm the happy recipient of 108 dBC, not 210. (Remember, doubling energy gets you 3 dB)
So, back to suppressors. How do they work? Simple; they extend the time over which the muzzle blast is released from the muzzle. Remember, sound is a compression wave.
A short, extra physics diversion. We all have clear recollections of some statement from a teacher or professor. One of mine is "Work is area under the curve." A tall, narrow curve can have as much area under it as a lower, wider one. The law of conservation of energy tells us we cannot change the muzzle blast, it will be the total it will be. But, there is a way out.
If we extend the wave, causing the compressed gases to exit the muzzle over a larger period of time, we lower the peak And since the peak is what causes our ears problems, we have, in effect, made the muzzle blast "quieter." We have not decreased the actual energy released, but we have increased the time it took to escape and apparent volume, and Thus transmission to the brain, providing the clues to location.
Also, the various frequencies in the compression wave provide additional information. The extra information is often called "speech' or "music" but sometimes is also "the shot came from over there."
The human car is incredibly sensitive, at least within the range from 20 Hertz to 20,000 Hz.
How loud can a sound be? When the volcano Krakatoa exploded in August of 1883, the sound was heard on Rodrigues Island in the Indian Ocean, 3,000 miles away. (It took some four hours and 10 minutes to get there.)
The measuring system used in sound is denoted in decibels. As in; .10 bel, a unit of sound named in honor of Alexander Graham Bell. The scale is logarithmic, and so a jump of only 3 decibels (symbol: dB) is a doubling of power.
However, while the human ear is very good at detecting sound out of a background, or faint sounds, it is, not so good at parsing minor differences in sound level. It is entirely possible for someone to listen to a set of sequential sounds, and not be able to correctly tell which was louder, unless the difference is more than a mere 2 dB.
Two examples; my gun club has a nearby club, 1.25 miles away. It is possible, if no one is shooting at my club, to hear the shots fired at the other club. Good luck getting them to register in a dB meter, but our ears can hear them. I recently watched a Prius drive by, on a windy day. I knew it was there, because even though it wasn't any louder than the background noise, the pattern of noise it made stood out.
Our brains are very good at picking out patterns, much better than in determining actual volume. I learned this when working as a researcher in a lab, trying to determine volumetric and pattern-related sound changes. Volume didn't matter, and speed of presentation hardly mattered, but apparent minor changes in the patterns, presented had a big effect on recognizing the different set out of a group.
Noise, while to a certain extent frequency-dependent, is, past a certain point, bad for you. OSHA requires that employers who find their employees are exposed to eight hours of 85 dB of frequency-weighted noise a day, must provide hearing protection.
That gives us a threshold to work with. 85 dB is'pretty easy to get a grasp of; If you have to raise your voice, to be heard by someone who is within arm's reach, the background noise is 85 dB or higher. A vacuum cleaner falls short of that, but a snow blower exceeds it.
What are some common noises? Well. My desk, when I'm not writing (my typing style has been unfairly compared to "search and destroy") is in the low. 40s on the dB scale. The burr grinder I just used to make more coffee' generates high 70s (at my ear level) when I impatiently wait for it to finish. Again, my gun club, with no shooting going on, and not much wind, has a Now, the Second law of Thermodynamics also tells us we can't get something for nothing, and this extension of release time comes at the cost of heat. The suppressor will heat up, and quickly.
Ok, on to the good stuff.
A decibel meter records the compression of air, the sound, and reports on what it observes. Except, it isn't as simple as all that. What frequencies? In what timeframe? As with so many things, the more you want, the more it costs. Also, since the distance from the source to the meter matters, decibel measurements are like realtors listings; location, location, location.
The relevant, and correct, measuring methodology as a result, requires laboratory-grade equipment, a trained operator, and careful use. The correct meters are no longer made, and when you can find them used, they can cost as much as a high-grade AR. The meters require a particular microphone, and that can cost you another couple of grand. All told; five grand, easy, for a full set-up.
Writing about guns pays well, but not that well.
Since the laboratory-grade equipment is out of reach for a lowly gun scribe, I'll have to make-do. And, since the maximum dB reading for a non-lab meter is in the 130-140 dB range, I'll have to adjust there, as well. But, this will be mentioned each time I'm testing a suppressor, so it will be clear, for example, if a manufacturer says their suppressor has a 30 dB decrease, but my measurement only turns up 20, that the difference in measurements between our methods is probably the reason.
How good are they?
Generally, all good suppressor manufacturers can knock enough of of the muzzle blast of a high-powered rifle to bring it down into the mid 130s in dB. But what is more important, the best suppressors change the frequency distribution, making the muzzle blast sound less like a gunshot, and more like some otherwise indistinguishable noise, somewhere off in the distance.
Different calibers have different requirements for what will work. If you park a.22 Long Rifle can on a.22-250, you risk blowing it off the muzzle. A suppressor that will handle a.22-250 could easily weigh as much as a.22 rifle.
However, if the cartridge/barrel length combination is such as to produce an already quiet report, a good suppressor can make an already quiet combo "ear-safe." That is, the suppressed blast, combined with the distance to your ears, makes it unlikely to harm your hearing, if your hearing is otherwise unprotected.
Those who have been shot at, and from far enough away to allow the distinction, report there are two components to a gunshot. The crack is the sound of the bullet going by (assuming it is supersonic) and the thump is the mink blast. Since the bullet travels over or well over the speed of sound, its sonic crack passes the observer when the bullet does. The slower muzzle blast, limited to the speed of sound, gives the careful (or very motivated) observer a big clue as to direction and distance.
This is why the military is so very interested in suppressors. Buy disguising the sound of the muzzle blast, a suppressor makes it very difficult to determine direction and distance.
So, we'll be giving you reviews of suppressors, in all the calibers available to me, and also background, design information and historical tidbits. This promises to be fun.
Ed. Note: With this issue we inaugurate a new column on suppressors and other NFA items by Pat Sweeney, a longtime writer for Guns & Ammo and other IMO titles and author of several books on shooting and gunsmithing.
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|Title Annotation:||noise produced by guns|
|Date:||May 1, 2013|
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