Ear vs Muzzle Decibel?

[Logicalpath]

I saw a post just now that linked to some suppressor testing and it brought up a question for me. When I first started considering a suppressor for Home Defense I reviewed the decibel reduction on suppressors using supersonic vs subsonic. Along the way I found that anything 140 over can cause permanent damage with one occurrence. Which prompted me to find out what the average decibel for a gunshot is 165. That doesn't seem like much of a difference to me, the lowest Surefire can produce is 132(going off of memory here).

Currently I am in the process of putting the paperwork together for a AAC 762-SDN-6 and I see that according to THIS testing chart (http://www.silencerforum.com/forum/showthread.php?649-5.56mm-suppressors-(and-7.62mm-QD-cans-on-5.56mm)-SOUND-TESTING-on-a-KAC-SR15E3-(16-quot-)) the "ear" decibel is average is 144.7. Which brings me to two questions:

1. What is the average decibel of a unsuppressed shot from an AR-15 shooting 5.56?

2. Shouldn't the Ear Decibel be lower than the Muzzle Decibel?

[vinsonr]

Along the way I found that anything 140 over can cause permanent damage with one occurrence. Which prompted me to find out what the average decibel for a gunshot is 165. That doesn't seem like much of a difference to me, the lowest Surefire can produce is 132(going off of memory here).

Decibels are logarithmic, not linear, so something 165db is much louder and damaging than 132. For every 3db increased, sound intensity goes up by 100%, and sound pressure goes up by 50%. So using the numbers above, that 33db reduction means 165db is 11 times as loud, and the pressure is 5.5 times as much.


1. What is the average decibel of a unsuppressed shot from an AR-15 shooting 5.56?

Too many factors exist to get an average. A shorter barrel is louder because there are more gasses and unburned powder that tend to expand rapidly once they escape the barrel, creating more sound when they do so. No suppressor will make 5.56 hearing safe, but I'd rather shoot a few rounds in home defense with the can on than without.


2. Shouldn't the Ear Decibel be lower than the Muzzle Decibel?

The bolt and carrier movement creates a very loud noise right next to the weapon and the shooter's right ear (for a righty). It's usually quite a surprise to people when I turn off the gas on my Switchblock and let them shoot without the bolt cycling. It's a much different experience from the shooter's perspective, even though it sounds the same from 15 yards or so away.

[Logicalpath]

Decibels are logarithmic, not linear, so something 165db is much louder and damaging than 132. For every 3db increased, sound intensity goes up by 100%, and sound pressure goes up by 50%. So using the numbers above, that 33db reduction means 165db is 11 times as loud, and the pressure is 5.5 times as much.



That is very interesting, thank you for that explanation. It definitely makes much more sense now. With that in mind I may have to reconsider the 8" AAC upper and go for for one of the longer AAC's.

[vinsonr]

300BLK rounds were designed for a pistol length gas system, and short barrel rifles. Although shorter is louder, 300BLK is not likely to be as bad off on SBR's because the powder is designed to burn up in those shorter barrels.

[Logicalpath]

300BLK rounds were designed for a pistol length gas system, and short barrel rifles. Although shorter is louder, 300BLK is not likely to be as bad off on SBR's because the powder is designed to burn up in those shorter barrels.

So would you say that going from an 8" to a 11" AAC may not be that big of a deal?

[vinsonr]

Any sounds over 80db or so can cause hearing loss with prolonged exposure. The higher the db, the shorter amount of time that your ears can cope.

Even a few shots fired indoors with pretty much any gun will very likely result in at least temporary effects on your hearing, including loss of hearing and ringing. If the exposure is loud enough or long enough, you could get permanent damage.

I don't think that 8" vs 11" will matter much when suppressed, but there are some guys on this forum with much more in depth knowledge of AAC products.

[JasonM]

So would you say that going from an 8" to a 11" AAC may not be that big of a deal?

In terms of sound, no, going from 8" to 11" will not make a noticeable difference in 300 BLK. I'd be very surprised if it was more than 1 dB.

And like vinsonr said, going from 165 dB to around 140 dB is a HUGE difference. But there are the other advantages as well- less recoil, tighter groups, no muzzle flash, greatly reduced muzzle blast, etc.

[JasonM]

Also, there is no hard rule about the ear/muzzle measurements- some cans have more backpressure that can increase the at-the-ear numbers, action noise of the particular weapon is a big player, etc...

[tb-av]

dB Calculator

http://www.sengpielaudio.com/calculator-levelchange.htm

EDIT: If you enter 33 in the (165-132) "level change" columns you will see the actual -pressure- is 45x higher. In other words the force on your ear drums ( I think this is what is going to give you permanent damage ). You hear it as being 10x louder though. The calculated intensity of that difference is ~2000x