Guns and the Noise They Produce

[Bobson]

A short while ago, something (I'm not sure what) got me thinking about my Howard Leight earmuffs I use for shooting. IIRC, they have an NRR (Noise Reduction Rating) of 30. They're the bigger, "I look like the guy at the airport" type made by HL. In comparison, the smaller, more compact version produced by Howard Leight (assuming they only make two - those are all I've ever seen at sporting good's stores I frequent) are (again, IIRC) NRR 23.

So I began wondering, what does the Noise Reduction Rating actually mean in terms of noise reduction? So I hopped on the Google machine and learned a couple things.

1. Sound is commonly measured in decibels (dB); most people know that.
2. NRR is not simply a 1:1 ratio of noise reduction.

If you're exposed to a noise measuring 100 dB, and you're wearing NRR 30 earmuffs, you might deduce that your actual exposure is to 70 dB. This is incorrect. NRR is just that - a noise reduction rating: a mathematical expression that, when evaluated for a specific NRR, provides a constant dB reduction. NRR 30 translates to a reduction of roughly 11.5 dB, meaning exposure to a 100 dB sound, with the protection of NRR 30 earmuffs, leaves the wearer exposed to about 88.5 dB. This seems somewhat insignificant. But is it really?

At least one of the sources below indicate that any sound measuring 120 dB (the level typically associated with a "close" clap of thunder) or louder has the ability to cause significant, permanent hearing loss. A gunshot is, on average, about 165 dB. Using hearing protection that rates NRR 30, the average shooter is exposed to 153.5 dB - still a rather dangerous level of noise.

So what about doubling up - using ear plugs under the muffs? That's got to help a lot, right? Not so much. OSHA says that in order to determine the actual reduction level when doubling up, the user should take the NRR of the higher-rated product (generally this will be the earmuff), and get the noise reduction level of that, by using the following formula: (NRR-7)/2. Then, you just add a constant of 5 to the value given by the first part, and you're done.

Example: NRR 30 earmuffs, NRR 18 ear plugs. (30-7)/2 = 11.5 +5 = 16.5. So the result of doubling up with these two specific pieces of hearing protection results in a reduction of 16.5 dB from a given sound. Thus, the shooter who doubles up on hearing protection, and is exposed to the average 165 dB from his firearm, is still exposed to 148.5 decibels. Remember that 120 dB or more can be significantly damaging. That's pretty concerning.

I thought I would share this information, as I've never seen anything about the actual meaning of NRR on this or any other firearm-related board. In discussing suppressors, certain people often cite hearing damage as a primary reason for the full legalization (removal from the NFA) of suppressors. This information may be a very beneficial aid toward that end.

As a brief side note, consider the following:

120 dB can be significantly damaging. The average gunshot is ~165 dB. What does that really mean? It seems like 165 dB may be just a little bit louder than 120 dB. Not so. Using some basic calculus (which you can view below the sources, if interested), we can determine that 165 dB is actually a whopping 45 times louder than 120 dB.

Sources: OSHA, Centers for Disease Control and Prevention, DangerousDecibels.org - a nonprofit funded by many state university medical centers, the CDC, and the National Institute for Occupational Safety and Health (NIOSH).

Math:
Sound intensity (I) is given by I=X(10^0.1L), where L is the loudness measured in decibels (dB), and X is the minimum intensity detectable by the human ear.

For a sound measuring 120 dB, I = X(0.1)ln10(10^0.1(120)); I = X10^120. This means a sound of 120 dB is 10^120 times louder than the minimum intensity detectable by the human ear. That’s a 1 with 120 zeroes behind it.

Likewise, a sound measuring 165 dB is equal to X10^165 (a 1 with 165 zeroes behind it) louder than the minimum intensity sound we can hear. So 165 dB is actually 45 times louder (^165 - ^120) than 120 dB – and not “just a bit louder.” Pretty intense stuff. Anyone know what a 1 with 165 zeroes after it is called? The best I can do is call it approaching infinity.

 

[herrwalther]

Decibels are exponentially related. Meaning (when graphed) they do not follow the 1:1 ration you mentioned. This is why something 121 dB sounds much louder than 120 dB. Or someone next to you is exposed to X decibels and you experience much lower just being a few feet away. I always found it really interesting that I used hearing protection when shooting a rifle, but could stand just 30 feet behind the firing line and be comfortable without them. Sound is a funny and very intriguing area of science that I wished I spent more attention with. I may not have hearing damage and tinnitus if I took the science of sound a bit more seriously.

 

[Impureclient]

That equation there makes zero sense. They are saying if you add something over one set it actually increases the sound getting through?
That's like saying if you have two fences around your home it will be easier to get in. Even better explanation would be if it's cold outside
that you'd be better off just wearing a warm coat rather than wearing that same coat with also a shirt underneath.

 

[thefish]

Well, I can't vouch for the math, but I always double up, and its quieter than not.

I think that's about the best I can do.

 

[lxd55]

the way I follow this in my life is, if it is uncomfortable it is not good. I also think the db noise is for sustained levels. I have shot in areas where the noise didn't bother me at all
but the concussions were palpable. I have 2 pair I use when I shoot my firearms and the most I "hear" is a pop/crack until one day I left the right side up and fired a .45 and my right ear rang for 15 minutes. really hurt!

 

[Bobson]

That equation there makes zero sense. They are saying if you add something over one set it actually increases the sound getting through?
That's like saying if you have two fences around your home it will be easier to get in.

Recall that NRR is a just a formula for a noise reduction constant.

Where NRRH = the NRR of the Higher of the two pieces of hearing protection: "[(NRRH-7)/2]+5" is the formula leading to the noise reduction constant. If you leave out the (+5) benefit of an extra layer of hearing protection, you're left with (NRR-7)/2. Let's look at an example:

Earmuffs with a noise reduction rating of 30: (30-7)/2 = 11.5
Ear plugs with an NRR of 18: (18-7)/2 = 5.5

So the benefit of the muffs alone indeed greater than the plugs alone. Likewise, the benefit of the two, together (11.5+5=16.5), is greater than either one by itself. It certainly doesn't make it detrimental in any way, as was the case with each of your counter-examples.

As for the inevitable question, Why don't you get the full benefit from each when they're worn together, for a total of 17? The truth is that I don't know. I got my information on this from OSHA, and I trust that they're correct. I've also been told that wearing two condoms at once isn't any safer than wearing one condom by itself - go figure.

I have shot in areas where the noise didn't bother me at all but the concussions were palpable.

One significant part of hearing loss is attributed to concussion, rather than sound. Your inner ear canal is lined with small fibers of hair which I forget the name of, but basically, in terms of the biology of your ear, these hair fibers play an extraordinarily important role in the processing of noise; without them, an otherwise perfectly intact/normal ear can't pick up any sound whatsoever, resulting in 100% deafness. Concussions heavy enough destroy these fibers permanently. So really, even if we had hearing protection that could fully negate the sound of a gunshot, if the concussions could still get through, you could still eventually suffer from extreme loss of hearing or even complete deafness.

Like herrwalther said, this is very interesting stuff; and even all the information here really just begins to scratch the surface. The more we learn, the more important it becomes that we get suppressors off the NFA.

 

[Tirod]

The truly signficant issue is that we have the "fix" available. They are called suppressors.

They were invented and are usually MANDATORY for automotive and motorcycle use (yeah, I know, nonetheless . . .) And in other countries they are considered a requirement for shooting in open public ranges.

Here, it's been politicized, and even tho other countries issue suppressors one for one in their armed forces (like Korea,) we still haven't got the "memo." What we do is pay for hearing compensation and disability for a preventable problem.

Like the amount of money we spend in college education, students could simply go for free, the government could underwrite the costs of a free suppressor and the savings would pay for it.

If the idealistic principle of government is to prevent evil, then that is the logical conclusion. What gets into the issue is politics and ignorance, when medical practicality should be our #1 interest.

Naturally, the point of having suppressors be entirely legal falls on deaf ears.

 

[drband]

Cilia- the little hair-like sensory structures in the inner ear.

 

[jmorris]

Why don't you get the full benefit from each when they're worn together, for a total of 17? The truth is that I don't know. I got my information on this from OSHA, and I trust that they're correct. I've also been told that wearing two condoms at once isn't any safer than wearing one condom by itself - go figure.

Sounds like you just accept what your told and thats it.

If I double plug (I have custom made in ear plugs molded to my ear canal) I cannot reliabily hear the beep of a shot timer.

If I go by what I read here.

http://www.gcaudio.com/resources/howtos/loudness.html

That was taken from the FAAA

A hand drill can damage my hearing and if I ride in an airplane (less than 100' from a jet engine) with hearing protection I can damage my ears.



and here is one for the "double glove" argument.

http://www.ncbi.nlm.nih.gov/pubmed/12137673

 

[hso]

If you search for Noise Reduction Rating here you'd find many threads on hearing protection and what it involves.

You're missing an important piece of information. Duration of exposure. While any exposure above 140dB may produce damage, it is the cumulative exposure that you're trying to reduce. Gunshot noise may be at 155-175 dB, but the duration is very short for a single gunshot. Fractions of a second, as opposed to continuous 8hr exposure. Several shots add to the cumulative exposure. While no one that cares about protecting their hearing and understands anything about wearing hearing protection would allow themselves to be exposed to a 140db+ gunshot since that alone risks damage wearing hearing protection and understanding that those short exposures below 140dB have to be added up to exceed safe levels.

BTW, HL makes many more muffs than just the ones seen in the sporting goods store. They're one of the major industrial safety equipment manufacturers.

 

[WestKentucky]

The reason you don't get to add nrr numbers is that the two separate layers of protection do not get the same exposure of noise. So if the muffs take an external hit of 165 and reduce it to say 125 then the plugs get a considerably lower noise exposure to work on. Exactly like that 30% off coupon at harbor freight. They give 30% off the sale price not the original price before marking the price down. So my tumbler was 60 bucks regular, and was on sale for 40. Mom used her 30% off on it and got it for 28. Same thing on hearing protection the muffs give the sales price reduction and then the canal caps give the coupon savings. Otherwise my tumbler would have been 22 bucks after 30% then 20 bucks off.

 

[Bobson]

Sounds like you just accept what your told and thats it.

I beg your pardon, sir.

There are those who believe everything they're told; and as evidenced by your critique, others who refuse to believe anything they're told. Not one sentence I wrote in this thread has been a matter of opinion. All the information here has come from THE respected authority on the subject.

 

[Bobson]

The reason you don't get to add nrr numbers is that the two separate layers of protection do not get the same exposure of noise. So if the muffs take an external hit of 165 and reduce it to say 125 then the plugs get a considerably lower noise exposure to work on. Exactly like that 30% off coupon at harbor freight. They give 30% off the sale price not the original price before marking the price down. So my tumbler was 60 bucks regular, and was on sale for 40. Mom used her 30% off on it and got it for 28. Same thing on hearing protection the muffs give the sales price reduction and then the canal caps give the coupon savings. Otherwise my tumbler would have been 22 bucks after 30% then 20 bucks off.

Thank you, this makes a lot sense.

 

[Bobson]

And hso, thank you for sharing your knowledge on the subject.

 

[Louca]

I am wondering about something, and it is along the lines of what WestKentuckian mentions (but not exactly what he said). And what follows is not pure fact as some want here, just something I am wondering about.

Could it be that a noise reduction device reduces noise non-linearly with respect to intensity? In other words, the actual reduction amount may not always be say 30 dB, but may instead be 30 dB at some sound pressure level say 100 dB ambient and maybe 25 dB at 120 dB ambient. I don't know, just wondering.

Bobson, (according to Wikipedia) the /2 in your formula is put in there as a reduction for discrepancies in wearing vs. testing positions (bolding added by me):

OSHA has defined in their training manual for inspectors that the adequacy of hearing protection for use in a hazardous noise environment should be derated to account for how workers typically wear protection relative to how manufacturers test the protector's attenuation in the laboratory.[14] For all types of hearing protection, OSHA’s derating factor is 50%. If used with C-weighted noise, the derated NRR will become NRR/2.[14] If used with A-weighted noise, OSHA applies the 7-dB adjustment for C-A weighting first then derates the remainder.[14] For example, a protector with 33-dB attenuation would have this derating: Derated NRR = (33 – 7)/2 ... For earmuffs, the NRR should be derated by 25%, for slow-recovery foam earplugs the derating is 50% for all other protection, the derating is 70%.

So for earmuffs, it looks like the formula would become: Modified NRR = ((original NRR - 7) x 3)/4.

 

[Bobson]

Bobson, (according to Wikipedia) the /2 in your formula is put in there as a reduction for discrepancies in wearing vs. testing positions (bolding added by me):

That's correct. On the OSHA page I linked as a source in the OP, OSHA strongly recommends the 50% reduction for all "real life" applications, due simply to the fact that everyday practical use will seldom, if ever, match laboratory results, for various reasons. Fit may not be perfect/ideal, hair or head garments could interfere, etc.

Ultimately, the bottom line is that, for any number of reasons, the Joe Average user of any product is unlikely to experience results like those obtained in a laboratory setting, where every possible inhibiting factor is considered and accounted for. The 50% reduction in effectiveness addresses that, and this is something that's done in many different ways, in nearly every discipline of science.

I'm not sure about your resulting formula; I didn't read what you quoted from Wikipedia, as I'm a bit busy atm. Without the 50% reduction, it should be simply (NRR-7).

 

[WestKentucky]

OSHA rating is halved due to the prevalence of people not properly wearing hearing protection in the form of not pushing canal caps in far enough, or pinching the earlobes with muffs allowing pathways for raw noise to enter the ear canal. Since a surprising amount of noise is transmitted up the jawbone, you can never soundproof the person without putting the entire person in a soundproof room. At that point the sound of various bodily functions is very clear from the obvious unpleasant ones down to heart and lung sounds.

 

[hso]

One thing you have to remember is that plugs are efficient over a different range of frequencies than muffs. Think of two curves with high attenuation for one of the low frequency end for one curve and high attenuation for the other on the high frequency end. Each curve drops and flattens out as they approach the opposite end of the scale. So, if you average them across the frequencies they aren't going to be like adding NRR A to NRR B to get an effective NRR of A+B. Other factors that come into play are whether the protection is worn the entire time and whether they're perfectly applied (challenging with eye protection with muffs and sloppy plug insertion).

Even more disappointing is the "field attenuation" vs. the lab attenuation. IOW, real world use vs. ideal.

For dual protection (ear muffs and plugs are used simultaneously) use the following:

1. Determine the laboratory-based NRR for the higher rated protector (NRRh).

2. Subtract 7 dB from NRRh if using A-weighted sound level data.

3. Add 5 dB to the field-adjusted NRR to account for the use of the second hearing protector.

4. Subtract the remainder from the TWA as follows:

Estimated Exposure (dBA) = TWA (dBC) - (NRRh + 5) , or

Estimated Exposure (dBA) = TWA (dBA) - [(NRRh- 7) + 5]

Example:

TWA=110 dBA, plug NRR=29, and muff NRR=25 dB

Estimated Exposure = 110 - [(29 - 7) + 5] = 83 dBA

OSHA's experience and the published scientific literature have shown that laboratory-obtained real ear attenuation for HPDs can seldom be achieved in the workplace. To adjust for workplace conditions, OSHA strongly recommends applying a 50% correction factor when estimating field attenuation. This is especially important when considering whether engineering controls are to be implemented. The equations above would then be modified as follows:

Single Protection:
Estimated Exposure (dBA) = TWA (dBC) - [NRR x 50%], or

Estimated Exposure (dBA) = TWA (dBA) - [(NRR - 7) x 50%]

Dual Protection:
Estimated Exposure (dBA) = TWA (dBC) - [(NRRh x 50%) + 5] , or

Estimated Exposure (dBA) = TWA (dBA) - {[(NRRh - 7) x 50%] + 5}

 

[pendennis]

I got to talk to one of NIOSH's engineers a couple of years ago, and spent quite a bit of time asking him some very basic questions, and picked up a lot of good information.

One of the things you also have to consider, is the shock wave which travels through the stock of a rifle or shotgun. Most folks think that it's just the noise. A rifle or shotgun stock besides transmitting a shock wave, also displaces all hearing muffs, no matter what the quality (dependent on whether you shoot right- or left-handed). And, if you have a beard, it can also break the seal. The other problem is shooting glasses. They all break the seal, so you can never be assured that you're getting anywhere near the protection that muffs offer.

I've worn a combination of custom molded ear plugs, and very high quality electronic muffs (Peltor Tactical Pro and Pro Ears Gold Magnum) for years, but I'm headed to the ENT on Thursday for tests. I know I have some hearing loss, and it's especially noticeable in my right ear (right-hand shooter).

 

[lxd55]

Recall that NRR is a just a formula for a noise reduction constant.

Where NRRH = the NRR of the Higher of the two pieces of hearing protection: "[(NRRH-7)/2]+5" is the formula leading to the noise reduction constant. If you leave out the (+5) benefit of an extra layer of hearing protection, you're left with (NRR-7)/2. Let's look at an example:

Earmuffs with a noise reduction rating of 30: (30-7)/2 = 11.5
Ear plugs with an NRR of 18: (18-7)/2 = 5.5

So the benefit of the muffs alone indeed greater than the plugs alone. Likewise, the benefit of the two, together (11.5+5=16.5), is greater than either one by itself. It certainly doesn't make it detrimental in any way, as was the case with each of your counter-examples.

As for the inevitable question, Why don't you get the full benefit from each when they're worn together, for a total of 17? The truth is that I don't know. I got my information on this from OSHA, and I trust that they're correct. I've also been told that wearing two condoms at once isn't any safer than wearing one condom by itself - go figure.


One significant part of hearing loss is attributed to concussion, rather than sound. Your inner ear canal is lined with small fibers of hair which I forget the name of, but basically, in terms of the biology of your ear, these hair fibers play an extraordinarily important role in the processing of noise; without them, an otherwise perfectly intact/normal ear can't pick up any sound whatsoever, resulting in 100% deafness. Concussions heavy enough destroy these fibers permanently. So really, even if we had hearing protection that could fully negate the sound of a gunshot, if the concussions could still get through, you could still eventually suffer from extreme loss of hearing or even complete deafness.

Like herrwalther said, this is very interesting stuff; and even all the information here really just begins to scratch the surface. The more we learn, the more important it becomes that we get suppressors off the NFA.

thanx for the concern and your post is spot on, there are levels of sound which can affect you or others.

 

[redneck2]

I don't see that anyone has linked to this.

...http://www.thehighroad.org/showthread.php?t=654267

 

[MachIVshooter]

OoooooooK..........

We're putting WAYYYYYYY too much emphasis on the logarithmic decibel measurement here, and almost completely ignoring every other aspect of acoustics.

dB are nowhere near the whole story, and that is especially true of gunshots. As well, the NRR of hearing protection only addresses the dB aspect, does not delve into the deflection aspect.

Sound is vibration, that's it. Yet acoustics is not a simple science at all. Decibels only measure intensity; it does not account for duration, pitch, vector, reflection or pressure. Those aspects matter tremendously; A "noise" can have a "safe" dB level but create pressure that damages the ear. And noises that are so high pitched (high-frequency; ultrasonic) we cannot detect them could completely obliterate the cilia. Noises that may be relatively safe in an outdoor setting, such as a .22 rifle (short exposure at ~135 dB) are damaging in a confined area due to reflection. Noises may transcend the boundary from safe to damaging if the exposure is prolonged (duration).

Where hearing protection benefits us the most is not the dB reduction, but the deflection rather than absorption of the vibration (sound waves). Think of it this way:

Fire a .22 rifle with a 20" or longer barrel without ears outdoors. This is not offensive to most people, and just shy of the threshhold of instant damage by dB rating.

Yet if your ear is only a few inches away and perpendicular to the muzzle, it will be painful.

Now, fire that same rifle inside a metal ship container. Ouch! The refection of those waves inside a confined area made of materials that do not readily absorb the waves makes the volume unpleasant, but the dB are no different.

Then try it outdoors again, but this time with the muzzle stuck through a tight fitting hole in a 4x8 plywood sheet. That plywood will deflect the noise very effectively. So much so that, using the above method, much louder firearms would be hearing safe for the shooter on the back side of the board without ear protection.

Basically, for the formula in the OP to apply, you'd have to put your head in a small steel box with & right next to the muzzle. And yes, under those circumstances, NRR 30 muffs will not prevent hearing damage.

In short, do not think of sound as something ambient. Noise has a vector, and it can be deflected/reflected. This is the primary mode of protection offered by both muffs and plugs, and is why muffs are more effective, with a hard and usually convex exterior that more readily deflects the waves. So even though the NRR calculations suggest that we will still suffer hearing damage from gunshots even with PPE, we can conclude both mathematically and through billions of hours of human use that ear protection does, in fact, reduce the noise of a gun shot to safe levels under most. circumstances.

 

[gearhead]

OSHA rating is halved due to the prevalence of people not properly wearing hearing protection in the form of not pushing canal caps in far enough, or pinching the earlobes with muffs allowing pathways for raw noise to enter the ear canal. Since a surprising amount of noise is transmitted up the jawbone, you can never soundproof the person without putting the entire person in a soundproof room. At that point the sound of various bodily functions is very clear from the obvious unpleasant ones down to heart and lung sounds.

Excellent point about sound transmission through the bones in the head. For instance, I find shooting an M4 much more offensive than shooting a SCAR or bolt action simply due to all the impact taking place right below my cheek weld in the M4. It really transmits a lot of energy that way.

 

[Sheepdog1968]

I double up for the reasons you pointed out. In the 1980s as a teenager, I went to see the Born in the USA tour by Bruce Springsteen. It was a great concert. However, I found the next day my ears were ringing a bit. I decided that this can't be good for me. Ever since I wear hearing protection for loud things such as vacuuming, lawn mowing, and power tools. I figure it can't hurt and may help.

Unrelated, I saw a show on TV that compared a 40 year old native who loved in the sun to an 80 year old monk who was inside a monastery. Their skin differences were amazing. As such, I became a shade dweller.

Don't get wrong. Plenty of things I do wrong but when I die I will still hear well and have nice skin. Lol. Or as my friend used to say, a pretty corpse.

 

[Lateck]

You folks can argue the scales etc.,,, all day long.
But only Tirod gave a very good solution, suppressors.
That way it helps everyone, mostly on a range and for new young shooters.
You would still need "ears" but if you had them rated for 30dbs then the issue is mute.
Safety for all.

Lateck,