Why doubling-up hearing pro only gains you ~5dB

[Vorpalis]

I'd read several places that doubling-up on hearing pro only gains you about 5dB additional attenuation above the higher-rated of the two, regardless of their ratings, and wondered why. (If you wear 25dB ear plugs, and then put on 30dB ear muffs, the total attenuation isn't 55db, it's actually ~35dB).

I spent several hours looking all over the internet for an explanation, and learned lots of interesting stuff about noise and hearing protection, except an explanation for this doubling-up question. Eventually, I stumbled on a PowerPoint presentation given my someone in the CDC's NIOSH. This gave me the idea to just contact them, since they are there to serve us tax payers. Lo and behold, I actually got an answer! And not just a form letter, either. The office I contacted said they didn't have a good explanation, so they had forwarded my question to a researcher in the Pittsburgh Mining Research Division, and a bona fide Auditory Doctor, too!

Here's why: bone conduction. Sound waves in the air around us don't just affect our ear drums, they also affect the rest of our bodies. No matter how much isolation / protection ear pro can lend our ear drums, some amount of any ambient sound will travel through our bodies to our ear drums. It's not a fixed value, either; a louder ambient sound will cause proportionally louder sound waves in our bodies, regardless of ear pro. So, if you're exposed to a 180dB sound while wearing 35dB-worth of ear pro, you're still going to be exposed to 145dB of sound through your body.

Related fun-facts I learned during my research:

• The amount of attenuation a piece of ear pro can provide varies according to frequency. For example, your 25dB-rated ear plugs might only provide 20dB of attenuation for sounds below 2kHz, but provide 28dB of attenuation for sounds above 8kHz. The 25dB rating you see on the package is either an average across all frequencies, or it's the highest attenuation it can achieve for a specific frequency range.
• The effectiveness of ear pro depends on using it correctly. People who stuff ear plugs into the outer pocket of their ear don't get much protection. You have to stuff them into your ear canal for them to be effective.
• The decibel scale is logarithmic. A change of ~3dB means a sound is either twice as loud or half as loud, depending. So, 23dB is twice as loud as 20dB, and 137dB is half as loud as 140dB. Also, a 10dB change is either ten times as loud or one-tenth as loud, depending. So, 110dB isn't 10% louder than 100dB, it's 10x as loud! And 90dB is one-tenth as loud as 100dB.
• Also, the actual unit of sound is the Bel. The "deci-" is metric for "one-tenth", like a decimal marks the tenths place in a number.

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[Stomper]

From god's lips to my ears, unfortunately I was wearing my ear-pro and missed what was said!

 

[MrMillspecer]

Dad used to hand us cigarette butts for ear plugs. Wouldn't recommend that.

When the wind is right you can still smell the Belair Menthol.

 

[11Charlie]

In my house you need to double up when the women start talking, either that or go out to the shop.

 

[Stomper]

In the Army we used to use .45acp rounds inserted bullet end first into our ears... TRUE STORY, LOL

 

[L84Cabo]

Bro, you didn't know all that? Why didn't you just ask me?

I kid...a bit. I knew the scale was logarithmic and that the amount of attenuation varies according to frequency. So you do have to be a little careful when buying ear pro and hope the rating the mfg gives is either the lowest rating across all frequencies or the rating for gun shots. To this end, if you've ever looked at the NRR for MSA Sordins...which are widely regarded as some of the best ear muffs out there, their NRR rating is pretty low. It's my understanding that they've provided the lowest rating tested. The NRR at gun shot frequency is supposedly much higher.

I did not, however, know about sound traveling through the body. That's very interesting to say the least.

 

[Diamondback]

Here's why: bone conduction. Sound waves in the air around us don't just affect our ear drums, they also affect the rest of our bodies. No matter how much isolation / protection ear pro can lend our ear drums, some amount of any ambient sound will travel through our bodies to our ear drums. It's not a fixed value, either; a louder ambient sound will cause proportionally louder sound waves in our bodies, regardless of ear pro. So, if you're exposed to a 180dB sound while wearing 35dB-worth of ear pro, you're still going to be exposed to 145dB of sound through your body.

Yup... this is exactly why fighter pilots wear brain-buckets, a metal eggshell around the head to block the worst of it. They actually have *no* impact-protection effect when you're smacked through the canopy or go thump after hitting the silk.

 

[Certaindeaf]

From god's lips to my ears, unfortunately I was wearing my ear-pro and missed what was said!

What an ear pro!

 

[BillGabby1]

Dad used to hand us cigarette butts for ear plugs. Wouldn't recommend that.

When the wind is right you can still smell the Belair Menthol.

Here goes the old fart.
LOL
Back in the day, (don't you just love that lead in?)
when I worked for the Poleece Dportment, we were given a ball of cotton or told to put fired cases in our ears or alternatively use cigarette filters, non of which did much good, nowadays I could probably sue them over not providing hearing protection if I was handed a wad of cotton. I know they didn't work because of the constant hissing and squealing in my ears. Oh there goes the siren again!

 

[Snorider]

I took a noise class once (I am a CSP), the book we used is 3" thick much of which is math. Noise is interesting, a lot of physics. Basically air pressures that are transferred into mechanical energy which causes a biochemical reaction.

 

[Vorpalis]

...(I am a CSP)...

What's a CSP?

 

[Snorider]

Certified Safety Professional. We work with occupational noise exposure. This thread is good info...nice work!

 

[Kruel J]

Huh?

 

[Vorpalis]

Certified Safety Professional. We work with occupational noise exposure. This thread is good info...nice work!

Thank you! I'm glad someone else found this interesting.

 

[41mag]

"the constant hissing and squealing in my ears"....

.....you got a wife around, yes?

 

[Member 5605]

I'm cheap, I give the kids tissues to stuff in their ears.

That's got to be good for at least 500db protection.

Back to topic:
If anyone plays a Jews Harp you know about bone conduction.
(fast forward to 4:35)

 

[PiratePast40]

I've noticed a perceptible difference when I double up. I wear sonic defenders under electronic muffs. Remove the muffs and the sound difference is significant. And if one of the plugs comes out, same thing. Can really tell the difference. Of course, still have freakin' tinnitus.

EarPro EP3 Sonic Defenders® Earplugs from SureFire
Howard Leight | Impact Sport Electronic Earmuff

 

[Vorpalis]

@PiratePast40 Same experience here. It's a HUGE difference going from double to single ear pro. Wearing double ear pro, the outside world gets so quiet that I can hear my own breathing, heartbeat, stomach gurgling, and joints moving. It seems incredible how much an extra ~5dB makes, but then I realize that with 30dB ear muffs on, ambient sounds are 1/1000th as loud. With another 5dB of attenuation from ear plugs, sounds are almost 1/4000th as loud!

 

[Nick Burkhardt]

Forest Service says drones hamper firefighters

 

[woody06]

Thanks for all the responses and some very good ideas.
Certaindeaf - yeah, but the link posted by BDA.45 explains it.
Caveman Jim - sounds like we have similar problems
Piratepast40 - tnx for the link on the EP3 sonic defender. They look interesting.

I'm still be interested in hearing from anyone with custom molded earplugs.