This is 99% invisible. I'm Roman Mars.

The United States Marine Corps buys a lot of earplugs.

Visit any military base and you'll find them under the bleachers at the firing range in the bottoms of

washing machines. They are cheap and effective at making noise less noisy.

Most earplugs reduce noise by about 30 decibels.

That is Mary Roach, author of a bunch of great books. Her latest one is called Grunt.

It's about the science of humans at war.

30 decibels can be significant. Every 3 decibel increase in a loud noise cuts in half the amount of time you can be exposed without risking hearing damage.

To put that another way, an unprotected human ear can spend eight hours a day exposed to 85 decibels without

incurring hearing loss. That's the level of freeway noise. 85 decibels is also the level of a crowded restaurant.

But if you go up to 115 decibels, things like a chainsaw, or a really loud concert.

Your safe exposure time is only half a minute before your hearing could be affected.

So without ear plugs, you should only be exposed to the sound of a chain saw for about 30 seconds.

But with ear plugs, you've got closer to eight hours.

In a military situation, a reduction of 30 decibels is especially helpful with a steady grinding background

din, like say a Bradley tank clattering over asphalt or the

thrum of a Black Hawk helicopter.

But there's a problem with earplugs as the solution on a battlefield.

I will tell you that I've never seen a marine wear earplugs in a combat operation.

That's Aaron Owansu.

He spent eight years as an infantry marine and served a tour of duty in Fallujah, Iraq.

And this is Eric Fallon.

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