How do you calculate the noise reduction rate when using earplugs and earmuffs at the same time?

How do you calculate the noise reduction rate when using earplugs and earmuffs at the same time?

If you have earplugs with a NRR of 33 dB and ear muffs with a NRR of 30 dB.

What would the correct total NRR be?

Please also explain your calculation

I tried googling this, but did not find any clear answer.

Thanks in advance!
/L

 

http://www.audiologyonline.com/articles/extra-protection-wearing-earmuffs-and-1218

"No empirical or theoretically derived equations are available that can predict the attenuation of an earplug and earmuff combination with sufficient accuracy to be useful"

What the hell?

 

Markku is right, that website is wrong