The isolation of the Shure E2C is approximately 20-25dB, if I'm not mistaken.

My questions is, knowing that dB is calculated on a logarithmic scale, how would you calculate the dB of the sound you DO hear with earbuds on?

For example, if external noise is 100dB and I put on my earbuds which provide, say, 20dBs of isolation, and DO NOT PLAY ANY MUSIC, how much reaches me? 80dB?

Also... if I start playing music, do I ADD that dB to the dB from the external source that reaches me?

Just trying to straighten things out.

Thanks so much!

Decibels are measured on a logarithmic scale. So, with 20 dB of isolation, you can simply subtract 20dB. With a 100dB source and 20dB of isolation, you'll hear 80dB.

On the other hand, two 80dB sources will -- at most -- sound twice as loud as a single 80dB source (and sometimes less than this, because soundwaves may cancel), so that is only a 3dB increase in sound.

3dB or 2dB?

One more question..

Let's say I have a 100dB source.
if I have two 100dB sources playing, does that mean the new combined rating would be 101dB?

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Two uncorrelated sources at the same level will add up to +3dB.

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No, 103dB, see above. With two correlated sources, say two speakers next to each other playing the same signal, you'll end up at 106dB.