The isolation/frequency curve is one factor, certainly, but a bigger problem is that the ear's response is logarithmic. Even if the earplugs or earmuffs have a

The isolation/frequency curve is one factor, certainly, but a bigger problem is that the ear's response is logarithmic. Even if the earplugs or earmuffs have a perfectly flat isolation/frequency curve, so that all frequencies are attenuated by 33 dB, you can still hear just fine; it's just not as loud. Only if you introduce some masking sound (even white or pink noise will do) in combination with the attenuation can you render the outside sounds inaudible.