Exponential in relation to what other value? Adding 1 to _____ causes a doubling of resistance to pressure, adding 2 is a quadrupling, adding 3 is an octupling

Exponential in relation to what other value? Adding 1 to _____ causes a doubling of resistance to pressure, adding 2 is a quadrupling, adding 3 is an octupling, what goes in the blank?