Choosing a value from within a frequency band that isn't specifically stated as a value in an array

Be gentle!

I am trying to create a tool for working out the radiation efficiency for a thin plate. There are three formulas, depending on the critical frequency of the material in question

lets say

• that Fc = the critical frequency
• the centre frequencies of the third-octave bands of interest are f = [50 63 80 100 125 160 200 250]

to work out the radiation efficiency in third octave bands there are 3 equations

1. where f<Fc
2. where f = Fc and
3. where f>Fc

the problem I have is where Fc is not equal to any of the values stated within the array f, but is within the third octave band. For example, an Fc value of 51Hz would be in the 50Hz band, a value of 85Hz would be in the 80Hz band etc...

As such, how do I create a script that assigns Fc to equation 2, when the value does not match that which is in the array.

Help most gratefully received!!!

M.