Evaluate a function using tabulated data

For a start you should create a worksheet with the tabled data and an attempt for the function definition and post it here.

You will also have to exlplain which frequency f.l and fp should be and why a function with the name  L.pl uses a variable(!!) with the same name in its definition. Guess that you don't have recursion in mind, don't you?

This is for predicting noise in a control valve, the variable Lpi is the internal sound pressure level at a reference pressure, fp is the peak frequency and both of these are known fi is the only changing variable.  The table is all the third octaves.