Express objective function in Optimization study of Frequency domain in Acoustics module

Hi,

I want to run an optimization study in frequency domain. I have experimental values of the variable alpha for specific frequencies and I want to adjust my control variable R in order to match these results. If my experimental results are for frequencies 20, 30, 40 kHz ... then the objective functions to be minimised are:

abs(alpha(20kHz) - expermentvalue1)
abs(alpha(30kHz)- experimentvalue2)
abs(alpha(40kHz)- experimentvalue3)
....

My question is: How do I express in Comsol "alpha(20 kHz)"? Is there an operator to do so?

Thanks