Window function in the frequency domain

Hello,

I am working on a project that measures the sound with a pressure sensor and then analyse the signal. For the analysis I was asked to get the amplitude of the FFT-response for certain frequency bands.

The way I am thinking to solve this is to get my signal to a N=2048 buffer, and do the FFT calculation on that buffer. Then I get a good spectrum with enough precision. But now I would like to get the mean value of a certain band in this frequency spectrum. I think about multiplying the spectrum with a window function. But I can not find a window function that works in the frequency domain. Can somebody help me?