Averaging over bins in frequency domain
One possibility is use something similar to an MFCC Mel triangular filterbank with your FFT input vector, but using log spacing instead of Mel spacing for the triangular filter centers.  See: http://cmusphinx.sourceforge.net/doc/sphinx4/edu/cmu/sphinx/frontend/frequencywarp/MelFrequencyFilterBank.html and https://www.mathworks.com/matlabcentral/answers/195651-creating-mel-triangular-filters-function, and modify the triangle frequencies as needed.