Comparing PC FFT Program ARTA Software to HP 3580A & iPhone FFT Program

It certainly seems the FFTanalyzer is the way to go to get frequency domain information of audio frequencies. Low frequencies, i.e. 20 Hz and even lower are displayed on the FFT analyzer much quicker than the traditional audio frequency spectrum analyzer. Above about 5 KHz I do experience the FFT analyzer disagreeing more and more as the frequency rises with the THD display of the Tektronix AA501 and the HP 8903. 50% disagreement occurs with a fundamental frequency of about 16 KHz.