In this paper, direct and indirect methods of speech transmission index (STI) estimation are compared. Two versions of the indirect method of the STI estimating are considered. In the first version of the indirect method, a pair of signals is used as a test signal. It is a Maximum Length Sequence (MLS) signal with a uniform spectrum and a noise signal with a speech spectrum. In the second version of the indirect method, the test signal is an MLS signal with a speech spectrum. The comparison is carried out by means of computer modeling and by carrying out a field experiment in a medium sized university auditorium. Both versions of the indirect method use the same basic computer programs for STI calculating. It is shown that for the second version of the indirect method, the average values of the STI estimates differ from ones for the direct method by no more than 0.06 for signal-to-noise ratios from minus 20 dB to plus 20 dB. For the first version of the indirect method, this difference is significantly larger and reaches 0.24.