A preliminary hearing, to my ears, left me dissatisfied with the muddy, cathedral-like sounding tone. Figure 7 shows a cluster of harmonics from a normal recording of Bulgarian women, while figure 8 shows a cluster of harmonics from the altered Latvian recording. The Bulgarian figure shows that normal harmonics never cancel each other out to zero, while the Latvian harmonics intermittently cut off to produce silences. The straight lines in Figure 8 show intermittent silences in the harmonics' waves. This research, though, is in its preliminary stages, and nothing can be asserted with certainty.
More studies have to be done.
But the results obtained show promise of a very rich field of research, that could supply some interesting answers about the role and meaning of the drone in different cultures. Bibliography Hall, Donald E. Musical Acoustics. Rice, Timothy Floyd. Polyphony in Bulgarian Folk Music.