Contact Information
last updated: 10/22/2010

Bulgarian Drone-Singing : Akerbergs

     Drone is an important technique widely incorporated into Bulgarian and Latvian vocal tradition, and is a "sound" that I suspect greatly contributes to the common vocal characteristics of these two cultures. When viewed as an acoustic phenomenon, the element of drone raises some intriguing questions. What are the aesthetics of unison singing in drone? What is it that makes drone singing so appealing and widespread for Bulgarian and Latvian traditional singing? The fascination of the single held tone is known throughout the world and in many different cultures, and is an important part of certain religious and spiritual practices. Can studying the acoustic characteristics of the drone give us some clues for the reasons behind its popularity?

     As a member of a Latvian-Swedish vocal group devoted to reproducing the traditional Latvian singing style, I noticed several important characteristics that were important for a Latvian style to be considered "traditional." One was a tight-lipped, nasal, piercing sound that could be heard from afar. Another was the desirability of a loud, long, powerful tone. Yet a third characteristic was a straight, intense, and concentrated sound. It is my desire in this study to shed some light on the acoustical correlates for these descriptive, stylistic terms.

In Timothy Rice’s study on Bulgarian music, he translates indigenous descriptions of the interval between the drone and melody as "ringing like a bell" (Rice 165). Rice suggests that singers often sing thirds slightly sharp, producing an interval "rings like a bell." The result of the sharpened third is an overall rise in pitch throughout the execution of a song, until the final verse is a higher in pitch than the first verse. Though Rice mentions several different intervals "ringing like a bell," he does not include the unison, though the return to a unison after singing thirds, fourths, fifths, but especially seconds, is one of the most appealing characteristics of drone-singing.

     It may be, however, that such a bell-ringing tone is exactly the element responsible for the drone’s appeal. Can a unison also "ring like a bell"? Unison voices can produce a "beating" effect. Rice touches only slightly upon this possibility in his dissertation and then immediately abandons it. Rice writes: "...these singers recognize the impossibility of a true unison in choral performance. When two or more singers attempt to sing the same musical line together, they cannot fail to produce some simultaneously contrasting sounds through rhythmic, pitch or timbral differences" (Rice 7). What Rice may be describing is the "chorus effect", resulting from several voices singing "in unison"; inevitably such voices hover around a target pitch without achieving an exact unison, so that the effect of many voices, each with harmonics of slightly different frequencies, is a slow beating. Thus several voices together producing many different beating rates simultaneously, some slower, some faster, all gradually changing results in a kind of timbral ebb and flow (Hall 85).

     It may be this beating–especially of upper harmonics–that is sought, if even unconsciously, by drone singers, and it is perhaps the same beating that makes the unison interval "ring like a bell." Preliminary work at the SAVAIL lab has revealed that many features of vocal style considered desirable by experienced singers are especially conducive to beating. Figures 1-3 show the harmonic structure–the relative intensity of fundamental and harmonics of a solo voice singing at a single moment during the execution of a sung vowel. Frequency is on the horizontal axis, with amplitude is on the vertical axis; duration is not indicated as the spectra represent only an instant. Figure 1 shows a Bulgarian soloist, figure 2 shows an older Latvian woman soloist, while figure 3 shows a contrasting style typical of much European and American folksing in a rendition of the Ballad of Nockenoor..

Figure 1 Figure 2 Figure 3

Bulgarian solo Latvian solo Irish ballad

     In a large proportion of traditional sung timbres, the harmonic structure mimics the structure of spoken timbre, or vowel quality. In speech and often with sung vowels, as in figure 3, the upper harmonics are typically greatly attenuated relative to harmonics below about 600 Hz. One might suppose the harmonic structure of spoken vowels to be the "most natural" to maintain in singing. In the Bulgarian and Latvian samples (figs 1 and 2), however, the upper and lower harmonics are almost equal in intensity. This unusual harmonic structure may indicate a deliberate effort on the part of the singers to modulate sung timbre by emphasizing the upper harmonics.

This perhaps points out the conscious effort of the Bulgarian and Latvian to emphasize the upper harmonics. This can be brought about by "brightening" the tone. One Latvian singing group specified the importance of singing the drone with a broad, tight mouth, as if singing on the vowel "e". This also would emphasize the upper harmonics. The essential effect of emphasizing the upper harmonics, is the enhancing of close frequencies in the upper frequency ranges, where the perceivable beating would be negligible given the normal rolloff of harmonics in this range. Thus the vocal style of the Latvian and Bulgarian samples shows a deliberate effort to intensify the beating of the "chorus effect", and of a resulting "ringing" of the tone, inevitable in unison drone singing.

Another way of enhancing the beating effect is by keeping the singing tone as vibrato-free, as intense and "straight" as possible, so as to eliminate extraneous frequencies which would ruin the chorus effect. Degree of vibrato can be analyzed by measuring the "jitter" of a tone, to see how much it wavers from a central pitch. Figures 4-6 show the results of samples from the same three people represented in Figures 1-3, but this time showing how much jitter each contains. A straight line, or one that is just slightly wavy, signifies no "jitter," while an erratic line with various peaks and valleys signifies the presence of much "jitter." The Bulgarian sample shows very little jitter (fig. 4), while the Latvian sample shows very little jitter except for the beginning moment, when it is possible that she is changing a vowel, thus upsetting the natural "jitterness" of the voice. Figure 6, on the other hand, shows the contrasting sample (Ballad of Nockenoor) with a wide jitter. Since a considerable degree of jitter, whether vibrato or more micromodulation, is almost inevitable with the spoken or sung voice, the lack of frequency modulation in the Bulgarian and Latvian samples may again indicate the effort to intensify beating as much as possible.

Figure 4

Bulgarian solo

Figure 5

Latvian solo

Figure 6

Irish ballad

     I must emphasize, though, that these are preliminary results of random samples. It is hoped that these are representative results.

Results from a preliminary analysis made on a young Latvian group shows a recording that has been manipulated by reverberation and equalization in the audio technician’s studio. A preliminary hearing, to my ears, left me dissatisfied with the muddy, cathedral-like sounding tone. Yet, a closer examination of the higher frequencies shows curious gaps of silence in the harmonic waves, that could point either to waves canceling each other out, or to a deliberate effort on the part of the technician to bring about silences in harmonic frequencies in order to enhance the flow and ebb of inter-meshing harmonics, and thus creating an artificial "chorus effect". 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.

Figure 7

Bulgarian women

Figure 8

Latvian women

     Since normal harmonics do not tend to cancel each other out, these examples lead one to suspect that the sound has been tampered with "artificially".  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.



Hall, Donald E. Musical Acoustics. Pacific Grove, CA: Brooks/Cole Publishing, c1991.

Rice, Timothy Floyd. Polyphony in Bulgarian Folk Music. Dissertation. Ann Arbor, Michigan: University Microfilms International, 1977.

last updated: 10/22/2010