during the nineteenth century, when in Bulgaria enters the influence of Russian polyphonic choral church music. Many Bulgarian composers (Dobri Hristov, Petar Dinev, etc.) create their works in the spirit of Russian polyphony. Today Orthodox music is alive and is performed both during church worship services and at concerts by secular choirs and soloists. Contemporary Bulgarian worldwide recognized choirs and singers in whose repertoire permanently takes place the orthodox music are: Yoan Kukuzel Choir, Plovdiv Boys' Choir 'Stefka Blagoeva', Sofia Boys' Choir, Madrigal Sofia Choir, Sofia Orthodox Choir, Sofia Priest Choir, etc., worldwide famous opera singers Boris Christoff and Nicola Ghiuselev. The distinctive sound of Bulgarian folk music comes partly from the asymmetric rhythms, harmony and polyphony, such as the use of close intervals like the major second and the singing of a drone accompaniment underneath the melody, especially common in songs from the Shopluk region in Western Bulgaria and the Pirin region. Bulgarian folk music is unique in its complex harmonies and highly irregular rhythms. These kinds of rhythms, also called uneven beats or asymmetric measures, were introduced to musicologists only in 1886 when music teacher Anastas Stoyan published Bulgarian folk melodies for the first time. Examples of such beats are , , , and , or composite ones like , and . Each area of Bulgaria has a characteristic music and dance style. Bulgarian folk music inspired and was used by musicians like Kate Bush and George Harrison. Bulgarian vocal style has a unique throat quality, while the singers themselves are renowned for their range. Their voices are low and soprano and the children love singing as well as anything artistic. Diatonic scales predominate but in the Rhodope Mountains, for example, pentatonic scales occur, while in Thrace chromatic scales with augmented intervals (similar to the music of Classical Greece). Also, the intonation varies, and is quite different from the modern Western equal temperament. Depending on whether the melody moves up or down, an interval can augment or decrease by a quarter tone. Musical instruments (also characteristic of the whole Balkan region) include gaida (bagpipe), kaval (rim-blown flute), zurna or zurla (another woodwind, similar to oboe typical among Roma), tambura (long-necked lute), gadulka (bowed instrument held upright). The gaida of Bulgaria is worthy of its own subsection. In Bulgaria the gaida has been a long symbol of the country and its heritage, and is one of the more well-known