To hear the Vietnamese monochord the first time is to wonder how such a broad range of expressive, evocative sounds can come from playing a single string. The instrument, unique to Vietnamese musical tradition, consists of a hardwood box just over a meter long, tapering from 8.5 cm to 11 cm in width and from 6-8.5 cm in thickness, and a single brass string pegged at one end and tied to a flexible stick of bamboo or buffalo horn, inserted through a hollow wooden knob and fixed at the other end.
The player plucks with the string dampened, like harmonics on a guitar, at six precise points along the string. These correspond to notes on the musical scale, two or three notes apart. To play the notes in between, the player moves the stick to the right, loosening the string, to flatten the note, or to the left, tightening the string, to sharpen it. Use of the stick also controls the length and range of the plucked note, making it quiver, slide, rise or fall.
In Vietnamese, bẩu means gourd. In the days before electricity could be used to amplify the sound, the monochord comprised a section of bamboo and the stick the string was attached to passed through half of a bottle gourd. Large and concave, the gourd was the sound box.
The đàn bầu first appears in historical records in 1770, but the simplicity of its original design suggests it had already been around long before that. Both men and women play it and while its sound is often associated with melancholy, it is equally effective at lively or romantic tunes. Above all, it is a quintessentially Vietnamese sound.
to hear various tunes played on the đàn bầu, see the selections at https://www.youtube.com/results?search_query=vietnamese+monochord