Building a Đàn Bầu — Vietnam's One-String Harmonic Zither

The đàn bầu is Vietnam's one-string instrument. It plays only pure, bell-like harmonic notes, and a flexible rod bends every one of them, so a single string can sing like a voice.

Build a long, narrow, hollow wooden box about 90 cm long. This is the resonator that carries the quiet harmonics.

At one end, fix a springy vertical rod (bamboo, or traditionally buffalo horn) that can flex back and forth. This rod is the whammy that bends the notes.

Thread a small hollowed gourd onto the base of the rod, where it meets the string. The gourd softens and colours the tone — and gives the đàn bầu ('gourd instrument') its name.

Fit a single tuning peg at the far end of the box.

Tie the single string to the top of the flexible rod, run it the whole length of the box, and wind it onto the peg. Tune it to a clear, low note.

Rest the SIDE of your hand lightly on the string exactly halfway along, pluck with a finger of that same hand, and lift instantly. A pure, flute-like note rings — a harmonic, an octave above the string's own note.

Touch and pluck at one-third, one-quarter and one-fifth of the string's length. Each point sounds a higher harmonic. The đàn bầu plays with these pure overtones alone.

While a harmonic rings, push the flexible rod toward the box to slacken the string and DROP the note, or pull it away to tighten the string and RAISE it. Bend slowly and smoothly.

Pluck a harmonic and waver the rod gently to make the note glide and shimmer — the way the đàn bầu shapes its tones to follow the rising and falling sounds of the Vietnamese language.

The đàn bầu rests on two beautiful pieces of physics. The first is the string harmonic. A plucked string does not only vibrate as a whole — it also vibrates in halves, thirds, quarters and more all at once, adding quiet higher notes, the harmonic series, on top of its main note. If you touch the string lightly at a still point (a node) of one of those patterns and pluck, you silence the main note and every pattern that moves at that spot, leaving only a harmonic that has a node there — so a single, pure, flute-like overtone rings out alone. Touch halfway for the note an octave up, at a third for the note higher still, and so on up the series; the đàn bầu plays with these harmonics only, which is why its voice is so pure and bell-like. The second idea is tension: a string's pitch rises when it is tightened and falls when it is slackened, and the flexible rod (the cần) is a lever for exactly that — flex it toward the box to loosen the string and bend the note down, flex it away to tighten and bend up — so any harmonic can be slid, wavered and swooped continuously, famously tracing the six tones of spoken Vietnamese. It is the same tension trick as the ektara's squeezed neck and the musical bow, and the same harmonic series that the kalyuka and conch hunt through inside a tube — here found on a single string. The little gourd softens the sound, and because bare harmonics are gentle, modern đàn bầu players usually add a magnetic pickup and amplifier.