The Khruang Sai orchestra combines some of the percussion and wind instruments of the piphat with an expanded string section including the so duang (a high-pitched two-string bowed lute), the lower pitched solaw (bowed lute) and the three-string jhakhe (a plucked zither). In addition to these instruments are the klhui (vertical fipple flute) in several sizes and ranges, a goblet drum (than) and, occasionally, a small hammered Chinese dulcimer (khim). The khruang sai ensemble is primarily used for instrumental indoor performances and for accompanying the Thai hoon grabok (stick-puppet theater), a genre deeply influenced by Chinese puppetry styles. Accordingly, the addition of Chinese-sounding string instruments in the khruang sai ensemble is imagined, by the Thai, to be a reference to the probable Chinese origins of this theater form.

Wong khruang sai (Thai: วงเครื่องสาย; literally "string ensemble") is a musical ensemble in Thai classical music which consists primarily of string instruments. A typical khruang sai ensemble features two two-string fiddles, one high and one low (saw duang and saw ou), a three-string zither called jakhe, a vertical duct flute called khlui, hand drums, and various cymbals. Depending on the size of the ensemble, instruments may be doubled or left out. A three-string spike fiddle called saw sam sai may be added as well. The khim (hammered dulcimer) has become popular and is also used in this ensemble. In the 20th century, Western instruments such as the violin or organ have also occasionally been integrated into khruang sai ensembles.