What sort of harp did David actually play?
The instrument David played is referred to in the Hebrew Bible as a 'kinnor'. The word is usually translated as 'harp', but it was actually a lyre (a member of the zither family).
Modern scholarship suggests that David played the so-called 'thin lyre'. This instrument had four to eight gut strings and was usually played with a plectrum in a similar manner to the way a modern guitar is strummed. However, according to Samuel I 16:23, "David took an harp, and played with his hand", which may suggest that David's technique was more advanced, and involved plucking with individual fingers harpist-fashion.