A sound post, particularly for string instrument, having a variable length is described. The sound post consists of a first tubular component having a first end and a second end and a second tubular component having a first end and a second end, wherein the second tubular component is at least partially arranged in the first tubular component in a movable manner, and wherein a first pivotable end piece is arranged at the first end of the first tubular component. The sound post also comprises a counter-element and a tension fastener element which is rotatably arranged and suitable to be countered against the counter-element by rotation. A pivotable end piece is also arranged at the second end of the second tubular component. A sound post tool set for mounting the sound post in an instrument and a method for mounting the sound post in an instrument are also disclosed.