A hollow wooden sounding box 1290 x 130 x 110 mm (length x width x height) provided with scales divided in mm between fixed bridges over which the wires are stretched. To one wire a spring balance reading to 100N x 5N is fitted and the tension exerted and measured can be finely adjusted by wing-nut. Another wire passes over a pulley and can be loaded as desired. A third wire is provided for comparison, its tension may be adjusted by means of a wristpin The wires supplied are of steel which is rustles. The distance between the fixed bridges is 720 mm Complete with three movable bridges, wrist-pin key and a set of four wires, length 1.5 m of 20, 22, 24, 26 SWG WITHOUT weight.