Comprises a hollow wooden resonance box 120 x 12.5 x 10 cms (L x W x H.) Two wooden rules 1 mtr. long subdivided in mms, between fixed bridges over wKPHh wires are stretched. A spring balance 100 x 5 N is fitted on one wire. Tension exerted can thus be measured and also adjustable finely by a wing nut resting against a plate at the end of the apparatus, second wire passes over a pulley and can be loaded as desired, a third wire is given for comparison and its tension adjustable by a wrest pin. The wires are made of rustless steel having uniform density. Distance between fixed bridges is kept 720 mm. Supplied complete with three movable bridges, a set of four wires 1.5 mtr. each. Supplied without weights.