[填空题]
A tuning fork is set into9 tn9-ca* xx(ncoi v;x vibration above a vertical open tube filled wita02hgxs o3p09 pi;t kth water (Fig. 12–35). The water level is allowed to drop slowly. As it does so, the air in the tube above the water level is heard to resonate with the tuning fork when the distance from the tube opening to the water level is 0i03 x9 2aok st0g;tphp.125 m and again at 0.395 m. What is the frequency of the tuning fork? Hz