In our previous paper, we described vibrations with circumferential mode number n=2 when a frequency of a pressure pulsation in a turbine coincides with the natural frequency of a steel penstock. We proposed the use of natural frequency calculation formulae based on numerical analyses that target n=2, 3 vibrations. We further pointed out that the vibration (n=2) prevention by circumferential stiffeners was not remarkable. Because a limited number of papers are available describing experimental or analytical vibration studies, we performed 1/4 scale model experiments on pressure pulsations with the parameters of plate thickness and the presence of stiffeners. Based on a comparative study of experiments and numerical analyses, we proposed modification of the vibration prevention specifications stipulated in the current technical standard.