In the enclosed-box type direct-radiation loudspeaker system that has been invented to improve radiation efficiency in the low frequency range, Takahashi has offered an effective system which uses a vapor-liquid coexistence system. If operated under ideal conditions, it prevents the enclosed cabinet system from having the disadvantage of raised fundamental resonance frequency, and improves radiation efficiency. We made an experiment using Freon 11 as working liquid and wick in the cabinet. The stiffness of the enclosed cabinet was observed to decrease to approximately 1/8 when the volume velocity is under 7 × 10-5 m3/s. This means that we can reduce the volume of the cabinet to about 1/8 of the conventional size without compromising the performance. © 1992, Acoustical Society of Japan. All rights reserved.