By the numerical analysis results using the Finite Difference Time Domain method in our previous studies, it was supposed that the spherical biconcave lens with an aperture diameter of 2.0 m has sufficient directional resolution (for example, the beam width is 1 deg at 60 kHz) for realizing the Ambient Noise Imaging (ANI) system. In this study, to confirm the directional resolution of the lens in a wide frequency band of 20‐100 kHz, we performed a reduced scale experiment of one‐fifth space in a water tank. The lens, made of acrylic resin, has an aperture diameter of 400 mm and a radius of curvature of 500 mm. A burst pulse of 25 cycles at 100, 200, 300, and 500 kHz, in which the frequency increases 5 times, was radiated from the sound source to the lens. The results show that the ‐3 dB area, whose pressure is 3 dB lower than the maximum at the image point, does not overlap each other at 300 and 500 kHz. It is supposed that the lens of 2.0 m aperture has the fine resolution over 60 kHz in the original scale.