This situation is normal. When the mobile phone is placed in the tank, the music becomes stereo because the tank has a surround single-element structure. The sound emitted by the mobile phone placed in the tank will form a fixed 17M back and forth. This back and forth is the oscillation of the echo frequency, and the echo is the basis for the formation of stereo sound. Conditions form the basic conditions for stereo sound, so there will be a phenomenon where the music becomes stereo when the mobile phone is placed in the tank.