Reflections of FM signals (whistles) of two Black Sea bottlenose dolphins (Tursiops truncatus) from the boundaries of a tank many times smaller than the spatial extent of the signals were recorded for the first time. It is shown that dolphins produce whistles with sound pressure levels (about 2 Pa) only slightly exceeding (by 3-8 dB) the acoustic noise amplitudes of the tank. It has been established that as they propagate, FM signals are repeatedly reflected from the tank boundaries; the shape of the emitted signal is distorted as a result of interference with signals reflected from the boundaries of the tank and acoustic noise. In contrast to the time domain, the FM signal spectrum is more resistant to interference; therefore, the signal-to-noise ratio of the spectral power densities (SPD) of the same signals and their reflections reaches 30-40 dB. The results indicate the advantages of analyzing FM signals and their echoes in the spectral domain.