Abstract: To understand the behaviour of dolphins a specific language of these animals must be decoded. Dolphins acoustic emissions can be classified into two general categories: tonal whistles and pulsed sounds or clicks. Spinner whistles are frequency-modulated sounds with a fundamental component usually below 20 [kHz], while the echolocation signals have bimodal frequency spectra with a low-frequency peak between 40 and 50 [kHz] and a high frequency peak between 110 and 130 [kHz]. Because of the fact that the signal of 100 kc/sec is much stronger absorbed by water than that of 20 kc/sec, at some longer distances higher harmonics are not audible for dolphins. Therefore, these harmonics probably do not play a significant role in long distance conversation. Another great problem is the noise produced by human activity that can impede the communication between dolphins or even prevent echolocation.