In the first part of this master thesis, the development of the basic concept of the sound through the history of science as well as the wave and sound phenomena in nature were described, frequency range of a human ear was determined, and ultrasound was explained. In the experimental part the several experiments were described. Firstly, the wave length of sound in some fluids was determined and speed of sound from a projected picture was calculated. Secondly, dependence of the speed of sound on the temperature of a fluid (water) was investigated. Finally, bat detector was constructed, and detailed instructions for that were presented. Bat detector is a device which detects if bats are present at a certain location and works on frequencies of ultrasound. The application of constructed bat detector was presented at 18th International Bat Night in Osijek ZOO. In conclusion, the obtained results from the experiments were discussed and some additional application of ultrasound in our lives were described.