In physics, the sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, the sound is the reception of such waves and their perception by the brain. Sound waves are longitudinal waves that travel through a medium like air or water. When we think about sound, we often think about how loud it is (amplitude, or intensity) and its pitch (frequency). An echo is a sound you hear when you make a noise, and the sound wave reflects off a distant object. Besides the novelty of hearing your words repeated, echoes can be used to estimate the distance of an object, its size, shape and velocity, as well as the velocity of sound itself. Free vibrations of an elastic body are called natural vibrations and occur at a frequency called the natural frequency. If the forced frequency is equal to the natural frequency, the vibrations' amplitude increases manifold. This phenomenon is known as resonance. So, if the obstacle is at a distance of 17 m at least, the reflected sound or the echo is heard after 0.1 seconds, distinctly. Hence, by measuring the time taken for the observer to listen to the echo and distance between the observer and obstacle can be used to determine the speed of sound.