How often have you shouted for someone by cupping your mouth with your hands? Do you know why we instinctively do so? The reason for doing so is that, the vibrations of vocal chords present in our throat produce sound waves that come out of our mouth and travel through the air in all directions. When these sound waves reach somebody’s ears, he hears our voice. However, when these sound waves spread in the air, the energy associated with them also spreads and the energy per unit area or intensity of the sound decreases. When we call someone at a distance, we cup our hands around our mouth. By doing this we prevent the sound waves from spreading in all directions, thus making them travel only in one direction. All the energy is directed in a particular direction and the intensity of sound does not decrease. Hence, it can reach longer distance and people standing at a distance can hear our voice clearly. So, to send our voice over longer distances, we cup our hands around our mouth.