When you speak or shout or sing or even whisper, the vibrations of your voice send sound waves through the air.
These sound waves travel out from your mouth in all directions, out straight, to the right, to the left, and to all the places in between.
But when you cup your hands around your mouth, you are pointing the sound waves in only one direction and keeping them from spreading out in all the others.
By doing this, you are giving your sound waves more energy and making them louder and stronger.
