If you hold a large seashell like a conch shell to your ear, you will hear a sound like that of the ocean roaring. Have you wondered whether it really is the ocean ? Actually, it is not .
Seashells especially large ones have many hard curved surfaces that are good reflectors of sound. As a result sound waves that are trapped inside a dry sea shell are reflected and re-reflected, creating a jumble of echoes. These echoes create a roaring sound, very similar to the roar of the ocean.