2. Some male frogs have external sound sacs that can expand and vibrate on both sides of their mouths. The vocal sac produces * * * sound, which makes the frog's song magnificent and loud. After the rain, they will hear the cries of male frogs echoing each other, one after another, and form a chorus. Chorus is much superior to solo because it contains more information; Chorus is loud and far away, which can attract more female frogs, so frogs often use chorus.