The male peeper draws air into the lungs and shuttles it back and forth across the larynx into a balloon-like vocal sac beneath the chin. The sac is not a reservoir of exhaled air but a resonator and radiator, coupling sound into the surrounding air far more efficiently than an open mouth could. Recycling the air lets a male call repeatedly without pausing to breathe in fresh air.
The calling muscles contract at rates among the highest measured in vertebrate muscle, and calling is metabolically among the most expensive activities a frog performs. Males call at roughly one peep per second, faster when the night is warm.
What carries is the chorus rather than the individual. Massed calling from a single wetland can be heard well over a kilometre away under still conditions, while one frog is much quieter. Peepers also do not call only in spring: scattered autumn calling is common and is not a sign of a second breeding season.