Frogs are calling in large numbers these days.
One one boardwalk I watch 7 or 8 in one pond as they called.
The circular wave patterns gave away the location of the frogs.
Frogs have a tympanic membrane, or tympanum. This is the circular patch of skin directly behind its eye is what is commonly called its eardrum.
The tympanum transmits sound waves to the middle and inner ear, allowing the frog to hear both in the air and below the water.
The frogs I was watching bulged the air sacks on the side of the neck to make sound. Others have throat sacks but the principal is the same.