Dolphins hear tones with a frequency up to 160 kHz with the greatest sensitivity.
Dolphins have a well-developed, acute sense of hearing. The auditory cortex of their brain is highly developed.
The dolphin's auditory nerve is about twice the diameter of the human eighth nerve (connecting the inner ear to the brainstem) leading to more rapid sound processing for dolphins. In addition, a dolphin's auditory nerve supply is about three times that of humans — possibly providing more ultrasonic information to a dolphin's central nervous system for echolocation.
Bottlenose dolphins hear tones with a frequency of up to 160 kHz with the greatest sensitivity ranging from 40 to 100 kHz. The average hearing range for humans is about 0.02 to 20 kHz. In other studies, the hearing range for the bottlenose dolphin has been measured in 75 to 150,000 Hz (0.075 to 150 kHz).
A dolphin's small external ear openings don't seem to be important in conducting sound. They lead to reduced ear canals that are not connected to the middle ears.
Soft tissue and bone conduct sound to a dolphin's middle and inner ears. In particular, fat lobes in a toothed whale's lower jaw appear to be an adaptation for conveying sound to the ears.
In dolphins, ears aren't attached to the skull. Ligaments hold each ear in a foam-filled cavity outside the skull. This separation of the ears allows a dolphin to localize sound, which is important for echolocation. Humans and most land mammals cannot effectively localize sounds underwater.