Some animals, such as dogs, dolphins, bats, and mice, are able to hear sounds which have a frequency higher than upper-limit of the human ear. This makes them able to hear ultrasounds. Children can hear some high-pitched sounds that older adults cannot hear, as in humans the upper limit pitch of hearing gets lower as a person gets older. This frequency limit is caused by the middle ear that acts as a low-pass filter. If ultrasound is fed directly into the skull bone and reaches the cochlea without passing through the middle ear, much higher frequencies (up to about 200 kHz) can be heard. This effect (sometimes called ultrasonic hearing) was first discovered by divers exposed to a high-frequency (ca. 50 kHz) sonar signal.