<P> The auditory system of a land mammal typically works via the transfer of sound waves through the ear canals . Ear canals in seals, sea lions, and walruses are similar to those of land mammals and may function the same way . In whales and dolphins, it is not entirely clear how sound is propagated to the ear, but some studies strongly suggest that sound is channeled to the ear by tissues in the area of the lower jaw . One group of whales, the Odontocetes (toothed whales), use echolocation to determine the position of objects such as prey . The toothed whales are also unusual in that the ears are separated from the skull and placed well apart, which assists them with localizing sounds, an important element for echolocation . </P> <P> Studies have found there to be two different types of cochlea in the dolphin population . Type I has been found in the Amazon river dolphin and harbour porpoises . These types of dolphin use extremely high frequency signals for echolocation . Harbour porpoise emits sounds at two bands, one at 2 kHz and one above 110 kHz . The cochlea in these dolphins is specialised to accommodate extreme high frequency sounds and is extremely narrow at the base of the cochlea . </P> <P> Type II cochlea are found primarily in offshore and open water species of whales, such as the bottlenose dolphin . The sounds produced by bottlenose dolphins are lower in frequency and range typically between 75 and 150,000 Hz . The higher frequencies in this range are also used for echolocation and the lower frequencies are commonly associated with social interaction as the signals travel much farther distances . </P> <P> Marine mammals use vocalisations in many different ways . Dolphins communicate via clicks and whistles, and whales use low frequency moans or pulse signals . Each signal varies in terms of frequency and different signals are used to communicate different aspects . In dolphins, echolocation is used in order to detect and characterize objects and whistles are used in sociable herds as identification and communication devices . </P>

What is the range in db level of normal human hearing