Hearing range
Routine investigation for hearing loss usually involves an audiogram which shows threshold levels relative to a normal.
For humans, the test involves tones being presented at specific frequencies (pitch) and intensities (loudness).
The compression and rarefaction of these waves set this thin membrane in motion, causing sympathetic vibration through the middle ear bones (the ossicles: malleus, incus, and stapes), the basilar fluid in the cochlea, and the hairs within it, called stereocilia.
[10] Individual hearing range varies according to the general condition of a human's ears and nervous system.
[12][13][14] Audiograms of human hearing are produced using an audiometer, which presents different frequencies to the subject, usually over calibrated headphones, at specified levels.
The levels are weighted with frequency relative to a standard graph known as the minimum audibility curve, which is intended to represent "normal" hearing.
The threshold of hearing is set at around 0 phon on the equal-loudness contours (i.e. 20 micropascals, approximately the quietest sound a young healthy human can detect),[15] but is standardised in an ANSI standard to 1 kHz.
[17][18] Cats do not use this ability to hear ultrasound for communication but it is probably important in hunting,[19] since many species of rodents make ultrasonic calls.
[20] Cat hearing is also extremely sensitive and is among the best of any mammal,[17] being most acute in the range of 500 Hz to 32 kHz.
[22][23] As with humans, some dog breeds' hearing ranges narrow with age,[24] such as the German shepherd and miniature poodle.
As dogs hear higher frequency sounds than humans, they have a different acoustic perception of the world.
[26] Each type reveals different information; CF is used to detect an object, and FM is used to assess its distance.
The pulses of sound produced by the bat last only a few thousandths of a second; silences between the calls give time to listen for the information coming back in the form of an echo.
Evidence suggests that bats use the change in pitch of sound produced via the Doppler effect to assess their flight speed in relation to objects around them.
[27] The information regarding size, shape and texture is built up to form a picture of their surroundings and the location of their prey.
The shape of a bird's head can also affect its hearing, such as owls, whose facial discs help direct sound toward their ears.
With the average pigeon being able to hear sounds as low as 0.5 Hz, they can detect distant storms, earthquakes and even volcanoes.
The differences in auditory systems have led to extensive research on aquatic mammals, specifically on dolphins.
[33] 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.
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.
Type II cochlea are found primarily in offshore and open water species of whales, such as the bottlenose dolphin.
In dolphins, echolocation is used in order to detect and characterize objects and whistles are used in sociable herds as identification and communication devices.
