The human ear can hear the sound between frequencies of 20 Hz to 20,000 Hz. Thus, audible range or range of hearing is between 20 Hz to 20,000 Hz; for human beings. However, children under 5 years of age can hear the sound upto 25000 Hz.
Sound, below the frequency of 20Hz is called infrasonic or infrasound. Infrasound is produced because of very slow vibration. For example; simple pendulum produces sound below 20Hz. Human being cannot hear infrasound as their ears are not adapted to hear the sound of such range.
Many animals; such as whale, elephant, rhinoceros, etc. can produce and hear sound having frequencies below 20 Hz.
Sound, above the frequency of 20000 Hz is called ultrasound. Humans cannot hear the ultrasonic sound. However, many animals such as dogs, cat, bat, monkey, deer, etc. can hear ultrasound.
Bats catch their prey by producing ultrasound. Bat produces ultrasound and detects the reflected sound waves coming from any obstacle; such as a prey. By detecting the reflection of ultrasound, bat understands the position and type of prey or of any obstacle in the way. Some aquatic animals, such as dolphin, also use ultrasound to catch their prey.
Ultrasound is sound waves of high frequency. Because of high frequency, ultrasound is associated with more energy and can penetrate upto a large extent. This characteristic of ultrasound makes it very useful for many purposes. Some of its uses are given here:
The full form of SONAR is SOund Navigation And Ranging. This is a device which is used to measure depth of sea bed, locate scraps, wrecks, submarines of enemies, etc. in the water by producing ultrasound. It is fitted over ships and submarines.
Ultrasonic sound waves are produced by SONAR and when these waves return after reflection from anything in water, they are analyzed with the help of computer. The shape and position of objects under sea and ocean is detected on the basis of speed and nature of reflected ultrasound waves.
The human ear can be divided into three main parts, viz. external ear, middle ear and internal ear.
External Ear: The external ear is outside the body and is also called pinna. It extends into the ear canal.
Middle Ear: The middle ear is composed of the ear drum or tympanum and the bone ossicles. There are three bone ossicles, viz. the hammer, the anvil and the strirrup.
Internal Ear: The internal ear is composed of a cochlea and three semi-circular canals. The cochlea makes the hearing apparatus and the auditory nerve from it goes to the brain.
Working of Human Ear: The external ear catches sound waves and channelizes them to the ear drum, via the ear canal. During compression, the pressure increases outside the ear drum which forces the eardrum to move inwards. During rarefaction, the pressure decreases outside the ear drum which forces the eardrum to move outwards. Thus, a vibration is produced in the eardrum. Further, the three bones amplify the sound wave, by vibrating in turns. In the inner ear, the vibrations are converted into electrical signals. These signals are transmitted by the auditory nerve to the brain. Finally, the brain interprets those signals as sound.
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