Structure and function of human ear: The hearing organs in humans are ears. It is divided into three parts - outer, inner and middle ear. Middle ear has a stretched structure called eardrum. When ears receive sound, it enters down the ear canal and reaches eardrum. Sound vibrations vibrate the eardrum and it sends vibrations to inner ear from which vibrations are sent to the brain for interpretation of sound.

The decreasing amount of air decreases loudness of sound. When air is removed completely from a vessel, it is said that there is a vacuum. If all air is sucked from the vessel the sound would stop completely. The sound cannot travel through a vacuum.

(i) Oscillation: The to and fro motion of an object is called vibration. This motion is also called oscillatory motion.
(ii) Hertz : The number of oscillations per second is called the frequency in Hertz (Hz). A frequency of 1 Hz is one oscillation per second.

The number of oscillations per second is called frequency. Its unit is Hertz. Frequency determines the shrillness of sounds. The high frequency makes the high pitch and if the pitch is low, the frequency is less. High pitched sound (females) is shrill whereas low pitched sound (males) is hoarse

Hearing impairment means not being able to hear properly. Partial disability is generally the result of a disease, injury or age. Children with impaired hearing need special care by learning sign language. Technological devices for the hearing impaired have made it possible for such persons to improve their quality of life.

(i) Vibration of metal surface
(ii) Flapping of wings
(iii) Air vibrates to produce sound.
(iv) Vibrations of membrane
(v) Vibrations of strings
(vi) Stretched membrane
(vii) Air column

Time period is time taken to complete one vibration (or oscillation). Now pendulum takes 4 seconds to complete 40 oscillations, so it will take \(4\over 40\) seconds or \(1\over 10\) second to complete one Vibration 40 and it will complete \({40\over 4}=10\) Vibrations in one second. So, Time period \(={1\over 10 }\) second. Frequency = 10Hz

In outer space, there are no gases to help transmit sound. To communicate with one another, astronauts make use of electromagnetic waves such as radio waves which can travel through vacuum.

As we know that the audible frequency range for human being is 20 Hz to 20 kHz. So, it is an obvious thing that the frequency of bat is beyond this range due to which, we are usually unable to hear the screams of a bat.

The following measures to limit noise pollution in our locality is given as:
(i) The industries which produce noise should be set up away from the residential areas.
(ii) The television and the music systems should be run at the low volumes.
(iii) The aircraft engines, transport vehicles, industrial machines and home appliances must be installed with silencing devices

Hearing impairment means a person has damaged hearing ability and cannot hear properly. There are some people who born with poor hearing ability. A person with very poor hearing ability is said to be deaf. So, deafness is total hearing impairment.

Since, range of vibrations is different by different objects. But we can hear the vibrations which lies between 20 Hz to 20,000 Hz, so sound of every vibration cannot be heard by us.

Sources of noise pollution are
(i) Horns of vehicles
(ii) Loudspeakers of temples
(iii) Generators running without silencers

Since, excessive noise in the surroundings can cause many health related problems. Lack of sleep, hypertension, anxiety and many more health disorders may be caused by noise pollution.
So, even if a person is exposed to a loud sound continuously then he may get temporary or even permanent deafness.

Since, the ultrasonic waves produce by the bats are high pitched which are not heard by the human beings. So, these ultrasonic waves after striking the insect send back an echo which is heard by the bat and finally it gets over on the insect and catches it