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.

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.

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

Lightning is seen earlier than the sound of thunder because light travels faster than sound. Hence, it reaches us before the sound does.

The frequency determines the shrillness or pitch of a sound. A drum vibrates with low frequency. Therefore, it produces low pitched sound. Whistle has a high frequency and therefore, produces a sound of higher pitch.

Yes, as the sound of children is shriller than the sound of adults. So, the frequency of sound produced by a child is more than that of an adult.

The factor on which the quality of sound depends is the looseness or the tightness of the vocal cord.

In most of the cases, we cannot see the vibrations because of its small amplitude.

Presence of excessive noise in the surrounding may cause noise pollution, which is harmful to human health. Here are some measures to limit noise pollution:
(i) Noise producing industries should be set up away from residential areas.
(ii) Silencing devices must be installed in aircraft engines, transport vehicles, industrial machines and heavy appliances.
(iii)Use of automobile horn should be minimized.
(iv) TVand music system should be run at low volumes.
(v) Trees must be planted along the sides of the roads and around buildings to cut down the sounds reaching the residents

As we know that the speed of sound in air is about 340 m/s and the speed of light in air is 300,000,000 m/s. It means that sound travels at a slow speed while light travels much faster than sound.

So, though sound may take a few seconds to travel a distance of a few hundred metres, light will take practically negligible time to reach a distance of even a few kilometres. e.g. in the rainy season, the flash of lightning is observed first and the sound of thunder is heard a little later (though both are produced at the same time in clouds) (see figure). It is because of the very high speed of light that we see the flash of lightning first and it is due to comparatively low speed of sound that the thunder is heard a little later.