Samacheer Class 8 Science - Sound

Ultrasonics – Frequency more than 20000 Hz
Speed of sound in air – 330 m
Infrasonics – Frequency below 20Hz
Sound propagation – Needs material medium

Lightning:
The most common example of showing that light travels faster than sound is lightning. Whenever a lightning strikes, you see the lightning first and then hear the thunder after some time.

Loudness of a sound depends on the amplitude of the vibration. So to increase loudness of sound by four times, the amplitude of the vibration also to be increased by four times.

Music:
The sound that provides a pleasing sensation to the ear.
It is produced by the regular patterns of vibrations.
Noise:
Sound that is unpleasant to the ear
It is produced by the irregular and non-periodic vibrations.

Noise may cause irritation, stress, nervousness and headache.
Long term exposure to noise may change the sleeping pattern of a person.
Sustained exposure to noise may affect hearing ability. Sometimes, it leads to loss of hearing.
Sudden exposure to louder noise may cause a heart attack and unconsciousness.
Noise of horns, loud speakers, etc., cause disturbances leading to lack of concentration.
Noise pollution affects a person’s peace of mind.

Plant parts such as stems, leaves, branches wood, etc., absorb sound.
Rough bark and thick, fleshy leaves are particularly effective at absorbing sound due to their dynamic surface area and helps in reducing noise pollution.

1. Aim:
To prove that sound cannot travel through vacuum and it needs a medium for propagation.
2. Materials Required:
Bell jar, mobile phone and vacuum pump.
Procedure:
Take a bell jar and a mobile phone.
Switch on the music in the mobile phone and place it in the jar.
Now, pump out the air from the bell jar using a vacuum pump.
As more and more air is removed from the jar, the sound from the mobile phone becomes feebler and finally, very faint.Conclusion:
This experiment proves that sound cannot travel in vacuum and it needs a medium.

Loudness
Pitch
Quality or Timbre
1. Loudness:
It is defined as the characteristic of a sound that enables us to distinguish a weak or feeble sound from a loud sound.
The loudness of a sound depends on its amplitude.
Higher the amplitude louder will be the sound and vice-versa.
When a drum is softly beaten, a weak sound is produced. However, when it is beaten strongly, a loud sound is produced.
The unit of loudness of sound is decibel (dB).
2. Pitch:
The pitch is the characteristic of sound that enables us to distinguish between a flat sound and a shrill sound.
Higher the frequency of sound, higher will be the pitch. High pitch adds shrillness to a sound.
The sound produced by a whistle, a bell, a flute and a violin are high pitch sounds.
3. Quality or Timbre:
The quality or timbre is the characteristic of sound that enables us to distinguish between two sounds that have the same pitch and amplitude.
For example in an orchestra, the sounds produced by some musical instruments may have the same pitch and loudness.

Strict guidelines should be set for the use of loudspeakers on social, religious and political occasions.
All automobiles should have effective silencers.
People should be encouraged to refrain from excessive honking while driving.
Industrial machines and home appliances should be properly maintained.
All communication systems must be operated in low volumes.
Residential areas should be free from heavy vehicles.
Green corridor belt should be set up around the industries as per the regulations of the pollution control board.
People working in noisy factories should wear ear plugs.
People should be encouraged to plant trees and to use absorbing materials like curtains and cushions in their home.

The outer and visible part of the human ear is called pinata (curved in shape).
It is specially designed to gather sound from the environment, which then reaches the ear drum (tympanic membrane) through the ear canal.
When the sound wave strikes the drum, the vesicles move inward and outward to create the vibrations.
These vibrations are then picked up by special types of cells in the inner ear. From the inner ear the vibrations are sent to the brain in the form of signals.
The brain perceives these signals as sounds.

Given data: time t = 2s
Speed of sound V = 330 ms -1
To find: Distance d = ?
Formula: Distance = Speed x time
Distance d = 330 x 2 = 660 m

Given data: Frequency n = ?
Velocity V = 25 ms -1
To find: Frequency n = ?