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 and noise are two different types of sound that can be distinguished based on their characteristics and effects. Music is sound that provides a pleasing and agreeable sensation to the ear and is generally enjoyed by listeners. It is produced by regular and periodic patterns of vibrations that follow a specific rhythm, melody, and harmony. The vibrations in music are organized and follow mathematical relationships, creating pleasant tones and sequences. Noise, on the other hand, is sound that is unpleasant and disagreeable to the ear and causes discomfort to listeners. It is produced by irregular and non-periodic vibrations that do not follow any organized pattern or rhythm. The vibrations in noise are random and chaotic, resulting in harsh and unwanted sounds. While music has a definite pitch and frequency that can be measured, noise consists of a mixture of different frequencies occurring randomly without any particular order or structure.

Noise pollution poses numerous hazards to human health and well-being. Exposure to noise may cause irritation, stress, nervousness, and headaches in individuals. Long-term exposure to excessive noise can disrupt and change the normal sleeping patterns of a person, leading to sleep deprivation and associated health problems. Sustained and continuous exposure to high levels of noise can gradually affect and damage the hearing ability of a person, and in severe cases, it may lead to permanent loss of hearing or deafness. Sudden exposure to very loud noise can cause severe physiological reactions such as a heart attack and unconsciousness in some individuals. Noise from horns, loudspeakers, construction equipment, and traffic causes significant disturbances that lead to a lack of concentration, making it difficult for people to focus on work or studies. Additionally, noise pollution negatively affects a person's peace of mind, mental health, and overall quality of life, contributing to anxiety and psychological stress.

Planting trees is an effective natural method for reducing noise pollution. Plant parts such as stems, leaves, branches, and wood have the ability to absorb sound waves and reduce their intensity. The rough bark of trees and thick, fleshy leaves are particularly effective at absorbing sound due to their dynamic surface area and irregular structure, which helps to trap and dissipate sound energy. When sound waves strike these plant surfaces, they are partially absorbed rather than reflected, thereby reducing the noise that travels further. A dense plantation of trees acts as a natural sound barrier, creating a buffer zone that prevents noise from reaching residential and sensitive areas. Trees also provide additional environmental benefits such as improving air quality, reducing air pollution, providing shade, and enhancing the aesthetic value of the surroundings. Therefore, large-scale afforestation and tree planting programs are recommended as an important strategy for reducing the harmful effects of noise pollution in urban and industrial areas.

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 human ear is a complex sensory organ responsible for hearing and is divided into three main parts: the outer ear, middle ear, and inner ear. The outer and visible part of the human ear is called the pinna, which has a curved shape specially designed to gather sound waves from the environment. Sound waves collected by the pinna travel through the ear canal and reach the eardrum, also called the tympanic membrane. When sound waves strike the eardrum, it vibrates, and these vibrations are transmitted to three tiny bones called ossicles (hammer, anvil, and stirrup) located in the middle ear. These bones amplify the vibrations and pass them to the inner ear. In the inner ear, the vibrations are detected by special sensory cells called hair cells located in a structure called the cochlea. These hair cells convert the mechanical vibrations into electrical signals or nerve impulses. From the inner ear, these electrical signals are sent to the brain through the auditory nerve in the form of signals. The brain receives and processes these signals and perceives them as different sounds, allowing us to hear and interpret various sounds in our environment.

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 = ?