Sound Online Test 8th Science Lesson 15 Questions in English

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Sound is produced when an object is set to vibrate.

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Vibration means a kind of rapid to and fro motion of an object. This to and fro motion of the body causes the substances around it to vibrate. Thus sound spreads to the surroundings.

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The substance through which sound is transmitted is called medium. Sound moves through a medium from the point of generation to the listener. Sound is a form of energy and it needs a medium to travel.

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Thomas Alva Edison in 1877 invented the phonograph a device that played the recorded sound.

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Sound cannot travel in vacuum and it needs a medium like air. Sound travels in water and solids also. The speed of sound is more in solids than in liquids and it is very less in gases.

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The speed of sound is the distance travelled by it in one second. It is denoted by ‘v’. It is represented by the expression, v = nλ, where ‘n’ is the frequency and ‘λ’ is the wavelength.

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Wavelength is the distance between two consecutive particles, which are in the same phase of vibration. It is denoted by the Greek letter. The unit of wavelength is meter (m).

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Frequency is the number of vibrations of a particle in the medium in one second. It is denoted by ‘n’. The unit of frequency is hertz (Hz).

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Given, v = 250 ms-1, n = 25 Hz

v = nλ

λ = 250 / 25 = 10m

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The speed of sound depends on the properties of the medium through which it travels, like temperature, pressure and humidity.

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In any medium, as the temperature increases the speed of sound also increases. For example, the speed of sound in air is 331 ms-1at 0°C and 344 ms-1 at 22°C.

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Speed of sound in different media at 25°C

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The amount of water vapor in the air is known as humidity. It is less during winter and more during summer. The speed of sound increases with increase in humidity. This is because the density of air decreases with increase in humidity.

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When a body vibrates, the particle of the medium in contact with the vibrating body is first displaced from its equilibrium position. It then exerts a force on the adjacent particle. This process continues in the medium till the sound reaches the ear of the person.

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When a vibrating tuning fork moves forward, it pushes and compresses the air in front of it, creating a region of high pressure. This region is called a compression (C). When it moves backward, it creates a region of low pressure called rarefaction (R). These compressions and rarefactions produce the sound wave, which propagates through the medium.

Explanation

Mechanical wave is a disturbance which propagates in a medium due to the repeated periodic motion of the particles of the medium from their mean position. The disturbance which is caused by the vibrations of the particles is passed over to the next particle. It means that the energy is transferred from one particle to another as a wave motion.

Explanation

Characteristic of wave motion: In wave motion, only the energy is transferred not the particles. The velocity of the wave motion is different from the velocity of the vibrating particle. For the propagation of a mechanical wave, the medium must possess the properties of inertia, elasticity, uniform density and minimum friction among the particles.

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There are two types of mechanical wave. They are Transverse wave and longitudinal wave.

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Transverse wave: In a transverse wave the particles of the medium vibrate in a direction, which is perpendicular to the direction of propagation of the wave. E.g. Waves in strings, light waves etc. Transverse waves are produced only in solids and liquids.

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In a longitudinal wave the particles of the medium vibrate in a direction, which is parallel to the direction of propagation of the wave. E.g. Waves in springs, sound waves in a medium.

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Longitudinal waves are produced in solids, liquids and also in gases.

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The seismic wave formed during earthquake is an example for a longitudinal wave. Waves travelling through the layers of the Earth due to explosions, earthquakes and volcanic explosions are called seismic waves. Using a hydrophone and seismometer one can study these waves and record them. Seismology is the branch of science that deals with the study of seismic waves.

Explanation

Loudness 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.

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Amplitude is the maximum displacement of a vibrating particle from its mean position. It is denoted by ‘A’.

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The loudness of a sound depends on its amplitude. The unit of loudness of sound is decibel (dB). The unit of amplitude is ‘meter’ (m).

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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.

Explanation

The voice of a female has a higher pitch than a male. That is why a female’s voice is shriller than a male’s voice. Some examples of low pitch sound are the roar of a lion and the beating of a drum.

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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. Yet, you can distinctly identify the sound produced by each instrument.

Explanation

According to the frequency we can classify the sound into three types. They are: Audible sound, infrasonic sound and Ultrasonic sound.

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Audible sound: Sound with frequency ranging from 20 Hz to 20000 Hz is called sonic sound or audible sound. These sounds can be heard by the human beings only. Human ears cannot hear sounds with frequencies below 20 Hz or above 20000 Hz. So, the above range is called as audible range of sound.

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A sound with a frequency below 20 Hz is called as subsonic or infrasonic sound. Humans cannot hear the sound of this frequency, but some animals like dog, dolphin, etc., can hear.

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Uses of infrasonic sound are given below. It is employed in the Earth monitoring system. It is also used in the study of the mechanism of the human heart.

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A sound with a frequency greater than 20000 Hz is called as ultrasonic sound. Animals such as bats, dogs, dolphins, etc., are able to hear certain ultrasonic sounds as well.

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Some of the uses of ultrasonic sounds are given below.

• It is extensively used in medical applications like ‘sonogram’.
• It is used in the SONAR system to detect the depth of the sea and to detect enemy submarines.
• It is also employed in dish washers.

Explanation

Another important application of ultra sound is the Galton’s whistle. This whistle is inaudible to the human ear, but it can be heard by the dogs. It is used to train the dogs for investigation.

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A bat can hear the sounds of frequencies higher than 20,000 Hz. Bats produce ultrasonic sound during screaming. These ultrasonic waves help them to locate their way and the prey.

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Some sounds are pleasing to the ear and make you happy. The sound that provides a pleasing sensation to the ear is called ‘music’. Music is produced by the regular patterns of vibrations.

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Musical instruments are categorized into four types as given below.

• Wind instruments
• Reed instruments
• Stringed instruments
• Percussion instruments

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In a wind instrument the sound is produced by the vibration of air in a hollow tube. The frequency is varied by changing the length of the vibrating air column. Trumpet, Flute, Shehnai and Saxophone are some well-known wind instruments.

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A reed instrument contains a reed. Air which is blown through the instrument causes the reed to vibrate, which in turn produces the specific sound. Examples of reed instruments include Harmonium and Mouth Organ.

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Stringed instruments make use of a string or wire to produce vibrations and hence the specific sound. These instruments also have hollow boxes that amplify the sound that is produced. The frequency of sound is varied by varying the length of the vibrating wire. Violin, Guitar, Sitar are some of the examples of stringed instruments.

Explanation

A guitar string has a number of frequencies at which it will naturally vibrate. These natural frequencies are known as the harmonics of the guitar string. The natural frequency, at which an object vibrates, depends upon the tension of the string, the linear density of the string and the length of the string.

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There are varieties of percussion instruments all over the world. Percussion instruments like the drum and tabla consist of a leather membrane, which is stretched across a hollow box called the resonator. When a membrane is hit, it starts vibrating and produces the sound.

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Percussion instruments produce a specific sound when they are struck scrapped or clashed together. They are the oldest type of musical instruments.

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In a human being the sound is produced in the voice box, called the larynx, which is present in the throat. It is located at the upper end of the windpipe.

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The larynx has two ligaments called ‘vocal cords’, stretched across it. The vocal cords have a narrow slit through which air is blown in and out.

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When a person speaks, the air from the lungs is pushed up through the trachea to the larynx. When this air passes through the slit, the vocal cords begin to vibrate and produce a sound.

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By varying the thickness of the vocal cords, the length of the air column in the slit can be changed. This produces sounds of different pitches.

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Male generally have thicker and longer vocal cords that produce a deeper, low pitch sound in comparison with females.

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The ear is the important organ for all animals to hear a sound. We are able to hear sound through our ears. The human ear picks up and interprets high frequency vibrations of air.

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Ears of aquatic animals are designed to pick up high frequency vibrations in water.

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The outer and visible part of the human ear is called pinna (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.

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When the sound wave strikes the drum, the ossicles 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.

Explanation

Any sound that is unpleasant to the ear is called noise. It is the unwanted, irritating and louder sound. Noise is produced by the irregular and non-periodic vibrations.

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Noise gives you stress. The disturbance produced in the environment by loud and harsh sounds from various sources is known as noise pollution. Busy roads, airplanes, electrical appliances such as mixer grinder, washing machine and un-tuned radio cause noise pollution. Use of loudspeakers and crackers during the festivals also contributes to the noise pollution.

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The major source of noise pollution is from the industries. Noise pollution is the bi-product of industrialization, urbanization and modern civilization.

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Noise creates some health hazards. Some of them are listed below.

• 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.
• It causes lack of concentration in one’s work. Noise of horns, loud speakers cause disturbances leading to lack of concentration.
• Noise pollution affects a person’s peace of mind. It adds to the existing tensions of modern living. These tensions results in disease like high blood pressure or short tempered nature.

Explanation

Noise pollution can be significantly reduced by adopting the following steps.

• 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 use absorbing materials like curtains and cushions in their home.

Explanation

The following are the symptoms of hearing loss, Ear ache, A feeling of fullness or fluid in the ear and Ringing in your ear.

Explanation

Hearing loss is caused by various reasons. Some of them are Aging, Ear infections if not treated, certain medicines, Genetic disorders, a severe blow to the head and loud noise.