Class VIII Science (Chapter-SOUND) PDF

# Ch - Sound ## Defines 1. Sound - Sound is a form of energy which produces a sensation of hearing in our ears. 2. Vibration - Vibration is a rapid back and forth movement of an object about its mean position. 3. Simple Pendulum - A simple pendulum consists of a small metal ball called bob suspend...

# Ch - Sound ## Defines 1. Sound - Sound is a form of energy which produces a sensation of hearing in our ears. 2. Vibration - Vibration is a rapid back and forth movement of an object about its mean position. 3. Simple Pendulum - A simple pendulum consists of a small metal ball called bob suspended by a long thread from a rigid support, such that the bob is free to swing back and forth. 4. Oscillation of a pendulum - The motion of the bob of a pendulum from one extreme position to the other extreme position and back is called oscillation of a pendulum. 5. Amplitude of the pendulum - The maximum displacement of the bob from its mean position on either side is called the amplitude of the pendulum. 6. Time period of the pendulum - The time taken by the bob of a pendulum to complete one oscillation is called the time period of the pendulum. 7. Frequency - The number of complete oscillations made by a simple pendulum in one second is called its frequency. The SI unit of frequency is Hertz (Hz). ## Question Answer **Q1. Write the characteristics of sound.** **Ans.** The characteristics of sound are: * **Pitch** - Pitch is the characteristic of sound by which you can distinguish a shrill sound from a grave (hoarse) sound even though the two sounds have the same loudness. * **Loudness** - Loudness of sound is directly proportional to the square of the amplitude of the vibration, producing the sound. The loudness of a sound is measured in a unit called decibel (dB). * **Quality** - It is the characteristic of a sound that enables us to distinguish between two sounds of the same pitch and loudness produced by two different sources. It depends upon the waveform of the sound. Quality of a sound is called its tone. **Q2. What are audible and inaudible sounds?** **Ans.** * **Audible sound** - The sound whose frequency lies between 20 Hz and 20,000 Hz, which we are able to hear is called audible sound. So, the audible range of human ear is 20Hz-20,000Hz. * **Inaudible sound** - These are of two types: * **Infrasonic sounds** - The sound (inaudible for humans) having frequency less than 20Hz are known as infrasonic sounds. * **Ultrasonic sounds** - The sounds (inaudible for humans) having frequency more than 20,000 Hz are known as ultrasonic sounds. **Q3. Differentiate between noise and music.** **Ans.** | **Noise** | **Music**| |---|---| | It produces an unpleasant effect on our ears and mind. | It produces a pleasant effect on our ears and mind. | | It is produced by irregular and non-periodic vibrations. | It is produced by regular and periodic vibrations. | | Long exposure to noise leads to serious health hazards. | It does not cause any health hazard. | | It has an irregular waveform. | It has a regular waveform. | **Q4. What is noise pollution? Mention its harmful effects and measure to reduce it.** **Ans.** Noise from any source that causes disturbance or discomfort of any kind in the environment is called noise pollution. **Harmful effects of noise pollution** Noise pollution can lead to many health hazards: 1. It causes nervous tension, irritation, earache, headache and high blood pressure. 2. It may cause lack of concentration in work or studies. 3. Loud music during night disturbs our sleep. 4. Prolonged exposure to high level noise can affect the eardrum and may lead to temporary or even permanent loss of hearing. **Measures to reduce Noise Pollution** 1. Automobiles should be fitted with silencers and soft horns. 2. The horns of motor vehicles should not be blown unnecessarily. 3. You should not play radio, television and stereo system too loudly. 4. Airports and noise-making factories should be shifted away from the residential areas of the city. 5. The use of loud speakers at social and religious functions should be restricted. 6. We should not play loud music in cars and other vehicles while driving. 7. Machines should be maintained in a good condition to reduce industrial noise pollution. 8. Avoid bursting firecrackers that made a loud noise. **Q5. Explain the structure of ear with the help of diagram.** **Ans.** Our ear is divided into three parts: outer ear or external ear, middle ear and inner ear. * **(a) Outer ear** - It consists of a flap-like cartilaginous structure called pinna. It collects sound waves. The pinna is further connected to ear canal, at the end of which eardrum is present. Eardrum (or tympanic membrane) is a stretched membrane that the outer ear from the middle ear. * **(b) Middle ear** - It consists of a chain of three bones - hammer, anvil and stirrup. Middle ear is connected to the throat through eustachian tube. * **(c) Inner ear** - The inner ear contains cochlea and semicircular canals. It is filled with a fluid and has many hair cells inside it. The receptors send impulses through sensory neurons to the auditory nerve which connects inner ear to the brain.