Chapter 12 Sound

12 Sound In the Chapter Sound is produced because of vibration of different objects. Sound travels as a longitudinal wave through a material medium. Sound travels as successive rarefactions and compressions in the medium. In sound propagation, it is the energy of the sound which travels and not the particles of the medium. Sound cannot move in vacuum. The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation. The distance between two consecutive compressions or two consecutive rarefactions is known as the wavelength, λ. The time taken by the wave for one complete oscillation of the density or pressure of the medium is known as the time period, T. The number of complete oscillations per unit time is called the frequency (v), v = 1/T. The speed v, frequency v, and wavelength A, of sound are related by the equation, v=λν. The speed of sound depends on the nature and the temperature of the transmitting medium. The law of reflection of sound suggests that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface and the three lie in the same plane. For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least 0.1 s. The persistence of sound in an auditorium is the result of repeated reflections of sound and is known as reverberation. Sound properties like pitch, loudness and quality are determined by the corresponding wave properties. Loudness is a physiological response of the ear to the intensity of sound. The amount of sound energy passing each second through unit area is known as the intensity of sound. The audible range of hearing for average human beings is in the frequency range of 20 Hz-20 kHz. Sound waves with frequencies below the audible range are called "infrasonic" and those above the audible range are called "ultrasonic". Ultrasound has many industrial and medical applications. Intext Exercises Page No. 162 1. How does the sound produced by a vibrating object in a medium reach your ear? Ans. Sounds after it is produced moves through a medium from the point of generation to the listener. When an object vibrates it sets the particle of the medium around it to vibrate. The particles do not travel all the way from the vibrating object to the ear. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle and get is displaced from its position of rest. After displacing the adjacent particle the first particle comes back to its original position. This process continues in the medium till the sound becomes feeble. The disturbance set by sound in the medium travels through the medium and not the particle of the medium. Page No. 163 1. Explain how sound is produced by your school bell. Ans. If it is an electric bell when the switch is pressed electric charges flow through the wire and the hammer strikes the gong. This produce vibrations. If it is a manual bell then vibrations are produced by hitting the round plate with a stick. When an object vibrates, then the air layers around it also start vibrating in exactly the same way. These layers of air carry sound waves from the sound producing object (bell in this case) to our ears. During the transmission no actual movement of air from the bell to our ear takes place. The air layers only vibrate back and forth and transfer the sound energy from one layer to the next layer till it reaches our ear. Why are sound waves called mechanical waves? 2. Ans. A wave is a disturbance that moves through a medium and the particles of the medium set by neighbouring particles into motion. They in turn produce similar motion in others and the particle moves away from its initial position. This exactly happens during propagation of sound in a medium. Hence sound can be visualized as a wave. Sound waves are characterized by physical motion of particles in the medium and are called mechanical waves. 3. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend? Ans. We cannot hear the sound produced by our friend because there is no atmosphere in the moon. It is all vacuum on the surface of moon. Sound needs a medium to travel. Page No. 166 1. Which wave property determines? (a) loudness (b) pitch Ans. (a) The loudness or softness of a sound is determined by its amplitude. (b) The pitch of a sound is determined by its frequency. 2. Guess which sound has a higher pitch-guitar or car horn? Ans. Car horn has a higher pitch. Page No. 166 1. What are wavelength, frequency, time period and amplitude of a sound wave? Ans. (a) The distance between two consecutive crests or two consecutive troughs is called wavelength. (b) The number of oscillations per unit time is called the frequency of the sound wave. (c)The time period of a wave is the time taken by a wave to travel through a distance equal to its wavelength. (d) The magnitude of the maximum displacement of the vibrating particles of the medium on either side of their mean undisturbed position is called the amplitude of the wave. 11. Can sound propagate through vacuum? Ans. No, it needs a material medium for its propagation. 12. Why does sound need a medium to travel? Ans. Sound needs a medium to travel because sound is vibrations of media (oscillations of media molecules relatively to a predefined system of coordinates.) 13. When the wire of a sitar is plucked, what type of waves are produced in the wire? Ans. Transverse waves are produced in the wire. 14. Which type of waves are produced when a stone is dropped on the surface of water in a pond? Ans. Transverse waves. 15. Which type of waves are produced when a freely suspended spring is pulled downward and released? Ans. Longitudinal waves. 16. How are waves classified on the basis of medium of propagation? Give one example of each. Ans. Mechanical wave: Waves which can be produced or propagated only in a material medium are called elastic waves or mechanical waves. Example: Waves on water surface, waves on stretched strings, coiled springs and sound waves are elastic or mechanical waves. Non-mechanical wave: The waves that do not require any material medium for their propagation are called non-mechanical waves. Example: All electromagnetic waves like lightwaves, radiowaves, X-rays, etc. 17. Give two examples of longitudinal waves. Ans. (i) Sound waves: When a sound wave travels, the air molecules oscillate in a direction parallel to the direction in which the sound waves propagate. Hence, sound waves are longitudinal waves. (ii) Waves produced in a spring when it is compressed and released are longitudinal waves. 18. Name the low pressure region of the sound wave. Ans. Rarefaction. 19. What is a trough? Ans. The depression (or the valley) in a transverse wave is called a trough. 20. What is the difference between music and noise? Ans. Noise is unpleasant to the ear, whereas music is pleasant to the ear and it is of rich quality. 21. What is a periodic wave? Ans. A wave in which the particles of the medium oscillate continuously while propagating the disturbance is called a periodic wave. 22. Write factor on which pitch of a sound depends. Ans. The pitch of a sound is directly proportional to its frequency of vibration. 23. Why is the roaring of lion different from the sound of a mosquito? Ans. A roaring lion has low pitch but greater loudness while a mosquito has a high pitch but lesser loudness. 24. How can we detect the filling of a bottle under a tap by hearing its sound at a distance? Ans. As the water level increases, the length of vibrating air column above water decreases thereby increasing the frequency. Multiple Choice Questions 1. Note is a sound: (a) of mixture of several frequencies. (b) of mixture of two frequencies only. (c) of a single frequency. (d) always unpleasant to listen. Ans. (a) 2. A key of a mechanical piano struck gently and then struck again but much harder this time. In the second case: (a) sound will be louder but pitch will not be different. (b) sound will be louder and pitch will also be higher. (c) sound will be louder but pitch will be lower. (d) both loudness and pitch will remain unaffected. Ans. (a) 3. In SONAR, we use: (a) ultrasonic waves (b) infrasonic waves (c) radio waves (d) audible sound waves Ans. (a) 4. Sound travels in air if: (a) particles of medium travel from one place to another. (b) there is no moisture in the atmosphere. (c) disturbance moves. (d) both particles as well as disturbance travel from one place to another. When we change feeble sound to loud sound, we increase its: 5. (a) frequency (b) amplitude (c) velocity (d) wavelength Ans. (b) 6. Earthquake produces which kind of sound before the main shockwave begins? (a) ultrasound (c) audible sound (b) infrasound (d) none of the above Ans. (b) 7. Infrasound can be heard by: (a) dog (b) bat (c) rhinoceros (d) human beings Ans. (c) 8. Before playing the orchestra in a musical concert, a sitarist tries to adjust the tension and pluck the string suitably. By doing so, he is adjusting: (a) intensity of sound only. (b) amplitude of sound only. (c) frequency of the sitar string with the frequency of other musical instruments. (d) loudness of sound. Ans. (c)