Chapter 11 Sound - PDF

# Chapter-11 Sound ## Question and Answer 1. How does the sound produced by a vibrating object in a medium reach your ear? Ans: - A vibrating object sets the particles of the medium around it vibrating during vibration. The particles in the medium in contact with the vibrating object displace from...

# Chapter-11 Sound ## Question and Answer 1. How does the sound produced by a vibrating object in a medium reach your ear? Ans: - A vibrating object sets the particles of the medium around it vibrating during vibration. The particles in the medium in contact with the vibrating object displace from its equilibrium position. It then exerts force on the adjacent particles. After displacing the adjacent particle, the first particle of the medium comes back in its original position. This force process continues in the medium till the sound reaches our ear. 2. Explain how sound is produced by your school bell. Ans: - It creates a series of compression and rarefactions making production of sound, when the bell continues to move forward and back word. 3. Why are sound waves called mechanical waves? Ans: - Sound waves need a material medium to propagate sound waves. Sound waves propagate through a medium because of the interaction of the particles present in that medium. 4. Which wave property determines (a) loudness, (b) pitch? Ans: (a) Amplitude (b) Frequency 5. Guess which sound has a higher pitch: guitar or car horn? Ans: - High the frequency higher is the hitch. Since guitar has a higher pitch than a car horn, because sound produced by the strings of guitar has high frequency than that of a car horn. ## # Chapter-11 Sound 6. What are wavelength, frequency, time period and amplitude of a sound wave? Ans: - Wavelength: The distance between two consecutive compressions or two consecutive rarefactions is known as wavelength. Its SI unit is meter (m). Frequency: The number of complete oscillations per unit time is known as the frequency of a sound wave. It is measured in Hertz (Hz). Time period: Time taken for one complete oscillation in the density of the medium is called the time period of the sound wave. Amplitude: The maximum height reached by the crest or trough of a sound wave is called its amplitude. ## 7. How are wavelength and frequency of sound wave related to its speed? Ans: - Speed, wavelength and frequency of a sound wave are related by the following equation: * speed (v) = Wavelength (λ) × Frequency (ν) V = λ × ν ## 8. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium. Ans: - The frequency of the sound wave: ν = 220 Hz Speed of the sound wave: V = 440 m/s For a sound wave, Speed = Wavelength × Frequency, V = λ×ν λ = V/ν = 440/220 = 2 m Hence, the wavelength of the sound wave is 2m. ## 9. A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions and rarefactions from the source? Ans: - The time interval between two successive compressions is equal to the time period of the wave. This time period is reciprocal of the frequency of the wave, and is given by the relation: T = 1/frequency Ans: - Frequency = 500 Hz T = 1/frequency = 1/500 = 0.002 s ## 10. Distinguish between loudness and intensity of sound. Ans: - Intensity of a sound wave is defined as the amount of sound energy passing through a unit area per second. Loudness is a measure of the response of the ear to the sound. The loudness of a sound is defined by its amplitude. The amplitude of a sound decides its intensity, which in turn is perceived by the ear as loudness. ## 11. In which of the three media; air, water or iron, does sound travel the fastest at a particular temperature? Ans: - The speed of sound depends on the nature of the medium. Sound travels fastest in solids. Its speed decreases in liquids and it is the slowest in gases. Therefore, for a given temperature, sound travels fastest in iron. ## 12. A sound is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m/s? Ans: - Speed of sound: V = 342 m/s Echo heard in time: t = 3 s Distance travelled by sound = V × t = 342 × 3 = 1026 m In the given time interval, sound has to travel distance that is twice the distance of the reflecting surface and source. Hence the distance of the reflecting surface from the source = 1026/2 = 513 m ## 13. Why are the ceilings of concert halls curved? Ans: - Ceilings of concert halls are curved so that sound after reflection (from the walls) spreads uniformly in all directions. ## 14. What is audible range of the average human ear? Ans: - The audible range of an average human ear lies between 20 Hz to 20,000 Hz. ## 15- What is the range of frequencies associated with: (a) Infrasound? (b) Ultrasound? Ans: - (a) Infrasound has frequencies less than 20 Hz. (b) Ultrasound has frequencies more than 20,000 Hz. # Exercise 1. What is sound and how is it produced? Ans: - Sound is a form of energy which gives the sensation of hearing. It is produced by the vibrations caused in air by vibrating objects. 2. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a sound source? Ans: - When a vibrating body moves forward, it creates a region of high pressure in its vicinity. This region of high pressure is known as compression. It creates a region of low pressure in its vicinity when it moves backwards. This region is known as rarefaction. As the body continues to move forward and backwards, it produces a series of compression and rarefactions, as shown in the figure below. ## 3. Why is sound wave called a longitudinal wave? Ans: - The air particles vibrate parallel to the direction of propagation as sound wave is produced by compressions and rarefactions in the air that is why it is called longitudinal wave. ## 4. Flash and thunder are produced simultaneously - But thunder is heard a few after the flash is seen, why? Ans: - The speed of sound (344 m/s) is less than the speed of light (3×10⁸ m/s). Sound of thunder takes more time to reach the earth as compared to light. Hence, a flash is seen before we hear thunder. ## 5. The frequency of a sound source of sound is 100 Hz. How many times does it vibrate in a minute? Ans: - Frequency = 100 Hz (given) This means the source of sound vibrates 100 times in one second. Therefore, the number of vibrations in 1 minute, i.e. in 60 s = 100 × 60 = 6000 times ## 6. Does sound follow the same laws of reflection as light? Explain. Ans: - Sound follows the same laws of reflection as light does. The incident sound wave and the reflected sound wave make the same angle with the normal to the surface at the point of incidence. Also, the incident sound wave, the reflected sound wave and the normal to the point of incidence call lie in the same plane. ## 7. Give two practical applications of reflection of sound waves. Ans: - Two practical applications of reflection of sound waves are: 1. Reflection of sound is used to measure the distance and speed of underwater objects. This method is known as SONAR. Working of sonar is based on reflection of sound. 2. Working of stethoscope is also based on reflection of sound. In a stethoscope, the sound of the patient's heartbeat reaches the doctor's ear by multiple reflection of sound. ## 8. What is reverberation? How can it be reduced? Ans: - The repeated multiple reflections of sound in any big enclosed space is known as reverberation. Reverberation can be reduced by covering the ceiling and walls of the enclosed space with sound-absorbing materials, such as fiberboard, loose woolens, etc. ## 9. What is loudness of sound? What factors does it depend on? Ans: - The effect produced in the brain by the sound of different frequencies is called loudness of sound. Loudness depends on the amplitude of vibrations. In fact, loudness is proportional to the square of the amplitude of vibrations. ## 10. How is ultrasound used for cleaning? Ans: - Objects to be cleaned are put in a cleaning solution and ultrasonic sound waves are passed through that solution. The high frequency of these ultrasound waves detaches the dirt from the objects. ## 11. Explain how defects in a metal block can be detected using ultrasound. Ans: - Defects in metal blocks do not allow ultrasound to pass through them and they are reflected back. This fact is used to detect defects in metal blocks. Ultrasound is passed through the metal block and detectors are placed on the other end. The defective part of the metal block does not allow ultrasound to pass through it. As a result, it will not be detected by the detector. Hence, defects in metal blocks can be detected using ultrasound. **Notes:** - The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength (λ). - The time taken by the wave for one complete oscillation of the density or pressure of the medium is called time period, T. - The number of complete oscillations per unit time is called the frequency (ν), ν = 1/T - The speed v, frequency v and wavelength λ, of sound are related by the equation, λ = v/ν - The law of reflection of sound states that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface at the point of incidence and the three lie in the same plane. - The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation. - The amount of sound energy passing each second through unit area is called the intensity of sound. - The audible range of hearing for average human beings is in the frequency range 20 Hz-20 KHz.