PHYSICS CLASS NOTES FOR CBSE Chapter 05. Sound Sound is that form of energy which makes us hear. A wave is a vibratory disturbance in a medium which carries energy from one point to another without there being a direct contact between the two points. There are two types of waves : longitudinal waves and transverse waves.
01. Sound Waves are Longitudinal Waves A wave in which the particles of the medium vibrate back and forth in the ′same direction′ in which the wave is moving, is called a longitudinal wave. Longitudinal waves can be produced in all the three media : solids, liquids and gases. The waves which travel along a spring (or slinky) when it is pushed and pulled at one end, are longitudinal waves. The waves produced in air when a guitar wire (sitar wire, tanpura wire or violin wire) is plucked are longitudinal waves. A rarefaction is that part of a longitudinal wave in which the particles of the medium are farther apart than normal, and there is a momentary increase in the volume of the medium.
Compression Rarefaction Compression Rarefaction of a longitudinal wave.
A rarefaction is that part of a longitudinal wave in which the particles of the medium are farther apart than normal, and there is a momentary increase in the volume of the medium.
02. Characteristics of a Sound Wave A sound wave can be described completely by five characteristic : Wavelength, Amplitude, Time-period, Frequency and Velocity (or Speed).
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CLASS NOTES FOR CBSE – 05. Sound
Characteristics of a sound wave
(i)
Wavelength : The minimum distance in which a sound wave repeats itself is called its wavelength. The Si unit for measuring wavelength is metre (m). (ii) Amplitude : The maximum displacement of the particles of the medium from their original undisturbed positions, when a wave passes through the medium is called amplitude of the wave. (iii) Time-Period : The time required to produce one complete wave (or cycle) is called time-period of the wave. The time taken to complete one vibration is called time-period. (iv) Frequency : The number of complete waves (or cycles) produced in one second is called frequency of the wave. The number of vibrations per second is called frequency. The SI unit of frequency is hertz, 1 hertz is equal to 1 vibration per second. (v) Velocity of Wave (or Speed of Wave) : The distance travelled by a wave in one second is called velocity of the wave (or speed of the wave). The SI unit for measuring the velocity of a wave is metres per second. Velocity of a wave = Frequency × Wavelength
03. Reflection of Sound The bouncing back of sound when it strikes a hard surface is called reflection of sound. Sound is reflected well from hard surfaces like a wall, a metal sheet, hard wood and a cliff. The laws of reflection of light are obeyed during the reflection of sound. We will now describe an experiment to study the reflection of sound.
04. Applications of Reflection of Sound (i)
Megaphone and Bulb Horn : A megaphone is a large, cone-shaped (or funnel-shaped) device for amplifying and directing the voice of a person who speaks into it. A megaphone works on the multiple reflection of sound. A bulb horn is a cone-shaped wind instrument which is used for signalling in bicycles, car, buses, trucks and boats, etc.
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CLASS NOTES FOR CBSE – 05. Sound
(ii)
Stethoscope : Stethoscope is a medical instrument used by the doctors for listening to the sounds produced within the human body, mainly in the heart and the lungs. A stethoscope works on the principle of multiple reflection of sound.
05. Characteristics of Sound A sound has three characteristics : loudness, pitch and quality (or timbre). In other words, sounds are recognised by three characteristics : loudness, pitch and quality (or timbre). Two musical sounds may differ from one another in one or more of these characteristics. We will now discuss all the characteristics of sound in detail, one by one. Let us start with loudness. (i) Loudness : Sounds are produced by vibrating objects. If less energy is supplied to an object by hitting it lightly (or by stretching it lightly), then the object vibrates with a smaller amplitude and produces a faint sound (or feeble sound). One the other hand, if more energy is supplied to an object by hitting it. Since the amplitude of a sound wave is equal to the amplitude of vibrations of the source which produces the sound wave, we can also say that : The loudness of sound depends on the amplitude of vibration of the source producing the sound waves. This point will become clear from the following example. When we strike a table lightly, then due to less energy supplied, the table top vibrates with a small amplitude and hence a faint sound (or soft sound) is produced. If, however, we hit the table hard, then due to greater energy supplied, the table top vibrates with a large amplitude and hence produces a loud sound. Thus, the amplitude of sound waves depends on the force with which an object is made to vibrate. The loudness of sound is measured in ′decibel′, written as dB. The softest found which human ears can hear is said to have a loudness of 0 dB (zero decibel). The loudness of sound in a very noisy factory is about 100 dB and the sound of a jet aircraft 50 metres away is said to have a loudness of about 130 dB.
(ii)
Pitch : We an distinguish between a man’s voice and a woman’s voice of the same loudness even without seeing them. This is because a man’s voice and a woman’s voice differ in pitch. A man’s voice is flat having a low pitch, whereas a woman’s voice is shrill having a high pitch. We can now say that : Pitch is that characteristic of sound by which we can distinguish between different sounds of the same loudness. (iii) Quality (or Timbre) : We can distinguish between the sounds (or notes) produced by a flute and a violin even without seeing these instruments. This is because the sounds produced by a flute and a violin differ in quality (or timbre).
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CLASS NOTES FOR CBSE – 05. Sound
It is the difference in the quality of sound which enables us to tell at once which instrument played the musical sound (or musical note). We can now say that : Quality (or timbre) is that characteristic of musical sound which enables us to distinguish between the sounds of same pitch and loudness produced by different musical instruments (and different singers).
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CLASS NOTES FOR CBSE – 05. Sound
CBSE Pattern Exercise (1) (Q 1 to 4) Very Short Type 1. Can sound travel through (a) iron (b) water 2. In which medium sound travels fastest : air, water or steel? 3. What is the nature of sound waves in air? 4. Name that part of ear which vibrates when outside sound falls on it. (Q 5 to 7) Short Answer Type 5. A cricket ball is seen to hit the bat first and the sound of hitting is heard a little later. Why? 6. Define the ‘frequency’ and ‘time-period’ of a wave. What is the relation between the two? 7. (a) What are longitudinal waves and transverse waves? (b) Give two examples each of longitudinal waves and transverse waves. (Q 8) Multiple Choice 8. If the speed of a wave is 340 m/s and its frequency is 1700 Hz, then λ for this wave in cm will be : (a) 2 (b) 0.2
(c) 20 (d) 200
(Q 9 to 10) High Order Thinking Skills 9. Write the full name of ‘SONAR’. Explain its working 10. (a) What is an echo? How is echo formed? (b) What is the minimum distance in air required from a sound reflecting surface to hear an echo (at 20°C)?
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CLASS NOTES FOR CBSE – 05. Sound
ANSWER Q1 Sound can travel is solids, liquids and gases. However, it cannot travel in vacuum. (a) Yes, because iron is a solid. (b) Yes, because water is a liquid. Q2 Sound travels fastest through solids. This is because molecules in a solid medium are much closer together than those in a liquid or gas, allowing sound waves to travel more quickly through it. In fact, sound waves travel over 17 times faster through steel than through air. Q3 Nature of Sound Waves. Sound is one kind of longitudinal wave, in which the particles oscillate to and fro in the same direction of wave propagation. Sound waves cannot be transmitted through vacuum. The transmission of sound requires at least a medium, which can be solid, liquid, or gas. Q4 Ear drum located in middle ear vibrates when sound reaches to it through the eustachian tube Q5 The ball is seen to hit the bat first and the sound of hitting is heard a little later due to the very high speed of light that we see the ball hitting the bat first and it is due to comparatively low speed of sound of hitting is heard a little later. Q6 Frequency and time period. ... T is the time it takes for one complete oscillation , it is measured in seconds. All waves, including sound waves and electromagnetic waves , follow this equation. For example, a wave with a time period of 2 seconds has a frequency of 1 ÷ 2 = 0.5 Hz. Q7 (a) Transverse waves are always characterized by particle motion being perpendicular to wave motion. A longitudinal wave is a wave in which particles of the medium move in a direction parallel to the direction that the wave moves. (b) Sound waves are longitudinal waves, meaning that the waves propagate by compression and rarefaction of their medium. ... Alternatively, transverse waves oscillate perpendicular to the direction of motion. Common examples of transverse waves include light and, to a basic approximation, waves on the ocean. Q8 (c) 20 cm
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CLASS NOTES FOR CBSE – 05. Sound
Q9 Sonar (originally an acronym for sound navigation ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, communicate with or detect objects on or under the surface of the water, such as other vessels. Q10 (a) The delay is directly proportional to the distance of the reflecting surface from the source and the listener. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single reflection of the sound source.
(b) The velocity of sound in dry air is approximately 343 m/s at a temperature of 25°C. Therefore, the reflecting object must be more than 17.2m from the sound source for echo to be perceived by a person located at the source. Answer: The minimum distance from reflector is 16.6 m.
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