Chapter Notes - Sound Class 9 Notes | EduRev

Science Class 9

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Class 9 : Chapter Notes - Sound Class 9 Notes | EduRev

The document Chapter Notes - Sound Class 9 Notes | EduRev is a part of the Class 9 Course Science Class 9.
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1. Sound is a form of energy which produces a sensation of hearing in our ears.
2. Sound energy also comes under the purview of conservation law of energy. Other forms of energies may be transformed into sound energy and vice-versa.
3. Sound is produced due to vibrations of an object. Vibrations of temple bell, vibrations produced in school bell, vibrations of tuning fork, vibrations of sitar/violin wire, vibrations of the air column in flute, vibrations of vocal cord in human beings etc., produce sound.
4. Sound requires a material medium either solid or liquid or a gas, for its propagation. Sound cannot be transmitted through a vacuum.
5. When an object vibrates, it sets the medium particles around it vibrating. These vibrating medium particles further set neighbouring particles in motion and so on. The process continues till the vibrations reach our ears and cause sensation of sound. This sort of propagation is called wave motion.
6. Wave motion is a form of disturbance which travels through a medium due to repeated vibrations of the medium particles about their mean positions and simultaneously, the motion is handed over from one particle to another.
7. Waves which require a material medium are called mechanical waves. So, the sound waves are mechanical waves.
8. Waves are of two types : (i) longitudinal waves and (ii) transverse waves.
9. In longitudinal waves, the individual medium particles move in a direction parallel to the direction of propagation of the wave. Sound waves in air are longitudinal waves.
10. In longitudinal waves, during wave propagation, alternate zones of compression and rarefaction (or zones of high and low pressure or density) are formed in the given medium. These waves can be graphically represented by pressure and density variation with distance.
11. In transverse waves, the individual particles of the medium move about their mean positions in a direction perpendicular to the direction of wave propagation. Water ripples formed on the surface of a pond, waves on a vibrating string are examples of transverse waves. Crests and troughs are alternately formed in transverse waves.
12. Light waves are also transverse waves but of a special nature, as light waves can travel even through vacuum. Waves which can travel even through vacuum are known as electromagnetic waves. Crests and troughs are alternately formed in terms verse waves.
13. The change in density (pressure) from its maximum value to the minimum value and again to the maximum value makes one complete vibration (or oscillation).
14. The time taken by a wave for one complete vibration (oscillation) of the density (or pressure) of the medium is called the time period T.
15. The number of complete vibrations (oscillations) per unit time is called the frequency of the wave. Frequency of a wave is reciprocal of its time period. Frequency is generally denoted by Greek letter v (nu).
Chapter Notes - Sound Class 9 Notes | EduRev
SI unit of frequency is hertz (Hz). Frequency is said to be 1 Hz, if the oscillating object completes one oscillation in 1s.
16. The distance between centres of two consecutive; compressions or centres of two consecutive rarefactions is called the wavelength. Wavelength is generally denoted by Greek letter λ (lamda). Alternately, wavelength is the distance through which a wave is propagated during the time of one complete vibration (oscillation). Its SI unit is metre (m).
17. The speed of sound is defined as the distance which a point on the wave, such as a compression or a rarefaction, travels forward in a unit time.
18. The speed (u), frequency (v) and wavelength (λ) of sound are related by the equation u=vλ
19. For an oscillating object the frequency of oscillation depends on the material and dimensions (size and shape) of object as well as conditions of vibrating.
20. A characteristic of sound depending on its frequency is called ‘pitch’. The manner in which the brain interprets the frequency of an emitted sound is called the pitch.
21. If frequency of wave increases, its pitch correspondingly increases. A sound of a higher pitch is called shrill sound (e.g., the sound of buzzing of bee) but a sound of lower pitch is called a hoarse sound.
22. The magnitude of the maximum disturbance (in density or pressure) in the medium on either side of the mean value is called the amplitude;of the wave (A). The amplitude of sound wave produced by an oscillating object depends upon the magnitude of force with which the object is made to oscillate. Its SI unit is same as that of density/pressure.
23. Loudness of a sound is the perception made by the; listener’s ear whether the given sound is a loud sound or a soft sound. The loudness or softness of a sound is basically dependent on its amplitude. Greater the amplitude, louder the sound will be. Lesser the amplitude, softer the sound will be.
24. A loud sound can travel a larger distance because it has more energy. As a sound wave spreads out from its source and moves away from the source, its amplitude and hence loudness gradually decreases. Loudness is usually expressed in ‘decibels’ (dB).
25. The amount of sound energy passing out through unijt area in unit time around a point is called the intensity of sound at that point. Its SI unit is watt/m2 (W m-2).
26. Sometimes we consider the terms loudness and intensity of sound to be same. But they are not. Loudness is a measure of the response of the ear to the sound intensity.
27. The quality (or timbre) of sound is that characteristic which enables us to distinguish sounds produced by two different persons or musical instruments, although their pitch and loudness are exactly the same.
28. A sound of single frequency is called a ‘tone’. A tuning fork produces a pure tone.
29. The sound produced due to a mixture of several frequencies is called a note.
30. Musical sound is pleasant to hear and is of rich quality. On the other hand, sound unpleasant to hear is called noise.
31. Speed of sound is much less than the speed of light. It is due to this reason that during rainy season flash of light due to lightning is seen earlier and the sound of thunder is heard a little later.
32. The speed of sound in a medium depends on the properties (namely, elasticity and density) of the medium. In general, speed of sound increases with increase in temperature.
33. The speed of sound is maximum in solids, lesser in liquids and least in gases. As an example, speed of sound at 25 °C in steel is 5960 ms-1, in pure water 1498 ms-1 and in air 346 ms-1.
34. If an object is travelling with a speed greater than the speed of sound, it is said to be travelling at a supersonic speed. Advanced jet planes, during their flight, travel with supersonic speeds.
35. Just like light, the sound gets reflected at the surface of a solid or liquid. It follows both laws of reflection, namely (i) the incident sound wave, the reflected sound wave and the normal to the reflecting surface are in the same plane, and (ii) angle of incidence and angle of reflection are equal.
36. When we stand some distance from a huge building or a cliff and speak loudly, we are able to listen our voice again after a definite interval of time due to reflection of a sound wave from that building or cliff. Such repetition of sound due to reflection is known as ‘echo’.
37. For hearing a distinct echo, the time interval between the original sound and the reflected one must be at least 0.1 s. In other words, a distinct echo is heard only when distance of reflector surface is atleast 17.2 m or more.
38. If the reflecting object is situated at a distance d, then the time of echo t =2d/v, where v is the speed of sound in air.
39. Persistence of sound in an auditorium or big hall due to repeated reflection from the walls and roof is called ‘reverberation of sound’. Excessive reverberation is undesirable. To reduce reverberation, the roofs and walls of the auditorium are covered with sound absorbant materials.
40. Megaphones, horns, stethoscope etc., make use of multiple reflection of sound for their working.
41. The ceilings of concert halls, conference halls and cinema halls are generally curved so that sound after reflection reaches all corners of the hall.
42. For average human beings the audible range of hearing is in the frequency range of 20 Hz to 20 kHz.
43. Sound waves below the audible range (i.e., sounds of frequencies below 20 Hz) are called infrasonic sound. Rhinoceroses, whales and elephants produce sound in the infrasonic range.
44. Sound waves of frequencies greater than 20 kHz are called ultrasonic sound or ultrasounds. Dolphins, bats and porpoises produce ultrasound. Moths and rats are also sensitive for ultrasound.
45. Being of high frequencies, ultrasounds can travel along well-defined paths even in the presence of obstacles. Due to this property ultrasounds are extensively used in industries and for medical purposes.
46. SONAR stands for Sound Navigation And Ranging. It is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects. It transmits ultrasonic waves and detects the waves reflected from underwater objects. The method is called echo ranging.
47. The SONAR technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarines, icebergs and sunken ships etc.
48. Bats search out prey and fly in dark night by emitting and detecting reflections of ultrasonic waves.
49. We are able to hear with the help of ear. The ear allows us to convert pressure variations in air with audible frequencies into electrical signals that travel to the brain via the auditory nerve.
50. Our ear consists of mainly three parts,
(i) outer ear,
(ii) middle ear, and
(iii) inner ear.
The outer ear collects the sound from the surroundings. The eardrum situated at the inner end of outer ear vibrates when sound from surroundings is received. The vibrations are amplified several times by middle ear. Then the middle ear transmits the amplified pressure variations to inner ear. Inner ear turns pressure variations into electrical signals, Auditory none send these electrical signals to the brain, which interprets them as sound.

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