CLASS 7 SOUND

1. Basic Concept
2. Nature of SOUND
3. Sources of SOUND
4. Propagation of SOUND
5. Types of SOUND: INFRASONIC, SONIC/AUDIBLE, ULTRASONIC
6. Characteristics of SOUND
7. Speed of SOUND in different medium.

1. WELCOME TO MY CLASS Mr. Souvik Chatterjee M.Sc (Physics)& B.Ed

2. SOUND

3. Sound is a energy which produces sensation of hearing into our ears .

4. SOUND PRODUCED BY VIBRATIONS The oscillatory motion in which a body assumes a new shape during its motion , is called vibratory motion In vibratory motion , a part of the body oscillates keeping their other part fixed. Sound is produced when a body vibrates. In other words source sound of the body is vibrating body.

5. EXAMPLE When you blow a whistle, the vibration in the air particles produce sound When we speak, our vocal cords vibrate to produce sound in the larynx (also known as voice box) which can be felt by placing your hand in throat. When a bell is struck, it vibrates and produce sound.

6. SOURCES OF SOUND Tuning Fork Tuning fork is a U-shaped metallic piece with a stem in the middle. The arms are known as prongs. They are set into vibrations when either of the two prongs is struck gently on a rubber pad and sound is produced.

7. MUSICAL INSTRUMENTS Musical instruments like whistle, flute, and clarinet which are in form of pipes which produce sound when air blown in them. Thus the column of air inside them vibrates and producing sound.

8. Musical instruments like harmonium and mouth organ contains metal reeds and they produce sound when air is blown through them. Musical instruments like sitar, guitar, piano, violin have strings stretched on them. In sitar and guitar the string is plucked. In piano string is struck. In a violin, a bow is drawn across the string, to make the string vibrate and produce sound In instruments like drum, tabla , dholak etc lather membrane is made to vibrate by striking with hammer or with hand, so as to produce sound.

9. HUMAN BEING Human beings produce sound when our vocal vibrates on blowing air through them by our lungs. Our throat has larynx. Larynx also called voice box. It is designed to produce voice. It is a box like structure with walls of tough tissue. Inside two fold of tissue, there is a gap. They are called vocal chord. When we breathe, the vocal chords become loose and the gap between them increases. When we talk, shout or sing, our chords become tight and hence they vibrate and produce sound.

10. KEY POINTS Some animals like birds, frogs etc also produce due to vibration of their vocal chords. But bees do not have the vocal boxes. They produce sound by moving their winds up and down very fast.

11. SOUND NEEDS A MEDIUM FOR PROPAGATION Sound can not travel through vacuum. Sound requires a medium for propagation i.e travelling from one point to the other. In moon or in space there is no atmosphere, So two persons can not hear each other.

12. PROPAGATION OF SOUND Sound travels in air in the form of longitudinal wave

13. Displacement-Distance Graph

14. Displacement-Time Graph

15. KEY POINTS Frequency = 1/Time-period Velocity of sound = Frequency X Wavelength. In Displacement-time graph if there are more number of waves in same distance then the wavelength is less In Displacement-time graph if there are more number of waves in same time then Time-period will be less i.e Frequency will be more.

16. IMPORTANT DEFINITIONS One Vibration: To and fro motion which constitutes one full wave, is called One vibration. Wavelength: The length of the wave corresponding to one vibration is known as its wavelength. The S.I unit of wavelength is metre (m). Amplitude: Maximum displacement of the wave from the either side of mean position is known as amplitude. The S.I unit of amplitude is metre (m). Time-period : Time taken by a wave to complete one vibration is known as its time-period. The S.I unit of time-period is second(s). Frequency: No of vibrations produced by the source of sound wave in one second is known as its frequency. The S.I unit of frequency is hertz(Hz).

17. KEY POINTS Amplitude of vibration of the of a body gradually decreases due to friction of air and ultimately dies out. Frequency of a wave independent of amplitude, but equal to frequency of source producing the sound.

18. AUDIBLE SOUND Human beings can not hear sound of all frequencies. We can hear sound of frequency range from 20 Hz to 20,000 Hz. We can not hear sound below 20 Hz and higher than 20,000 Hz. The range of frequency from 20 Hz to 20,000 Hz is called audible range for the normal human ear.

19. ULTRASONIC SOUND Sound of frequency more than 20,000 Hz, is called ultrasonic sound. We can not hear ultrasonic sound. Some animals like dogs, bats, monkeys, deers,leopards can hear ultrasonic sound. Dogs can hear sound upto 50,000 Hz. Bats can hear sound upto 1,00,000 Hz.

20. INFRASONIC SOUND Sound of frequency lower than 20 Hz are called infrasonics . We can not hear the infrasonic sounds. For example the pendulum of a clock makes one vibration in 2 second. The frequency of sound produced due to its vibrations is 0.5 Hz. Hence we can not hear it.

21. CHARECTERISTICS OF SOUND We have the following two charecteristics of sound (1) Loudness (2) Pitch

22. LOUDNESS The loudness of sound depends on the amplitude of vibration of the vibrating body producing sound. Greater the amplitude of vibration louder is the sound The loudness also depends on area of the vibrating body. Greater the area of the vibrating body, louder the sound produced. Loudness is the characteristic of sound which distinguishes a loud sound from a feeble sound.

23. PITCH Pitch of a sound depends on its frequency ( i.e number of vibrations produced per second by the body) A sound of higher frequency is said to have higher pitch while a sound of low frequency said to have low pitch. Higher the pitch shriller the sound, but if the pitch is low, the sound is flat or grave. The voice of a girl is shriller than that of boy because the voice of the girl is of a higher pitch. The voice of a child is shriller than that of an adult. Pitch is a characteristic of sound which distinguishes a shrill sound from a flat or grave sound.

24. KEY POINTS Two sounds of same quality and loudness may differ in quality or waveform. The quality is the characteristic of sound which distinguishes between two different sounds having same loudness and pitch.

25. SPEED OF SOUND IN DIFFERENT MEDIUM Speed of sound in gas/vacuum = 330 m/s Speed of sound in Water = 1500 m/s Speed of sound in Iron = 5000 m/s

26. KEY POINTS Speed of sound will increase with increase in temperature. Speed of sound will increase by 0.61 m/s with an increase in 1 degree C of temperature. Speed of sound is more in humid air than dry air.