Class 9 Sound Notes

CSCE TUTORIAL

SOUND

 

1. Sound is a form of energy which produces a sensation of hearing in our ear.

2. Production of sound – Sound is produced by vibration of an object.

3. Vibration means to and fro motion of an object.

4. The sound of the human voice is produced by the vibrations of the vocal cords.

5. Sound produced by a bee is due to flapping of its wings. When a bird flaps its wings, sound is produced.

6. Propagation of sound – Sound is produced by a vibrating object. The matter or substance through which sound is transmitted is called a medium. When an object vibrates, it sets the particles of the medium around it vibrating. The particles do not travel all the way from the source of sound to the listener. A particle of the medium in contact with the vibrating object first displaces from its equilibrium position which then exerts force on the next adjacent particle. After displacing the second particle the first particle comes to rest. This disturbance thus passes to through the medium until it reaches our ear. Thus, only energy gets transferred in the form of sound waves and not the particles as the sound propagates.

7. A sound wave is a disturbance that moves through a medium when the particles of the medium set the neighbouring particles into motion. Since sound waves are characterized by the motion of particles in a medium, they are called the mechanical waves.

8. Sound as a series of compressions and rarefaction – Air is the most common medium through which sound travels. When a vibrating body moves forward, it pushes and compresses the air in front of it creating a region of high pressure called compression (C). This compression starts to move away from the vibrating body. When the vibrating body moves backwards, it creates a region of low pressure called rarefaction (R). As the object moves to and fro rapidly, this sets a series of compressions and rarefactions in the air. This makes the sound to propagate in the air.  ( Pressure is related to the number of particles of the medium in the given volume) More density of air means more pressure while less density of air means less pressure. Thus, propagation of sound can be visualized as propagation of density variations or pressure variations in the medium.

9. Sound needs a medium to travel – Sound waves are called longitudinal waves as they travel in the form of a series of compressions and rarefactions. In these waves the individual particles of the medium move in the direction parallel to the direction of motion of the wave. The particles of the medium are not actually moving, they only vibrating back and forth about their position of rest.

   Since the sound moves through the medium by to and fro motion of the particles, thus sound requires the presence of the material medium for its propagation.

10. Transverse wave – In a transverse wave particles do not oscillate along the direction of propagation but oscillate up and down about their mean position as the wave travels. Thus, in case of transverse wave, the particles move up and down perpendicular to the direction of motion of the wave. For example, if we drop a pebble in a pond of water, the ripples formed on the surface of water is an example of the transverse wave.

    Another example of the transverse waves is light wave. In light wave, however, no material medium is required for the propagation of wave, so they are not mechanical waves.

11. Characteristics of a Sound wave – There are three characteristics of sound.
12. Frequency – The number of oscillations per second is called the frequency of a wave. The SI unit of frequency is Hertz (Hz). It is usually represented by ‘𝛎)’ (Greek letter ‘nu’)
13. Wave length – Sound waves are a series of compressions and rarefactions. Compressions are a region of high pressure represented by the upper portion of the curve shown below:
    Rarefactions on the other hand are the regions of low pressure where particles are far apart and are shown by lower portions of the wave.
    A peak is called a crest and a valley is called a trough of the wave.
    Wavelength is the distance between two consecutive crests or two consecutive troughs and is represented by the (Greek letter ‘Lambda’) and its SI unit is m.  – Greek letter ‘Lambda’.
    Time period – The time taken by two consecutive compressions or rarefactions to cross a fixed point. The time taken to complete one oscillation is called the time period of the sound wave. It is represented by the letter ‘T’. The SI unit of time period is second (s).
    Relation between frequency and time period – Time period and frequency are reciprocal of each other.

14. Pitch – The shrillness of sound is called pitch. It is how our brain interprets the frequency of a sound. It depends on the frequency of sound. The faster is the vibrations of a source, the higher is its frequency and thus higher is its pitch. Thus, a high pitch sound corresponds to more compressions and rarefactions passing through a fixed point per unit time.
15. Loudness – The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented as ‘A’. For sound its SI unit is the SI unit for density or pressure.
    The loudness of sound is determined by its amplitude which further depends on the force with which an object is made to vibrate. Larger is the amplitude louder is the sound produced. A sound wave spreads out from its source in all directions. As it moves away from the source its amplitude as well as the loudness decreases because the particles lose the energy associated with them.
16. Quality or timbre of sound – The quality or timbre of sound is that characteristic which enables us to distinguish one sound from the other even if they have same pitch and loudness. A sound that is more pleasant is said to be of rich quality.
17. Tone – Sound of a single frequency is called a tone. The sound which is produced due a mixture of several frequencies are called a note. And is pleasant to our ear. Unpleasant sound is called noise.
18. Speed of sound – The distance which a point on a wave such as compression or rarefaction, travels per unit time.
    Speed, v = distance / time
    Speed = wavelength x frequency
    Speed of sound remains almost same for all frequencies in a given medium under the same physical conditions.

Ex 12.1) A sound wave has a frequency of 2 kHz and wave length 35 cm.

How long will it take to travel 1.5 km?

Ans: Frequency = 2 kHz = 2000 Hz

Wavelength = 35 cm = 35/100 m = 0.35 m

Speed  = Frequency x Wavelength
= 2000 x 0.35
= 700 m/s

Distance = 1.5 km = 1.5×1000 m = 1500 m

V = d/t

t = d/v = 1500/700 = 2.1 s

 

In text questions (page number 166)

3. wavelength = ?

Frequency = 220 HZ

Speed = 440 m/s

Speed = frequency x wavelength

Wavelength = speed / frequency

= 440 / 220 = 2 m

 

4. 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 from the source?

Ans:  Frequency = 500 Hz

Time period = 1 / frequency

= 1/500 s = 0.002 s

17. Intensity of sound – The amount of sound energy passing each second through a unit area is called the intensity of sound. Loudness and intensity are not same but related. Loudness refers to the measure of the response of our ear to the sound. Even though two sounds are of equal intensity but one may appear louder than the other because our ear detect it better.
18. Speed of sound in different media – Speed of sound varies in different media and also depends on the temperature of the media. Speed of sound decreases as we go from solid to liquid to gaseous state. It increases with increase in temperature. For example, the speed of sound in air is 331 m s^–1 at 0 ^°C and 344 m s^–1 at 22 ^°C .

19. Reflection of Sound – Sound bounces off after striking a surface (hard). Like light sound also follow the same laws of reflection.
20. The angle of incidence of sound wave is same as the angle of reflection.
21. The incident wave, the reflected wave and the normal to the point of incidence all lie in the same plane.

20. Echo – Reflection of sound after striking a hard surface which can be heard is called echo. The sensation of sound persists in our brain for about 0.1 s. to hear a distinct echo the time interval between the original sound and its reflection must be at least 0.1 second. If the speed of sound is taken to be 344 m/s at any given temperature e.g., 22^o C in air, the sound must go and reach back after reflection after 0.1 s
    Speed    = distance / time
    Distance = speed × time

=    344 × 0.1 = 34.4 m
So, for hearing an echo. The surface should be at least 17.2 m away from the source of sound.

Echoes may be heard more than once. This happens due to multiple reflections.  

21. Reverberation – A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no more audible. The repeated reflection that results in the persistence of sound is called reverberation. In an auditorium or a big hall, excessive reverberation is undesirable. To reduce reverberation, the roof and walls of the auditoriums are generally covered with sound – absorbent materials like compressed fibre board, rough plaster or draperies. The seat material is also chosen on the basis of their sound absorbent properties.

Ex 12.2) A person clapped his hands near a cliff and heard the echo after 5 s. What is the distance of the cliff from the person if the                 speed of the sound, v is taken as 346 m s^–1? 

Sol: Given, Speed of sound, v = 346 m s^–1

Time taken for hearing the echo, t = 5 s

Distance travelled by the sound = v × t = 346 m s^–1 × 5 s = 1730 m

In 5 s sound has to travel twice the distance between the cliff and the person.
Hence, the distance between the cliff and the person = 1730 m÷2 = 865 m.

22. Uses of Multiple Reflection of sound–

• Megaphones or horns, trumpet etc. are designed to send sound in a particular direction without spreading it in all direction. Multiple reflections take place in a tube followed by a conical opening to guide most of the sound waves in forward direction towards the audience.
• Stethoscope is a medical instrument used for listening to sounds produced with in the body mainly heart and lungs.
• The ceilings of concert halls, cinema halls etc. are curved so that sound after reflection reaches all corners of the hall.

23. Range of hearing – The audible range of sound for human beings is from 20 Hz to 20,000 Hz. Children under the age of 5 and some animals like dogs can hear upto 25 kHz. As people grow older their ears become less sensitive to higher frequencies.

26. Infrasonic – Sounds below the frequency of 20 Hz is called infrasonic. Rhinoceros communicate with each other at frequencies as low as 5 Hz. Whales and elephants also produce sounds in infrasonic range.
    It is observed that some animals become disturb before an earthquake. The earthquakes produce low frequency sound before the main shock waves begin to alert the animals.
27. Ultrasonic – Frequencies higher than 20 kHz are called ultrasonic waves. Dolphins, bats and porpoises produce ultrasounds. Moths of certain families have very sensitive hearing equipment. These moths can hear the ultrasounds produced by bats to avoid their capture by the bats. Rats also play games by producing ultrasonic waves.
28. Applications of Ultrasound –

• Ultrasounds can travel along the well-developed paths even in the presence of obstacles. It is therefore used to clean parts located in hard to reach places. For example, spiral tube, odd shaped parts etc. Objects to be cleaned are placed in a cleaning solution and ultrasonic waves are sent into the solution. Due to high frequency, the particles dirt, grease etc get detached and drop out.
• Ultrasound can also be used to detect cracks and flaws in the metal blocks. Huge metal blocks that are used in construction industry, if cracked can reduce the strength of the structure. Ultrasonic waves are allowed to pass through the metal blocks and detectors are used to detect the transmitted waves if there is a crack in the metal block, the ultrasound will get reflected back indicating the defect in the block.
• Ultrasounds are also used in medical diagnosis. It is used take images of various parts of heart. This technique is called echocardiography.
• Ultrasound scanner is an instrument used to get the images of internal organs such as liver, kidney etc. In this technique ultrasound travels through various organs and is reflected from a region where there is a change in the tissue density. These waves are then used to generate images by converting into electrical signals. These images then can be printed on a film. This is called ultrasonography.
• Ultrasonography is also used during pregnancy to detect any congenial defects and growth abnormalities in the foetus.
• Ultrasound may be employed to break small stones formed in the kidneys into fine grains which later get flushed out with urine.

 

27. SONAR – The acronym SONAR stands for SOund  NAvigation and Ranging. SONAR is a device that uses the ultrasonic waves to measure the distance, direction and speed of underwater objects.

• SONAR consists of a transmitter and a detector installed in a boat or ship. The transmitter produces and transmits ultrasonic waves which travel through water and get reflected after striking the object and is sensed by the detector. The detector converts the ultrasonic waves to electrical signals. Which are then appropriately interpreted.
• The distance between the object and the ship is calculated by using the formula               2d = v × t
• Where d is the distance between the object and the ship, v is the velocity of sound through sea water and t is the time interval between the transmission and the reception of the ultrasonic sound
• The above method is called echo ranging.

28. Uses of echo ranging / SONAR–

• It is used to determine the depth of the sea
• It is also used to locate underwater hills, valleys, submarines, ice bergs, sunken ships etc.

29. How do bats catch their prey?

    Bats search out their prey and fly dark night by emitting and detecting high pitched ultrasonic squeaks, which are reflected from the obstacles or prey and returns to the bat’s ear. The nature of reflection tells the bat where the obstacle or prey is and what it is like.

• Porpoises also use ultrasounds for navigation and location of food in the dark.