Sound Class 9th Notes - Free NCERT Class 9 Science Chapter 12 Notes - Download PDF

Have you ever put your ear to the railroad track and heard the distant rolling of an approaching train before you could actually hear its whistle? This straightforward experience demonstrates the travel of sound in varying media- a major concept in Class 9 Science Chapter 11: Sound. This chapter describes the formation of sound waves, their path through a solid, liquid and gas matric, and the properties of frequency, wavelength, amplitude and speed of sound that are responsible in giving us what we hear. These concepts are fundamental to excellent scores in the CBSE and foundation to upper‑level physics and competitive exams such as JEE and NEET where wave properties can be found in more refined ways.

Class 9 Science Notes of Chapter 11 designed by experienced teachers and contain concise summaries of topics like production of sound, longitudinal waves, echo and reverberation, ultrasound, and the functioning of human ear, important formulas (e.g.,
$v=f\lambda$) and relations for quick recap, well explained diagrams depicting waveforms, echoes and multiple reflection of sound and sample problems to improve problem-solving ability. These Class 9 Sound notes can give you a quick brush up before an exam or a more in depth understanding of the concept.

Also, students can refer,

• NCERT notes Class 9 Science
• NCERT Solutions for Class 9 Science Chapter 11 Sound
• NCERT Exemplar Class 9 Science Solutions chapter 11 Sound

NCERT Class 9 Science Chapter 11

• In our daily life , humans, birds, bells, machines etc produce sounds.
• Sound can be defined as a kind of energy that generates a sensation of hearing in our ears. The production of sound is aided by plucking, scratching, rubbing, blowing or shaking any object. Sound is produced by a vibrating body. Human beings produce sound by vibrations in the vocal cords.

Propagation of Sound

• Medium is anything which is capable of transmitting a sound. A medium can be any solid, liquid or gas. Sound travels from it's point of generation to the listener via a medium. When an object vibrates it enables the medium particles to vibrate. They do not move from the vibrating object to the ear, instead, one particle comes in contact with the object and gets displaced from its equilibrium position. A force is exerted on the adjacent particle which results in the displacement of this particle which enables the first particle to come back to its initial position. This process is repeated until the sound reaches our ears.
• A disturbance that moves through any medium when its particles cause the neighbouring particles into motion is called a wave. In turn, they develop similar motions in other particles. The medium particles do not travel forward themselves, instead, the disturbance makes them move forward. This is the mechanism of propagation of sound in a medium, thus the sound can be visualized as a wave. Sound waves are mechanical waves that enable the motion of particles in the medium. The most common medium by which sound travels is the air.
• When a vibrating body moves forward, it creates a region of high pressure called compression (C). The air begins to move away from the vibrating object. When this vibrating object moves back, it creates a region of low pressure called rarefaction(R). Thus, air comprises a series of compressions and rarefactions due to the movement of objects backwards and forward rapidly. Thus, sound waves are propagated through the medium.

Pressure = number of particles in the medium/volume.

More density produces more pressure and vice versa.

So, sound propagation can be seen as the propagation of variations in density or variations in pressure in the medium.

Medium of Sound waves

• Air, water, Steel are the material medium needed by sound to travel and propagate. Sound is unable to travel through a vacuum. Let us understand this through this experiment.
• An electric bell and a glass ( airtight ) were taken. The electric bell was hung inside this airtight bell jar which was further connected to a vacuum pump. When the switch was pressed on the sound of the bell was heard. When the air in the jar started pumping out, the sound became lower even though the same current was passing through the bell. Gradually when little air was left inside the bell jar a very weak sound was heard.

• Sound waves are longitudinal:

Compressions (C) are the regions where the coins become closer while rarefactions(R) are the regions where the coils become further apart.

Sound travels in a series of compressions and rarefactions in a medium. Sound waves are longitudinal waves where the individual particles of the medium travel in a direction parallel to the direction of propagation of the disturbance. These particles are immovable and cannot travel from one place to another. They oscillate backwards and forward about their resting position. This is the property of longitudinal waves and hence sound waves are regarded as longitudinal waves.

Characteristics of a sound wave:

• A sound wave is demonstrated by its frequency, amplitude and speed.

• The graphical representation of a sound wave shows how density and pressure change whenever the sound wave travels through a medium. The density and pressure at a given time changes with distance above and below the average value of density and pressure. These figures show the variations in density and pressure when a sound wave travels in the medium.

• Region of compressions contains the particles present together which is represented by the upper portion of the curve. Peak is known as the region of maximum compression. The density and the pressure is always high in the regions of compressions. The areas of lower pressure where the particles are seen spreading and they are shown by a valley known as rarefactions which is situated in the lower portion of the curve. Crest is the peak of the graph while the valley of the graph is called the trough of a wave.

• Wavelength: It means the distance between two consecutive compressions (C) or two consecutive rarefactions (R). is the symbol for wavelength. The SI unit of wavelength is meter(m).

• Frequency: is the number of times a body vibrates.
• The maximum value of density to its minimum value and then again to its maximum value form one oscillation. The number of oscillations per unit time is the frequency of the sound wave. It is also the number of compressions or rarefactions that cross per unit time. The symbol for frequency is while the SI unit of frequency is hertz(Hz).
• Time period: It is the time occupied by two compressions or rarefactions one after the other, to cross a specific point.

t is the time taken for one complete oscillation in the density of the medium. T is the symbol of the time period and its SI unit is second (s).

• Relation between frequency and time period:

T= 1/f

Terms related to sound waves:

• Pitch: It is the interpretation of the frequency of an emitted sound.

When the vibration of the source is faster, the frequency and therefore the pitch becomes higher. High pitch represents more compressions and rarefactions passing a fixed point per unit time.

• Amplitude( A) : The magnitude of the maximum displacement is called amplitude. The unit of amplitude is the same as the unit of density or pressure. The amplitude determines the loudness or softness of a sound and depends upon the force with which an object is vibrated. When a sound wave moves away from its source, its amplitude and loudness decrease.

• Quality or Timber: It helps us to differentiate one sound from another sound of the same pitch and loudness. Pleasing sound is said to be of rich quality while an unpleasant sound is regarded as noise. Music is soothing to our ears and is of rich quality. A sound of a single frequency is known as a tone while a sound that is developed because of a mixture of many frequencies is called a note.
• Speed (S)= Distance/Time = d/T, SI Unit: meter/second (m/s)
• Wavelength (λ ) of the sound wave is the distance moved by the sound wave in one time period (T) of the wave.

S= λν

speed = wavelength × frequency

• Intensity of sound: It is the amount of sound energy travelling each second through the unit area.
• Loudness: It is defined as a measure of the response of the ear to the sound.

Speed of sound in different media:

• The speed of the sound is dependent on the characteristics of the medium through which it propagates/travels.
• Speed of sound is dependent on the temperature of the medium. For example the speed of sound reduces when an object is changed from solid to gaseous state.
• A table is given which represents the speed of sound in various media.

Reflection of Sound

• The reflection of sound occurs at the surface of a solid or liquid. Sound also follows the laws of reflection.
• Laws of reflection:

1. The incident ray, the reflected ray and the normal lies in the same plane, that is, the reflecting surface

2. The angle of incidence is always equal to the angle of reflection.

Echo:

• When our sound reaches a suitable reflecting object such as a mountain, an echo is heard. It remains for only 0.1 seconds.
• Conditions for hearing echo:

1. The time difference(interval) between the original sound and the reflected sound must be at least 0.1 seconds.

2. The minimum distance of the reflecting surface from the source of sound must be 17.2 m.

• Due to successive or multiple reflections, echoes are heard again and again. Example- rolling of thunder.

Reverberation:

• The sound caused by repeated reflections from a reflecting object is called reverberation.
• The roof and walls of a big hall are generally covered by sound-absorbent materials like compressed fiberboard.
• Uses of multiple reflection of Sound:

1. Musical instruments like megaphone and a horn are made up of a tube whichis followed by a conical opening.

2. It is used in a stethoscope which is used for hearing sounds developed within the body. The sound of the patient’s heartbeat is heard by the doctor’s ears by multiple reflections of sound.

3. Ceilings of auditoriums are made curved aiming that sound after reflection reaches all corners of the hall.

Range of Hearing:

• Audible range of hearing of sound for humans varies from about 20 Hz to 20000 Hz.
• That sounds whose frequencies are below 20 Hz are known as infrasonic sounds.
• The communication of rhinoceros is done at 5 Hz. Whales and elephants also communicate through infrasonic sounds.
• The frequencies which are higher than 20 khz are known as ultrasonic sounds or ultrasounds.
• Dolphins and bags produce ultrasounds.
• Children who are under the age of 5 and some animals like dogs have the capacity to hear up to 25 kHz

Ultrasound

• Ultrasounds have frequencies which are capable of travelling every part even in the presence of obstacles. They are mainly used in industries and in the medical sector.

1. These high frequencies clean spiral tubes, electronic components and several other places which are hard to reach. The objects which are needed to be cleaned are put in a cleaning solution and the ultrasonic waves are passed in the solution. Because of the high frequency of these particles, the dust particles and the dirt get removed and the object becomes clean

2. Ultrasonic vibrations are used to find cracks and flaws in metal blocks. These metal blocks are used for construction of buildings and scientific equipment. The floors or the cracks inside these are invisible from outside. It reduces the strength of the buildings and machines. Ultraviolet waves are passed through these metal blocks to cure this and the detectors function to detect the propagated waves. When any small defect is encountered these ultrasound reflects back which indicates the presence of any crack.

3. Ultrasonic waves are used in the technique of echocardiography. This technique makes the ultrasonic waves reflect from various parts of the heart and develop its image.

4. Ultrasonic waves are also used in ultrasound scanners to produce images of internal organs of the human body. It is used to detect any fault in the liver, gallbladder, uterus, etc. The technique by which the images are displayed on a monitor is called ultrasonography.

5. The ultrasonic waves are also used to break small stones formed in the kidneys or liver which get removed with the urine of the individual.

Sonar

• The expanded form of Sonar is Sound navigation and ranging.
• It is an instrument which uses the ultrasonic waves to measure the distance, direction and speed of objects inside water.
• Working:

Sonar constitutes a transmitter and a detector which are installed in a boat or a ship. Ultrasonic waves are produced and transmitted by the transmitter which travel through water and strike the object on the seabed. The waves are reflected back and are sensed by the detector. The detector converts the waves into electrical signals for interpretation. The object's distance which reflects the sound wave can be measured by determining the speed of sound in water and the time period between transmission and reception of the ultrasound.

If the time interval is t, and the speed of sound is S, the total distance, 2d, travelled by the ultrasound becomes,

2d= S×t

This technique is known as echo ranging.

The Sonar technique is also used in various other determinations to calculate the depths of the sea and to detect the location of underwater hills, submarines, sunken ships, etc.

Sound Previous year Question and Answer

Q1: 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?

Answer:

The time interval between successive compressions from the source is equal to the time period, and time period is the reciprocal of the frequency. Therefore, it can be calculated as follows:

T= 1/F

T= 1/500

T = 0.002 s.

Q2: A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 m s⁻² and speed of sound = 340 ms⁻¹.

Answer:

Height (s) of tower = 500 m

Velocity (v) of sound = 340 m s⁻¹

Acceleration (g) due to gravity = 10 m s⁻¹

Initial velocity (u) of the stone = 0

Time (t₁) taken by the stone to fall to tower base

As per second equation of motion:

s= ut₁ + (½) g (t₁)²

500 = 0 x t₁ + (½) 10 (t₁)²

(t₁)² = 100

t₁ = 10 s

Time (t₂) taken by sound to reach top from tower base = 500/340 = 1.47 s.

t = t₁ + t₂

t = 10 + 1.47

t = 11.47 s.

Q3: A sonar device on a submarine sends out a signal and receives an echo 10 seconds later. then what is the speed of sound in water if the distance of the object from the submarine is 7200 m?

Answer:

Time (t) taken to hear the echo = 10 s

Distance (d) of an object from submarine = 7200 m

Total distance travelled by SONAR during reception and transmission in water = 2d

Velocity (v) of sound in water = 2d/t = (2 × 7200) / 10 = 1440 ms^-1

NCERT solutions of class 9 subject-wise

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• NCERT solutions for class 9 Science

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Class 9 Chapter Wise Notes