Sound is one of the most important chapters of Physics that needs a proper understanding of the concepts to excel. CBSE Class 9 Physics Chapter 11 Sound notes includes the propagation, production of sound, speed of sound. This note focuses on the application of ultrasound, sound reflection while discussing the structure and working of human ears.
Definition of Sound
Sound is a type of energy that enables us to hear.
Sound travels in the form of a wave
How is a Sound Produced?
When an object vibrates, it produces sound.
Several outside sources, such as wind, make the vibration in an object
Propagation of Sound
- Sound travels through a substance called a medium.
- The medium may be liquid, solid, or gas.
- When an object vibrates and moves forward, it compresses the surrounding air and creates a high pressure known as compression(C).
- When an object vibrates and moves backward, it forms a low-pressure region called Rarefaction (R).
Characteristics of Soundwave
- Wavelength()- Wavelength is the distance between the centers of two consecutive rarefactions or compressions.
The SI unit of wavelength is the metre.
- Frequency ()- Frequency refers to the number of oscillations passed in one second is called frequency.
The S.I. unit of Frequency is Hertz(Hz)
- Time period – The required time to pass through a point by compression or rarefaction is referred to as the time period.
T stands for time period
- Amplitude – The magnitude of the displacement of a particle in a medium from its original position is known as the amplitude of a wave.
Amplitude is denoted by A, and the SI unit of Amplitude is metres (m).
- Velocity – A wave that covers a distance in one second is called its velocity.
Velocity = wavelength x frequency
The SI unit is meter per second (ms-1).
Sound has characteristics like Pitch and Loudness.
1. Pitch- Pitch of sound depends on factors such as the frequency of sound, the size, as well as the type of object that produces sound.
Pitch is proportional to the frequency of sound.
2. Loudness – Loudness denotes the measurement of the sound that reaches our ears per second. It depends on the amplitude of sound.
Loudness can be measured in decibels (dB).
Read more: CBSE Physics Chapter 9 Force and Laws of Motion Class 9 Notes
Speed of Sound in Different Media
The speed of sound is different in various media. The properties of a medium, such as density, pressure, and temperature, impact the speed of sound.
- An increase in temperature enhances the speed of sound.
- The speed of sound increases with the increasing humidity in the air.
- The speed of sound in the air is 344 m/s at 22° C and 331m/s at 0°C
Sonic boom
When an object passes through the air at a high speed, which is greater than the speed of sound, it produces a loud sound like an explosion. It produces a shock wave in the air by exerting high pressure on the air. The loud sound produced by the shock wave is called a sonic boom.
Aircraft, rockets travel at supersonic speeds, and this sound energy can damage buildings.
Law of the Reflection of Sound
When sound falls on a plane surface, it bounce back is known as the reflection of sound.
- The incident sound wave, reflected sound wave, as well as the normal at the point of incidence fall in the same plane.
- The angle of incidence sound wave and the reflection of the sound are equal.
Application of the Reflection of Sound
A stethoscope works by the multiple reflections of a sound wave.
Loudspeaker, bulb bullhorn also works because of the reflection of sound.
Echo
The reflection of sound creates a repetition of sound called an echo.
The distance between the obstruction and the reflected surface must be at least 17.2 m to create an echo.
Reverberation
The persistence of sound is created by the repeated sound reflection in a big hall from the ceiling, floor in the hall is called reverberation.
SONAR
It is a device that helps to identify the distance, speed, and direction of an underwater object.
SONAR is installed at the bottom of a ship.
It includes a transmitter that produces as well as transmits ultrasonic waves and a receptor that is used to detect the depth of the sea and the location of underwater hills.
Read more: CBSE class 9 Physics Chapter 10 Gravitation Notes
Range of hearing
- The range of hearing of a human is 20 Hz to 20000 Hz.
- Infrasonic sound refers to the sound of frequency lower than 20 Hz.
- Ultrasonic sound waves are the sound of frequencies higher than 20 kHz.
Hearing Aid
A hearing aid is an electronic device that helps to hear sound to a person with a hearing disability. It involves a microphone that converts sound into electrical signals, and an amplifier is used to amplify the signal. Then the signal is sent to the speaker that converts the signal to sound and sends it to the ear of the person.
Application of Ultrasound
- It is used in industries to detect damage and cracks in metal blocks.
- The images of the internal body parts can be taken through ultrasonography.
- The image and function of the heart can be checked by Echocardiography.
The working of Human ear
- The human ear consists of the outer ear or Pinna, the auditory canal, the eardrum, the middle ear, and the cochlea.
- When a sound wave strikes the eardrum, the pressure outside the ear increases and it pushes the eardrum.
- The eardrum goes outwards and starts to vibrate.
- The middle ear transmits the pressure to the inner ear.
- The cochlear nerve turned the pressure into electrical signals and sent them to the brain through the auditory nerve.
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Conclusion
This CBSE Class 9 Physics Chapter 11 Sound notes serves as the best resource to get a clear concept on Sound. Characteristics of soundwave, sound reflection and its practical uses are illustrated through examples for the ease of the students. All the important definitions and laws are described briefly here.
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