Sound Class 9 Notes

What is wave?

Sound Class 9 Notes – A wave is a specific kind of energy disturbance that moves through a medium without causing any net particle movement. It can manifest itself in several ways, such as pressure variation or elastic deformation. The water’s ripple caused when a pebble enters it and causes a rubber cork to move up and down is a good illustration of a wave. As the wave moves from one place to another via the medium, it carries the energy of the disturbance.


There are two types of waves –

a. Transverse Waves
b. Longitudinal Waves.

Transverse Waves – Transverse waves are waves in which the particles of the medium move perpendicular to the direction of the wave. Examples of transverse waves include waves on a string, water waves, and electromagnetic waves such as light.

Longitudinal Waves – Longitudinal waves are waves in which the particles of the medium move parallel to the direction of the wave. Sound waves are a common example of longitudinal waves, where the air particles vibrate back and forth along the direction of the wave.

Sound Class 9 Notes

What is Sound?

When matter vibrates, it releases energy in the form of sound. As an object vibrates, the surrounding medium, such air or water, is disturbed. This disturbance propagates as a series of compressions and rarefactions, or sound waves. When these waves pass through the medium and enter our ears, the eardrum vibrates, sending messages to the brain that enable us to hear the sound.

Propagation of Sound

Sound is created by vibrating objects and travels through a medium. It creates regions of high pressure and low pressure as it moves, which form a wave that propagates through the medium. Air is the most common medium for sound, and a series of compressions and rarefactions are created in the air as the object vibrates, forming the sound wave. Sound is a mechanical wave that travels through density or pressure variations in the medium.

Sound Needs a medium to travel

Sound needs a medium to travel because it is a mechanical wave. Sound required physical medium for the propagation of a mechanical wave such as air, water, or solids.

Sound waves are longitudinal waves

One form of wave that moves through a medium is a sound wave, which moves through the medium by forcing the particles to vibrate back and forth in the same direction. As a result, particles move perpendicular to the wave’s path, making sound waves an example of longitudinal waves. When someone makes a sound, the air is vibrated, which causes a series of compressions and rarefactions that together make up a sound wave. Although rarefactions are areas where air particles are split apart and result in low pressure, compressions are areas where air particles are forced closely together and produce high pressure.

Characteristics of sound wave

Some of the characteristics of sound waves include:

  1. Frequency: The frequency of a sound wave refers to the number of vibrations per second and is measured in Hertz (Hz). This characteristic determines the pitch of the sound, with higher frequency waves producing higher-pitched sounds and lower frequency waves producing lower-pitched sounds.
  2. Amplitude: The amplitude of a sound wave refers to the intensity or loudness of the sound and is measured in decibels (dB). This characteristic is determined by the amount of energy in the wave, with higher amplitude waves producing louder sounds.
  3. Wavelength: The wavelength of a sound wave is the distance between two successive compressions or rarefactions in the wave. This characteristic is related to the frequency of the wave, with higher frequency waves having shorter wavelengths and lower frequency waves having longer wavelengths.
  4. Speed: The speed of a sound wave depends on the medium through which it travels, with sound waves traveling faster in denser mediums. In air at room temperature and pressure, sound travels at approximately 343 meters per second (m/s).
  5. Phase: The phase of a sound wave refers to its position relative to a reference point at a given moment in time. In the case of sound waves, this can refer to the position of a particular air molecule in the wave at a specific time.
  6. Timbre: The timbre of a sound refers to the unique quality or tone of the sound, which is determined by the harmonics and overtones that make up the sound wave. This characteristic allows us to distinguish between different sounds, even if they have the same frequency and amplitude.

Softness or Loudness of Sound

The softness or loudness of sound is determined by its amplitude, which is a measure of the amount of energy the sound wave carries. The amplitude of a sound wave is typically measured in decibels (dB), with 0 dB representing the minimum sound level that a human ear can perceive. Sounds with amplitudes above 0 dB are considered louder, while sounds with amplitudes below 0 dB are considered softer.

Speed of Sound in Different Media

The speed of sound varies depending on the medium through which it is traveling. Here are some examples of the speed of sound in different media:

speed of sound
Speed of Sound

The speed of sound formula

speed of sound formula
So, Speed = wavelength x frequency

Reflection of sound

Reflection of sound refers to the phenomenon where sound waves bounce off a surface and return back towards their source. This happens when the sound wave encounters an obstacle that is large compared to the wavelength of the sound.

The reflection of sound plays an important role in how we hear the world around us. For example, in a room with hard walls and floors, sound waves will bounce off these surfaces and create echoes that can interfere with our ability to hear the original sound clearly. On the other hand, in a room with sound-absorbing materials like carpets and curtains, the sound waves will be absorbed and there will be less reflection and fewer echoes.

Reflection of sound is also used in various technologies. For example, sonar and radar systems use sound waves to detect objects by measuring the time it takes for the sound to bounce off an object and return to the source.


If the speed of sound in air is assumed to be 344 m/s and the persistence of sound in our brain is approximately 0.1 seconds, then the minimum distance required to hear an echo from a reflecting wall or surface should be half of the product of these values, which is:

(344 m/s) x (0.1 s) / 2 = 17.2 m

This means that the minimum distance required to hear an echo is 17.2 meters. In other words, if you produce a sound in an open space or a large room and the nearest reflecting surface (such as a wall or a cliff) is at a distance of 17.2 meters or more, you may hear an echo of the sound. If the reflecting surface is closer than 17.2 meters, the time delay between the original sound and the reflected sound will not be large enough to be perceived as an echo.


When sound is produced in a room, it bounces off the walls, ceiling, and floor, creating multiple reflections that blend together and gradually decay over time. The sum of all these reflections is called the reverberant sound field, which can persist for several seconds depending on the size and shape of the room, the surfaces of the walls, and the materials inside the room.

Range of Hearing in humans

  1. Humans hearing sound capacity is 20 Hz to 20,000 Hz (20 kHz).
  2. Sound below 20 Hz are called infrasonic.
  3. Sound above 20000 Hz are called ultrasonic.

Applications of Ultrasound

  • Ultrasound waves are high frequency waves that can travel along well-defined paths even in the presence of obstacles, making them useful in industries and medical fields.
  • Ultrasonic waves can be used for cleaning parts located in hard-to-reach places by sending waves into a cleaning solution, detaching particles of dust, grease, and dirt.
  • Ultrasounds can detect cracks and flaws in metal blocks by passing through them and detecting any reflected waves, which indicates the presence of a defect.
  • Echocardiography uses ultrasonic waves to form an image of the heart by reflecting waves from various parts of it.
  • Ultrasonography is a technique that uses ultrasonic waves to get images of internal organs of the human body, helping doctors detect abnormalities and growth abnormalities, especially in the fetus during pregnancy.
  • Ultrasonic waves can also be used to break small stones in the kidneys into fine grains, which are later flushed out with urine.
Ultrasound is reflected back inside a metal block

SONAR is an acronym for Sound Navigation and Ranging.

  • It is an underwater navigation, communication, and object detection method that makes use of sound waves.
  • A SONAR device emits sound waves that move through water and reflect off nearby objects.
  • The SONAR system then picks up on the returning echoes and uses them to calculate the position, size, and shape of objects in the water.
  • SONAR is utilised for a variety of tasks, such as underwater mapping, underwater hazard detection, navigation, and tracking marine life.
  • Also, it is utilised in military applications like mines and submarine detection.
  • SONAR can be utilised for image and object detection in air and space in addition to underwater applications.
Ultrasound sent by the transmitter and received by the detector

Structure of Human Ear

  • The human ear is involved in detecting, transmitting, and transducing sound, as well as maintaining balance.
  • The outer ear is the visible part of the ear called the pinna, which collects sound from the surroundings.
  • Sound travels through the auditory canal to the eardrum, which vibrates in response to the incident sound waves.
  • Three bones in the middle ear—the hammer, anvil, and stirrup—amplify and further transfer vibrations to the inner ear.
  • The cochlea transforms pressure inputs into electrical signals in the inner ear.
  • Electrical signals are transmitted by the auditory nerve to the brain for interpretation.

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