CBSE Class 9 Science Notes Chapter 11 Sound

Sound

• The energy that produces the sensation of hearing in our ears is called sound. We hear sound from various sources such as humans, birds, bells, machines, vehicles, television, radios, etc.

How is sound produced

• Sound is produced when an object vibrates. Making an object vibrate requires energy, which comes from an external source.
• Example; when you clap, the mechanical energy in your hands is converted into sound energy.
• That is, energy is required to produce sound.

Other ways of producing sound:

• we can produce sound by friction, scraping, rubbing, blowing air, or shaking or striking objects.

Vibration:-

• Vibration means the rapid, repeated movement of an object.

Transmission of sound:-

• The transmission of sound from a source to our ears is called transmission of sound.
• In other words, the process of sound reaching from one place to another is called transmission of sound.
• Note:

A  medium is necessary for the transmission of sound.

Sound does not propagate in vacuum.

Channel:-

• The substance through which sound travels is called the medium. This medium can be solid, liquid, or gas. Sound travels fastest in solids, slower in liquids, and slowest in gases.

How is sound transmitted?

• When an object vibrates, it creates disturbance in the air particles around it. Consequently, these particles begin to vibrate. These particles do not travel from the object to our ear on their own, but instead create vibrations and disturbances in the particles near them.
• First, the particles in the medium in contact with the vibrating object are displaced from their equilibrium state. These exert a force on the particles near them, resulting in the displacement of the nearby particles from their rest positions. After displacing the neighboring particles, the initial particles return to their original positions.

This process continues, and the vibrations reach particles near the listener’s ear. Ultimately, these particles cause vibration in the listener’s drum (diaphragm). Thus, sound is transmitted through the air as longitudinal waves.

Wave:-

• A wave is a type of disturbance that travels through a medium and causes the particles of the medium to move in neighboring particles. The particles of the medium do not move to their own; rather, the disturbance is carried forward by the particles of the medium.

Compression:-

• When an object vibrates forward, it brings air particles closer together, creating an area of high pressure. The is called compression(c ).

Rareness:-

• When the object vibrate backward, the air particles are pushed apart and an area of low pressure is created. this is called rarefaction (R).

Types of waves:-

• Waves can be broadly divided into two types:-

i. Mechanical  waves:-    waves that require a physical medium ( solid, liquid, or gas ) for their propagation are called mechanical waves. For example, waves generated in water, waves generated in air, etc.

ii. Electromagnetic waves:- those waves which do not require any medium for their transmission are called electromagnetic waves, such as radio waves, light waves, etc.

Mechanical waves:-

• Sound waves are characterized by the motion of particles of the medium and are called mechanical waves.
• These are two types-

i. Longitudinal waves

ii. Transverse waves

i. Longitudinal waves:-  waves in which the displacement of the particles of the medium is parallel to the direction of propagation of the disturbance are called  longitudinal waves.

ii. Transverse waves:-  waves in which the particles of the medium move to at their mean position perpendicular  to the direction of propagation of the waves are called transverse waves.

Difference between transverse and longitudinal waves:-

Wavelength:-

The distance between two consecutive compressions (c) or two consecutive rarefactions (R)  is called wavelength.

• sign: wavelength is usually denoted by.λ (Greek latter lambda).
• SI units: its SI units is meter (m).

Wavelength of sound:-

The minimum distance over which the density or pressure of a medium repeats its value in a periodic manner is called the wavelength of sound (λ).

Completion of one oscillation:-

One oscillation is completed when the density change from maximum  to minimum and then again to maximum.

Characteristics of sound waves:-

A sound wave has the following characteristics: wavelength , frequency, amplitude, time period and wave velocity.

Frequency:-

The number  of times an event occurs (repeats) from a fixed point in  a unit time ( 1 second ) is called the frequency  of that event.

• In simple word: the total number of oscillations in unit time is called the frequency of the sound wave.
• Example: if you beat a drum 5 times in a second, then the frequency of the drum= 5 hertz ( Hz).
• sign: it is usually denoted by v (Greek latter,nu)
• SI units: its SI unit is hertz ( symbol Hz ).

Periodicity:-

The time taken for two consecutive compressions or two consecutive rarefaction to pass a given point is called the time period of the wave.

• In simple words: the time taken for one complete oscillation of density in the medium is called the time period of the sound wave.
• sign: it is denoted by the letter T.
• SI units: its SI unit it second (s).

Pitch:-

Pitch is the sound  characteristic based on which we hear a sound as sharp or soft.

• In simple words: how the brain perceives the frequency of an emitted sound is called pitch.
• Example; in an orchestra, a violin and a flute may be played at the same time and the sound may travel at the same speed, but because their frequencies are different, their pitches are perceived differently.
  • Important rules:

Dimensions:-

The maximum disturbance in a medium on either side of the original position is called the amplitude of the wave.

• Sign: it is usually denoted by the letter A.
• SI units: for sound, its unit will be the units of pressure or density.

Loudness of sound: 

The loudness or softness of sound depends mainly on the amplitude of the wave.

• Example: If we hit a table gently, we hear a soft sound because we produce a low-energy sound wave. If we hit the table hard, w
• When gently struck on the table low amplitude soft sound
• Hitting the table hard more amplitude louder sound

Tone, pitch and noise:

• Tone: Sound of a single frequency is called a tone.
• Voice: The sound produced by the mixture of many frequencies is called tone.
• Noise: Sound of irregular frequencies is called noise.
• Tone and pitch produce pleasant sounds in musicianship, while noise is unpleasant.

Wave velocity: –

The distance covered by a point on the wave, such as a compression or a rarefaction, in unit time is called the velocity of the wave.

• Formula:
• Speed=distance/(time )

If a wave travels a distance equal to its wavelength (λ ) in one time period T), then-

• Velocity (v) =l/T
• since 1/T = in(frequency),
• Therefore, v = λν
• That is: Wave velocity = wavelength frequency

conclusion: In the same medium and under the same conditions, the velocity of sound remains approximately the same for all frequencies.

Intensity of sound:-

The sound energy passing through a unit area in one second is called the intensity of sound.

• Note: Although we sometimes use the words ‘loudness’ and ‘intensity’ interchangeably, they do not mean the same thing.

Speed of sound in different mediums:-

Sound travels at a certain speed, which depends on the properties of the medium.

• The speed of sound is much less than the speed of light.

Example: The flash of a firecracker or the thunder of lightning is seen first, but the thunder is heard later.

• Speed of sound depending on the properties of the medium:
• In solids → the speed of sound is highest.
• In liquids → the speed of sound is medium.
• In gas → the speed of sound is the lowest.
• Reason: In a solid, the particles are very close together, so vibrations are transmitted quickly.

Effect of temperature on the speed of sound:-

In any medium, the speed of sound increases with increasing temperature, and decreases with decreasing temperature. This is why we hear louder and clearer in summer than in winter.

For example: The velocity of sound in air at 0°C is 331 m s’ and at 22°C it is 344 m s’.

Reflection of sound: –

The return (reflection) of sound after striking a solid or liquid is called reflection of sound.

• In simple words: Sound bounces back after hitting a solid or liquid in the same way as a rubber ball bounces back after hitting a wall.
• Laws of reflection of sound: –
• Angle of incidence = angle of reflection.
• The incident ray, the reflected ray and the normal – all three lie in the same plane.
• Conditions of reflection:
• A large object is required to reflect sound waves.
• Sound can reflect off both shiny and rough surfaces.
• Small objects or thin surfaces do not reflect sound sufficiently.

Echo: –

• When a sound is reflected from a reflecting surface (like a wall, mountain, building etc.) and is heard back after some time, then that reflected sound is called an echo.

Conditions for hearing a clear echo:

There should be a gap of at least 0.1 second between the original sound and the reflected sound, because the sensation of sound in our brain lasts for about 0.1 second.

If we assume the speed of sound to be 344 m/s at a given temperature, such as 22°C, then the sound must travel 0.1 s between reaching the obstacle and reflecting back to the listener. Therefore, the total distance traveled by the sound from the listener to the reflecting surface and back must be at least (344m / s) * 0.1s = 34.4m

Therefore, to hear a clear echo, the minimum distance of the obstacle from the source of sound must be half the total distance covered by the sound i.e. 17.2 m.

• Note:
• As the temperature increases, the speed of sound increases, so the required distance also changes slightly.
• Repeated reflection of sound can cause us to hear multiple echoes. Thunder is an example of this, as the sound bounces back repeatedly from multiple reflecting surfaces (clouds, ground, etc.).

Anuranan :-

When the sound produced in a large hall or assembly hall gets reflected repeatedly from the walls, ceilings etc. and persists for some time, then this phenomenon is called reverberation.

• Reverberation problem: Excessive reverberation in an auditorium or large hall reduces the clarity of sound. This is highly undesirable because the speaker’s voice becomes unclear.
• Ways to reduce reverberation: Installing sound absorbing materials on the ceiling and walls of the hall, such as compressed fiber board, rough plaster, heavy curtains and choosing sound absorbing materials for the seats.

Range of Hearing:-

• Human hearing: Humans can hear sounds from about 20 Hz to 20,000 Hz (20 kHz). Children under 5 years of age and some animals, such as dogs, can hear up to 25 kHz. The ability to hear higher frequency sounds decreases with age.

Infrasonic sound: –

Sounds with frequency less than 20 H2 are called infrasonic sounds.

If we could hear infrasonic sound, we would be able to hear the vibrations of a pendulum just as we can hear the vibrations of a fly’s wings.

• Example:

o Rhinoceros: Rhinoceros communicates using infrasonic sounds of frequency up to 5 Hz.

o Elephant and Heel: Heel and Elephant produce sounds in the infrasonic range.

Audible sound or Para sound:-

Sounds with frequencies above 20 kHz are called ultrasonic or ultrasound. Ultrasounds are high-frequency waves. Ultrasounds can travel along a specific path even in the presence of obstacles.

• Example: Animals like dolphins, bats and porpoises produce ultrasound.

Hearing Aid Device (Hearing Aid) :-

• Objective: People with hearing loss need this device. It is a battery-operated electronic device.
• Structure: It consists of a small microphone, an amplifier and a speaker.
• Functions: When sound strikes a microphone, it converts the sound waves into electrical signals. An amplifier amplifies these electrical signals. These signals are then converted back into sound waves by a speaker. These sound waves are incident on the ear’s diaphragm, allowing the person to hear the sound clearly.

Applications of ultrasound: –

Ultrasound is widely used in industries and medicine.

• 1. Use in industries
• Cleaning: Used to clean difficult objects such as spiral tubes, odd-shaped parts, electronic components.
• Process: The object is immersed in a cleaning solution and ultrasonic waves are sent through the solution. The ultrasonic waves break up dust and dirt.
• In detecting defects in metal: Ultrasound can be used to detect cracks and other defects in metal blocks (ingots).
• Process: Ultrasonic waves pass through a metal block. If there is even a slight defect, the ultrasound waves are reflected, indicating the presence of the defect.
• • Note: Ordinary sound is not useful for this purpose because its wavelength is longer.
  • 2. Uses in Medicine

  Echocardiography: Ultrasonic waves are reflected off

  • different parts of the heart to create an image of the heart. This technique is called echocardiography (ECG).
  • Ultrasonic detector: This detector can capture images of a
  • patient’s organs, such as the liver, gallbladder, uterus, kidneys, etc. This detector helps detect abnormalities in the body, such as gallstones and kidney stones, and tumors in various organs.
  • Ultrasonography: In this technique, ultrasonic waves travel through body tissues and are reflected from areas where there is a change in tissue density. These waves are then converted into electrical signals to create an image of the organ. These images are displayed on a monitor or printed on film. This technique is called ultrasonography.
  • Uses: Ultrasonography is used to examine the fetus during pregnancy and detect congenital defects and growth irregularities.
  • Breaking up kidney stones: Ultrasound can also be used to
  • break up small kidney stones into fine particles. These particles are then passed out in the urine.