CBSE Physics Notes of Sound Class 9th

Sound:Definition:

Sound is a form of energy which makes us hear.

Sound travels in the form of waves:

A wave is a vibratory disturbance in a medium which carries energy from one point to another point without there being a direct contact between the two points. There are two types of waves.

1. Longitudinal waves: A wave in which the particles of the medium vibrate back and forth in the same direction in which the wave is moving is called a longitudinal wave. Longitudinal waves can be produced in solids, liquid and gases. The longitudinal waves propagates in the form of Compressions and Rarefactions.

Compressions: Compression is that part of a longitudinal wave in which the particles of the medium are closer to other particles.

Rarefactions:A rarefaction is that part of a longitudinal wave in which the particles of the medium are further apart.

Examples:

 v The waves which travel along a spring when it is used or cold at one end or longitudinal waves.

 v  The sound waves in air are longitudinal waves.

2. Transverse waves: A wave in which the particles of the medium vibrate up and down at right angles to the direction in which the wave is moving, is called transverse wave. Transverse waves can be produced only in solids and liquids. The transverse waves propagates in the forms of Crests and Troughs.

Crest: The elevation or hump in a transverse wave is called crest. A crest is that part of the transverse wave which is above the line of zero disturbance of the medium.

Trough: The depression or Hollow in a transverse wave is called trough. A trough is that part of the transverse wave below the line of zero disturbance.

Graphic representation of sound waves:

1. Longitudinal waves: Longitudinal waves are represented by density-time graph

2. Transversal Waves: Transversal waves are represented by displacement-distance graph

Characteristics of sound wave

A sound wave can be described completely by five characteristics.

1.   Wavelength

2.   Amplitude

3.    Time period

4.    Frequency

5.  Velocity or speed  

1. Wavelength: The minimum distance in which a sound wave repeats itself is called its wavelength. The wavelength is denoted by  ‘𝝺’. The distance between centres of a compression and an adjacent refraction is equal to the half of wavelength. The SI unit of wavelength is metre(m).

2. Amplitude: The maximum displacement of the particles of the medium from their original position is called amplitude. The SI unit of amplitude is metre(m).

3. Time period: The time required to produce one complete cycle is called time period of the wave. The time period of a wave is denoted by capital T. The SI unit of time period is second(s).

4. Frequency: The number of complete cycles produced in one second is called frequency of the wave. The frequency of the wave is denoted by ‘f’. The SI unit of frequency is “per second” or ‘hertz (Hz)”.

5. Velocity or speed: The distance traveled by a wave in one second is called velocity of the wave. Velocity is denoted by ‘v’. the SI unit of the velocity is metre per second or ms^-1_.

Relationship between frequency (f) and time period (T)

 

Relationship between wavelength, frequency and speed of the sound wave:

Production of Sound

The sound is produced when an object vibrates. The energy required to make an object vibrate and produce sound is provided by some outside source like our hands, wind etc. The sounds of human beings is produced by the vibrations of two vocal cords in our through which works by the air coming from the lungs. 

Sound needs a material medium to travel

The substance through which sound travels is called a medium. A medium can be solid, liquid or gas substance. “Since the sound waves need a medium to travel, therefore sound waves are called mechanical waves also.” Sound cannot travel in vacuum.

Reflection of sound

Definition: The bouncing back of sound when it strikes a hard surface is called reflection of sound. The sound is reflected well from hard surfaces like a wall, a metal sheet, hardwood and a cliff.

Laws of reflection of sound:

There are two laws of reflection of Sound:

1.    The incident sound wave, the reflected sound wave and the normal at the point of incidence, all lie in the same plane.

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

Applications of reflection of sound:

1.    Megaphone and bulb horn

A megaphone is a large cone-shaped device for amplifying and directing the voice of a person who speaks into it. The megaphone is also known as loud hailer or speaking tube. A megaphone works on the multiple reflection of sound. A bulb horn is a cone-shaped wind instrument which is used for signalling in bicycles, cars, buses, trucks and boats. A bulb horn works on the multiple reflection of sound.

Megaphone

A bulb horn is a cone-shaped wind instrument which is used for signalling in bicycles, cars, buses, trucks and boats. A bulb horn works on the multiple reflection of sound.

Bulb Horn

2.    Stethoscope:

Stethoscope is a medical instrument used by the doctors for listening the sound produced within the human body, mainly in the heart and the lungs. The Stethoscope has two main parts.

      i.   Two ear pieces.  These are put by the doctors into his ears.

ii.   The rubber tube which joins the chest piece of the ear pieces.

Stethoscope works on the principle of multiple reflection of sound

Stethoscope

3.    Soundboard:

The sound board is a concave board which is placed behind the speaker in large halls or auditoriums so that its speech can be heard easily even by the person sitting at a considerable distance. The soundboard works on the multiple reflection of sound.

Sound board

Echo:

The repetition of sound caused by the reflection of sound waves is called an echo. An echo is simply a reflected sound. The minimum distance from a sound reflecting 

surface to hear an echo is 17.2 metres.

Reverberation:

The persistence of sound in a big hall due to repeated reflections from the walls, ceiling and floor of the hall, is called reverberation.

The range of hearing in human:

The range of frequency from 20 Hz to 20,000 Hz is known as the frequency range of hearing in humans.

Infrasonic sound: The sounds of frequencies lower than 20 Hz are known as infrasonic sounds. Infrasonic sounds cannot be heard by human beings.

Earthquakes and some animals like Whales, elephants and rhinoceroses produce infrasonic sounds.

Ultrasonic sound or ultrasound: The sounds of frequencies higher than 20,000 Hz are known as ultrasonic sounds or ultrasound. The ultrasonic sounds cannot be heard by human beings.

Applications of ultrasound:

1.   Ultrasound is used in industry for detecting floors in metal blocks: The flows like internal cracks in the metal blocks are detected by using ultrasound.

      

         2.  Ultrasound is used in industry to clean ‘hard to reach’ parts of objects such as as spiral tubes, odd shaped machines and electronic components.

         3.  Ultrasound is used to investigate the internal organs of the human body such as Liver, Gallbladder, Pancreas Kidneys, Uterus and Heart: The technique of obtaining pictures of internal organs of the body by using Echoes of ultrasound pulses is called Ultrasonography. The use of ultrasound waves to investigate the action of the horse is called Echocardiography.

    

      4. Ultrasound scans are used to monitor the development of foetus inside the Mother's uterus.

        5.  Ultrasound is used to break kidney stones into fine grains.

        6.  Ultrasound is used in SONAR apparatus to measure the depth of sea and to locate under sea objects like Shoal of fish, shipwrecks, submarines sea-rocks and hidden Icebergs in the sea.
    
     SONAR

     The full form of SONAR is SOund Navigation And Ranging. SONAR is an Apparatus which is used to find the depth of a sea or to locate the underwater things like shoal of fish, shipwrecks and enemy submarines. A SONAR apparatus consists of two parts:

     i. Transmitter: The transmitter emits the   Ultrasonic waves to the target.

i   ii. Receiver: The receiver receives the sound waves sent by the transmitter after reflection from the target.

   7.     Bats use ultrasound to fly at night and to search their prey.

   The method used by some animals like bats porpoises and dolphins, to locate the objects by hearing the echoes of the ultrasonic squeaks is called Echolocation.

i.  Birds fly in the darkness of night without colliding with other objects by the method of echolocation.

ii. Bats search their prey like flying insects at night by the method of echolocation.