Class 9 CBSE Board Chapter Sound

Sound

PHYSICS

Chapter 12: Sound

Sound

• Sound is a form of mechanical energy which produces the sensation of hearing.
• It is produced due to vibrations of different objects. It travels in the form of waves.
• Sound is a form of energy which produces a sensation of hearing in our ears.

Introduction to waves

A wave is a disturbance in a medium which moves from one point to another and carries energy without a net movement of particles. It may take the form of elastic deformation or a variation of pressure.

E.g: Rubber cork on the water that goes up and down when a rock falls in the water creates a ripple.

Propagation of Sound

• A material medium is necessary for the propagation of sound. It can be solid, liquid or gas.
• The disturbance which moves through a medium when the particles of the medium set the neighbouring particles into motion is known as a wave.
• A sound wave can be considered the propagation of pressure or density variations in the medium, i.e. it propagates in a medium as a series of compressions and rarefactions.
• A region of compressed air (increased density or pressure) is called a compression (C) and that of rarefied air (decreased density or pressure) is called a rarefaction(R).
• A vibrating object produces a series of compressions and rarefactions in the medium.

Example: When the prongs of a tuning fork move forward, compression is formed, and when the prongs move backwards, rarefaction is formed.

• As sound propagates, it is the sound energy which travels in the medium and not the particles of the medium.

Sound waves are longitudinal waves as the particles of the medium through which the wave propagates vibrate in a direction parallel to the direction of propagation ofwaves.

There are many types of waves like mechanical waves, electromagnetic waves, matter waves.

Mechanical Wave: Mechanical wave is periodic disturbances which require material medium like solid, liquid and gas for its propagation.

Some examples of mechanical waves includes

• Sound waves
• Water waves
• Waves produced in stretched string
• Waves produced in slinky or a long string

There are two types of mechanical waves

Transverse wave:In these waves the individual particles of the medium move in a direction perpendicular to the direction of propagation of the disturbance. The particles do not move from one place to another, but they simply oscillate back and forth about their position of rest

Longitudinal wave:In these waves the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another, but they simply oscillate back and forth about their position of rest

Sound waves are longitudinal waves as in sound waves, particles moves in a direction parallel to the direction of propagation of the disturbance.

Introduction to sound waves

Sound needs a medium to propagate. The matter or material through which sound propagates is called a medium. When particles vibrate about their mean positions, it pushes a region of compressed air, creating a region of high pressure, followed by a region of low pressure as the particle retreats to its mean position. The sound wave propagates by compressions and rarefactions of particles in a medium. Sound propagation can be visualised as the propagation of pressure variations in the medium.

Characteristics of Sound Waves

Wavelength

The distance between two successive crests or troughs (or) successive compressions and rarefactions is called as wavelength (λ).  The SI unit of wavelength is metre (m).

Time period

Time taken by two consecutive compressions or rarefactions to cross a fixed point is called a Time period (T). The SI unit of time in seconds (s).

Frequency

The number of compressions or rarefactions per unit time is called frequency (𝛎).

The SI unit of frequency is Hertz. The SI unit is Hertz (s−1)

V=1/T

Speed (v), wavelength (λ) and frequency (𝛎) are related as v=λ𝛎

Amplitude

The magnitude of disturbance in a medium on either side of the mean value is called an amplitude (A).

As shown in the figure below, the unit of amplitude will be the density or pressure. Distance between mean position and crest (maximum displacement).

Pitch

The number of compressions or rarefactions per unit time. Directly proportional to frequency.

Volume

Volume or loudness of a sound depends on the amplitude. The force with which an object is made to vibrate gives the loudness.

Higher force → higher amplitude → louder sound

The amount of sound energy flowing per unit time through a unit area is called the intensity of sound.

Note and Tone

A sound of a single frequency is called a tone. A sound produced with a mixture of several frequencies is called a note.

Quality of sound

The richness or timber of sound is called the quality. Sound with the same pitch and loudness can be distinguished based on the quality. Music is pleasant to the ears while noise is not. But they both can have the same loudness and pitch.

Speed of sound

Sound travels through different media with different speeds. Speed of sound depends on the properties of the medium: pressure, density and temperature

Speed of sound: Solids > Liquids > Gases

Speed of sound in air = 331 m/s at 0⁰C and 344 m/s at 22^∘ C

When a source emits sound with a speed greater than the speed of sound in air, it creates a sonic boom which produces shockwaves with lots of energy. They produce a very loud noise which is enough to shatter glass and damage buildings.

Reflection of Sound Waves

Like light, sound also follows laws of reflection, it bounces off the surface of solid and liquid.

Echo

The phenomenon where a sound produced is heard again due to reflection is called an echo.

E.g: Clapping or shouting near a tall building or a mountain.

To hear distinct echo sound, the time interval between original and reflected sound must be at least 0.1s. As sound persists in our brain for about 0.1s. Minimum distance for obstruction or reflective surface to hear an echo should be 17.2 m. Multiple echoes can be heard due to multiple reflections.

Sonar and Radar

SONAR – Sound Navigation And Ranging.

It is a technique that uses sound or ultrasonic waves to measure distance. The human range of hearing is 20Hz- 20kHz.

What are Ultrasonic sounds?

Ultrasonic sounds are high-frequency sound having a frequency greater than 20kHz (inaudible range).

Applications of Ultrasound

(i) Scanning images of human organs

(ii) Detecting cracks in metal blocks

(iii) Cleaning parts that are hard to reach

(iv) Navigating, communicating or detecting objects on or under the surface of the water (SONAR).

Sonar consists of a transmitter and detector mounted on a boat or ship. The transmitter sends ultrasonic sound waves to the seabed which gets reflected back and picked up by the detector. Knowing the speed of sound in water, distance can be measured using:2d=v×t. This method is called echolocation or echo ranging.

Reverberation

Persistence of sound because of multiple reflections is called reverberation. Examples: Auditorium and a big hall.

Excessive reverberation is undesirable and to reduce this, halls and auditoriums have sound-absorbing materials on the walls and roofs.  E.g: Fibreboard and rough plaster.

Doppler’s effect

If either the source of sound or observer is moving, then there will be a change in frequency and wavelength for the observer. The frequency will be higher when the observer moves towards the source and it decreases when the observer moves away from the source.

Example: If one is standing on a street corner and an ambulance approaches with its siren blaring, the sound of the siren steadily gains in pitch as it comes closer and then, as it passes, the pitch suddenly lowers.

Variations in Pressure and Density of a Medium due to Sound Waves

• The variations of pressure and density when a sound wave moves in a medium are as shownbelow:
• The portion of the medium where density (or pressure) has a value larger than its average value is called acrest.
• The portion of the medium where density (or pressure) has a value smaller than its average value is called atrough.
• The magnitude of maximum disturbance in the medium on either side of the mean position is called the amplitude (A).
• When a sound propagates through a medium, the density of the medium oscillates between a maximum value and a minimumvalue.
• The change in density (or pressure) from the maximum value to the minimum value and again to the maximum value is called anoscillation.
• The number of complete oscillations per second is called the frequency ( ν ) of the sound wave. Its unit is hertz(Hz).
• Thetimetakenforonecompleteoscillationinthedensity(orpressure)ofthemediumiscalledthe

time period(T) of the wave.

• Thedistancebetweentwoconsecutivecompressionsortwoconsecutiverarefactionsiscalled

wavelength (λ) of the wave. Its SI unit is metre (m).

Frequency(ν)andtimeperiod(T)arerelatedas

• Speed of sound is the distance travelled by the sound wave per unit time.

• Therelationbetweenthespeedofsoundwave(v),itsfrequency(u)andwavelength(λ)is v = uλ

Sound, travels as a wave

• From the previous section we have already established that sound is produced by vibrating objects. Term vibration refers to the rapid to and fro motion of an object.
• If we throw a piece of stone in a pond of still water then expanding circle of ripples or water waves are formed over the surface of water. These water waves moves on an outward direction on the surface of water.
• This happens because when stone hits water surface it disturbs the particles of water surface. As a result water particles began to vibrate about their means positions.
• These vibrating particles collide with the neighboring particles and make them vibrate.
• This process continues and the disturbance travels through the water.
• The disturbance travel in water due to the repeated periodic motion of the particles of water about their mean positions.

Speed of Sound in Different Media

• Speed of sound is finite and is much less than the speed oflight.
• Speed of sound in solids > speed of sound in liquids > speed of sound ingases
• The speed of sound increases with increase in temperature.

Characteristics of Sound

• Sounds can be distinguished from each other by three characteristics—loudness (intensity), pitch (frequency) and quality(timbre).

The intensity of sound at any point is the amount of sound energy passing per unit time per unit

• area in a direction perpendicular to the area. Its unit is watt/metre²(W/m²).
• The physiological response of the ear to the intensity of sound is called loudness. It is determined by the amplitude of the wave.
• Pitch is the physiological sensation which helps in distinguishing a shrill sound from a flat sound. It is determined by the frequency of thewave.
• Quality (timbre) distinguishes one sound from another sound of the same pitch and loudness. It is determined by the wave form of thesound.
• A sound of single frequency is called atone.
• The sound produced by a mixture of several frequencies is called anote.

Reflection of Sound

• The laws of reflection for sound are the same as those forlight.
• The repetition of sound caused by reflection of sound waves from an obstacle is known as anecho.
• The time interval between the original sound and the reflected one must be at least 0.1 s for an echo to be hearddistinctly.
• Multiple echoes are heard when sound is repeatedly reflected from several obstacles at suitable distances.
• The phenomenon of persistence or prolongation of audible sound after the source has stopped emitting it is calledreverberation.

Uses of Multiple Reflection of Sound

• In megaphones, horns, musical instruments and stethoscopes, the mechanism of multiple reflection of sound isused.

Sound needs a medium to travel

• Sound cannot travel through vacuum.
• This is because when sound travels from one place to another then energy is transferred from one particle to another particle of the medium.
• This means that sound needs a material medium like solid, liquid or gas for its propagation.
• Visit this link for demonstration that shows sound waves cannot travel through vacuum.

Application of Ultra Sound

• Ultrasound finds applications in industry, medical science and communication(SONAR).
• SONAR stands for Sound Navigation AndRanging. It is used to measure the distance, direction and speed of objects under the sea. It is also used in ship-to-shipcommunication.

• The outer ear which collects the soundwaves.
  • The middle ear which amplifies the sound waves about 60times.
  • The inner ear which converts the amplified sound energy into electrical energy and conveys it to the brain as nerve impulses forinterpretation.

Range of Hearing

• Audible sounds are those that can be heard while inaudiblesounds are those that cannot be heard.
• Human can hear sounds with frequency between 20Hz and 20,000Hz.
• Low frequency sounds which cannot be hear arecalled infrasonic.
• Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. Whales and elephants produce sound in the infrasound range. It is observed that some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before the main shock waves begin which possibly alert the animals
• Objects that vibrate at frequencies of above 20,000Hz produce sound which also cannot be heard by us. Such sounds are called.

ultrasonics

Ultrasound is produced by dolphins, bats and porpoises.

Human Ear Parts

The human ear parts are explained below:

External Ear

The external ear is further divided into the following parts:

Auricle (Pinna)

The auricle comprises a thin plate of elastic cartilage covered by a layer of skin. It consists of funnel-like curves that collect sound waves and transmits them to the middle ear. The lobule consists of adipose and fibrous tissues supplied with blood capillaries.

External Auditory Meatus

It is a slightly curved canal supported by bone in its interior part and cartilage in the exterior part. The meatus or the canal is lined with stratified epithelium and wax glands.

Tympanic Membrane

This membrane separates the middle ear and the external ear. This part receives and amplifies the sound waves. Its central part is known as the umbo.

Middle Ear

The middle ear comprises the following parts:

Tympanic Cavity

It is a narrow air-filled cavity separated from the external ear by tympanic membrane and from inner ear by the bony wall. The tympanic cavity has an auditory tube known as the eustachian tube in its anterior wall.

Eustachian Tube

The eustachian tube is a 4cm long tube that equalizes air pressure on either side of the tympanic membrane. It connects the tympanic cavity with the nasopharynx.

Ear Ossicles

These are responsible for transmitting sound waves from the eardrum to the middle ear. There are three ear ossicles in the human ear:

Malleus: A hammer-shaped part that is attached to the tympanic membrane through the handle and incus through the head. It is the largest ear ossicle.

Incus: An anvil-shaped ear ossicle connected with the stapes.

Stapes: It is the smallest ossicle and also the smallest bone in the human body.

Inner Ear

It comprises two parts:

Bony labyrinth

Membranous labyrinth

Bony Labyrinth

The bony labyrinth comprises a vestibule, three semi-circular canals, and spirally coiled cochlea. It is filled with perilymph.

Membranous labyrinth

The bony labyrinth surrounds the membranous labyrinth. It comprises sensory receptors responsible for balance and hearing. The membranous labyrinth is filled with endolymph and comprises three semi-circular ducts, cochlear duct, saccule and utricle. The sensory receptors include cristae, an organ of corti, and ampullaris maculae.

Function of Ear

Following are the important function of the ear:

Hearing

The mechanism of hearing involves the following steps:

• The sound waves pass through the auditory canal and reach the eardrum.
• The vibrations produced pass through the tympanic membrane to the tympanic cavity.
• The ear ossicles in the tympanic cavity receive the vibrations and the stapes pushes the oval window in and out.
• This action is passed on to the organ of corti, the receptor of hearing, that contains tiny hair cells that translate the vibrations into an electrical impulse that are transmitted to the brain by sensory nerves.

Balance

The eustachian tube and the vestibular complex are the important parts of the ear responsible for the balance.

• The eustachian tube equalizes the air pressure in the middle ear and maintains the balance.
• The vestibular complex contains receptors that maintain body balance.

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