Standard 9

Chapter 15

The Music of Sound

GURUDATTA WAGH

Std 9 Chapter 15 The Music of Sound

15.1 Nature of soundSound is a form of energy which produces sensation of hearing in our ears

15.2 Production of soundSound is produced by striking, plucking, blowing, scratching, rubbing, flapping, shakingVibrations in the object are responsible to produce sound

15.3 Propagation of soundWave It is produced at the point of disturbance. Particles of the medium do not change their positions. They vibrate about the mean position when the disturbance reaches that point. Wave is a disturbance that moves through a medium when particles of medium set neighbouring particles into motion, i.e. the wave motion is a mode of transmission of energy through a medium.

Vibration To and fro motion of an object

Propagation Sound also gets propagated in the form of waves. Sound is produced when an object is disturbed and starts vibrating.

Sound gets transmitted through matter or substance called medium, e.g. solid, liquid, gas.

Sound cannot travel through vacuum

Particles do not travel from the vibrating object to the listener. A particle of medium in contact with a vibrating object is first displaced from its equilibrium position. Then it exerts a force on the adjacent particle.

The adjacent particle gets displaced from its position at rest. The first particle comes back to its original position after displacing the adjacent particle.

The process goes on repeating from source to listener and sound reaches the listener in the form of waves.

15.4 Sound waves are longitudinal waves

Transverse wave Wave produced in a string. Particles oscillate up and down about their mean positions. They do not oscillate along the line of wave propagation but perpendicular to direction of propagation.

Longitudinal waves In a slinky the propagation of disturbance is in the form of longitudinal waves. Individual particles of the medium move in a direction parallel to direction of propagation of disturbance. Particles do not move from one place to other, they just oscillate back and forth about the mean position, e.g. sound waves.

Slinky

15.5 Characteristics of sound wavesWhen sound waves travel through a medium, there is change in density and pressure of the medium.

Compressions Regions where particles are crowded together; density and pressure is high

Rarefactions Regions where particles are spread apart; density and pressure is low

Wavelength Distance between two consecutive compressions or rarefactions. SI unit metre, Greek letter lambda ƛ

Frequency Change of density from maximum to minimum to maximum value, makes one oscillationFrequency of wave Number of oscillations/ unit time, SI unit hertz Hz, Greek letter nu ʋ

Oscillation The term 'vibration' is precisely used to describe mechanical oscillation but used as a synonym of 'oscillation' too, e.g. a swinging pendulum and alternating current.

Heinrich HertzFirst to broadcast and receive radio waves; produced electromagnetic waves, sent them through space and measured their velocity and wavelength

Time period of wave/ Time period of sound wave T Time taken by two consecutive compressions and rarefactions to cross a fixed point, i.e.Time taken for one complete oscillation in the density of mediumSI unit - secondRelation between frequency and period ʋ = 1/T

Pitch is the frequency of vibrations of a sound source. For faster vibrations frequency is higher and hence pitch is higher.High pitch sound corresponds to more number of compressions and rarefactions passing through a fixed point per unit time.

Pitch We can distinguish a male voice from a female voice due to different frequencies produced by them. Pitch high = shrill sound; pitch low = soft sound

Amplitude of the wave (A) Magnitude of maximum displacement of particle in the medium on either side of the mean value‘A’ changes with density or pressure; depends upon the force with which an object is made to vibrateWhen sound waves move away from a source its amplitude and loudness decreases

Amplitude Determines the loudness or softness of sound

Timbre Enables us to distinguish one sound from another having the same pitch and loudness. Sound which is more pleasant is said to be of rich quality.

Tone A sound of single frequency

TONE

Note Sound produced due to a mixture of different frequencies. Pleasant to listen

Intensity Amount of sound energy passing per second through unit area

Intensity

Loudness Measure of response of the ear to sounds

15.6 Speed of soundDistance which a point on a wave, such as compression or rarefaction travels per unit time is the speed of soundSpeed = Distance/ Time

If we consider the distance travelled by the sound wave in one time period, thenSpeed = wavelength/ Time periodV = ƛ/ TV = ʋƛ because 1/ T= ʋSpeed = frequency x wavelength

Under the same physical conditions, for all the frequencies in a given medium, the speed of sound remains almost the same.

The physical conditions of the given medium are related to the state of medium and temperature.The speed of sound decreases from solid to gaseous medium.

If we increase the temperature of any medium, the speed of sound also increases.

15.7 Range of hearing in humans

Ultrasound/ Ultrasonic Bats can produce and hear sounds of frequency higher than 20000Hz which humans cannot hear, e.g. dolphins, rats. Moths of certain families can hear the ultrasound produced by bats and they can protect themselves from capture by bats

Infrasound/ Infrasonic Human ear cannot hear sound of frequency less than 20 Hz, e.g. sound produced before an earthquake by vibrations of the earth surface

Audible range of sound human beings 20–20000 HzChildren < 5 years, dogs can hear up to 25000 Hz

As people grow older, ears become less sensitive to higher frequencies

A decibel dB(A) is a unit of measurement that indicates how loud a sound is. Humans can hear sounds between 0 and 140 decibels. 0 decibel does not mean that there is no sound, merely that we cannot hear it. 0 decibel is the so-called hearing threshold for the human ear.

Decibel levels

Ultrasound Able to travel along well defined paths even in presence of obstacles

Uses Ship to ship communication. Welding plastic surfaces. Kill bacteria in liquids like milk. Echocardiography. Scan and obtain images of internal organs of the human body. Clean parts located in places which are difficult to reach. Detecting cracks and flaws in metal blocks

15.8 Reflection of soundSimilar to light waves, sound waves get reflected at the surface of solids or liquids.

Follow the same laws of reflection.Light waves

Laws of reflection

Only requirement for reflection of sound is an obstacle of large size having a rough or polished surface.

Direction in which sound is incident and reflected makes equal angles with the normal to the reflecting surface at the point of incidence and these are in the same plane.

15.8.1 Echo Repetition of sound due to reflection of original sound from any surface

The minimum distance of the reflecting surface from the source of sound must be 17.2 m for related temperature 22 °C. At 22 °C the speed of sound is 344 m/ s in air.Sensation of sound persists in our brain for 0.1 s.Total distance covered by sound from the point of generation to the reflecting surface and back should be at lease 34.3 m, sinceDistance = velocity x time = 344 x 0.1 = 34.4 m

For hearing different echoes, the minimum distance between the source of sound and obstacle must be half the distance, i.e. 17.2 m.Distances are different for different temperaturesDue to successive or multiple reflection, echoes may be heard a number of times. Gol Gumbaz, Bijapur, Karnataka. Reflection of sound – measuring the depth of oceans.

15.9 Reverberation is the collection of reflected sounds from the surfaces in an enclosure like an auditorium.

Persistence of sound The time interval between the successive arrival of the same sound signal goes on decreasing. The intensity of sound gradually decreases. These reflected sounds interfere and cause a jumbling and prolongation of sound in the room. Due to this pitch of the music will not be heard distinctly.

Some theatres, auditoriums are considered acoustically poor because of reverberation.

Reducing reverberation Roofs and walls of auditoriums are generally covered with sound-absorbent materials like compressed fibre-boards, rough plaster or draperies (curtains). The material for the seat is selected such that it is a good absorber of sound waves.

15.10 SONAR

Sound Navigation And Ranging (Acronym an abbreviation formed from the initial letters of other words and pronounced as a word, e.g. NASA )

Uses To measure the distance, direction and speed of under water objects using ultrasonic waves; determine the depth of the sea; locate underwater hills, valleys, submarines, sunken ships, etc.

SONAR = transmitter + detector•Installed in a ship or a boat•Transmitter produces and transmits ultrasonic waves•Waves travel through water

• Waves get reflected after striking the object on the sea bed

• Reflected waves are sensed by the detector• Detector converts the ultrasonic waves into electrical

signals which are interpreted• Time interval between transmission and reception of

the ultrasound is recorded• By knowing the speed of sound in water and considering

the above time interval, the distance of the object that reflected the sound wave can be calculated

15.11 The human ear

Converts pressure variations with audible frequencies into electrical signals that travel to the brain via the auditory nerve

3 major parts 1) external, 2) middle, 3) inner

External (Pinna) Collects the sound which is carried by auditory canal to the ear drum.The ear drum is a thin membrane which is set into vibrations due to the sound reaching it.When a compression of medium reaches the ear drum, the pressure outside the membrane increases and forces the ear drum inwards.The ear drum moves outwards when a rarefaction reaches it and the ear drum vibrates.The vibrations are amplified and carried to the inner part of the ear.Auditory nerve connects the part of inner ear to the brain.

Inner ear

Spiral chamber like a snail shell called cochlea is present.Cochlea is responsible for recognizing the qualities of sound, i.e. loudness and pitch.Vibration received from the ear drum are converted into nerve impulses or electrical signals, which are sent to the brain through the auditory nerve.The brain interprets them as sound.The amplitude of vibration of membrane gives the brain a measure of loudness of sound being heard.

THANK YOU

SSC Std 10th TextbookCBSE Std 10th Textbook

YouTubeGoogle

Wikipedia

Suggestions and Appreciations welcome

gkwagh@gmail.com