Sound (ध्वनी)
➥ Sound is a form of energy which is produced by vibration and makes us hear.
➥ The to and fro or back and forth motion of an object is called vibration.
➥ Sound needs a medium for propagation –
➥ Note- The sound cannot travel through a vacuum.
➥ Speed of sound ➯ Solid > Liquid > gas
Types of Sound:-
- Audible (श्रव्य) – Frequency: 20 Hz to 20,000 Hz
- Ultrasonic (पराश्रव्य) – Frequency: 20 kHz or above (20,000 Hz+)
- Infrasound (अपश्रव्य) – Frequency: Below 20 Hz
Production of Sound
➥Sound is produced when an object vibrates or sound is produced by vibrating objects.
- By vibrating string (e.g., sitar)
- By vibrating air (e.g., flute)
- By vibrating membrane (e.g., tabla, drum)
- By vibrating plates (e.g., bicycle bell)
- By friction in objects
- By scratching or scrubbing the objects
Propagation of Sound
➥ The substance through which sound travels is called a medium. The medium may be solid, liquid, or gas.
- Sound travels through a medium (solid, liquid, or gas).
- When an object vibrates, it makes air particles vibrate in the same way.
- The vibrating air particles push nearby particles, making them vibrate too.
- This process continues until sound reaches our ears.
- Sound travels as waves, carrying energy through the medium.
- These waves are called mechanical waves.
Sound Waves are Longitudinal Waves
- Compression (C) is the part of a wave where particles of the medium are closer together, forming high pressure.
- Rarefaction (R) is the part of a wave where particles are further apart, forming low pressure.
- When a body vibrates, it creates a series of compression and rarefaction, resulting in a sound wave.
Sound Needs Medium for Propagation
➥ Sound waves are mechanical waves and require a material medium for propagation, like air, water, steel, etc. It cannot travel in a vacuum. For example, when an electric bell is placed in an airtight bell jar and the air is pumped out, no sound is heard, demonstrating that medium is necessary for propagation of sound.
Sound Waves as Longitudinal Waves
- In a longitudinal wave, particles move in the same direction as the wave.
- Pushing and pulling a slinky creates compression (close turns) and rarefaction (far turns).
- The particles vibrate in the wave's direction.
Transverse Waves
- Moving a slinky up and down creates a transverse wave.
- The slinky moves horizontally, but particles vibrate up and down at right angles.
- Light waves are transverse and don’t need a medium to travel.
Characteristics of Sound Wave
- The characteristics of sound waves are: wavelength, frequency, amplitude, time period, and velocity.
- When a wave travels in air, the density and pressure of air change from their mean position.
- Compression is shown by the crest, while rarefaction is shown by the trough.
- Compression is the region of maximum density or pressure.
- Rarefaction is the region of minimum density or pressure.
➥ The combined length of a compression and an adjacent rarefaction is called its wavelength.
➥ The distance between the centers of two consecutive compressions or two consecutive rarefactions is also its wavelength.
➥ It is denoted by the Greek letter lambda (λ). Its SI unit is meter (m).
➥ The number of complete waves produced in one second or the number of vibrations per second is called frequency.
➥ The number of compressions or rarefactions passing in one second is also frequency.
➱The frequency of the wave is the same as the frequency of the vibrating body that produces the wave.
➱ The SI unit of frequency is hertz (Hz), and the symbol for frequency is ν (nu).
➱ 1 Hertz (Hz) = 1 vibration per second.
➱ Bigger unit of frequency: Kilohertz (kHz) = 1000 Hz.
➥ The time taken to complete one vibration is called the time period.
➥ The time required to pass two consecutive compressions or rarefactions through a point is the time period.
➮ The SI unit of time period is second (s), and time period is denoted by T.
➮ The frequency of a wave is the reciprocal of the time period: v = 1/T
➥ The maximum displacement of the particle of the medium from their original undisturbed position is called amplitude.
➥ Amplitude is denoted by A, and its SI unit is meter (m).
⇨ Sound have characteristics like pitch and loudness and timbre.
a. Pitch of Sound
➥ The pitch of sound depends on its frequency (vibration).
➥ Higher frequency = higher pitch, lower frequency = lower pitch.
➮ A woman's voice has a high pitch, while a man's voice has a low pitch.
➮ High pitch sound has more compressions and rarefactions passing a fixed point per unit time.
➥ Loudness depends on the amplitude of the sound wave.
➥ Greater amplitude = louder sound, smaller amplitude = softer sound.
➥ Loudness is measured in decibels (dB).
➥ Timbre depends on the shape of the sound wave.
➥ It helps distinguish sounds of the same pitch & loudness.
➥ A single frequency sound is a tone, and a mixture of frequencies is a note.
➥ Noise is unpleasant, music is pleasant and of good quality.
➥ Velocity is the distance travelled by a wave in one second.
➥ Its SI unit is meter per second (m/s).
Speed of Sound in Various Mediums
- Sound travels slowest in gases, faster in liquids, and fastest in solids.
- Increases with temperature and humidity.
- Speed of light is faster than sound.
- In air, sound travels at 344 m/s at 22ºC.
Sonic Boom:- When objects travel at speeds greater than the speed of sound, they create shock waves, producing a loud noise called a sonic boom. eg-Supersonic objects (aircraft, bullets)
Reflection of Sound
- Sound reflects off hard surfaces, obeying the laws of light reflection.
- Incident wave, reflected wave, and normal lie in the same plane.
- Angle of incidence equals angle of reflection.
Echo
➥ The repetition of sound caused by the reflection of sound waves is called an echo.
⇨ We can hear echo when there is a time gap of 0.1 second in original sound and echo (reflected sound).
⇨ Echo is produced when sound reflected from a hard surface (i.e. brick wall, mountain etc.) as soft surface tends to absorb sound.
Reverberation
➥ The persistence of sound in a big hall due to repeated reflection of sound from the walls, ceiling and floor of the hall is called reverberation.
➥ If it is too long, sound becomes blurred, distorted and confusing.
Methods to Reduce Reverberation
- Use panels made of felt or compressed fiber board on walls and ceilings to absorb sound.
- Hang heavy curtains on doors and windows.
- Place carpets on the floor.
- Make seats using materials that absorb sound.
Applications of Reflection of Sound
- Megaphones and Loudspeakers: Funnel-shaped designs reflect sound towards the audience, amplifying it.
- Stethoscope: Reflects sound of heartbeats through a rubber tube to the doctor's ears.
- Sound Board: Used in auditoriums to reflect sound waves towards the audience, improving speech clarity.
- Curved Ceilings in Concert Halls: Reflect sound to ensure it reaches all areas of the hall.
Range of Hearing
- Human Hearing Range: 20 Hz to 20,000 Hz. Children and dogs can hear up to 25 kHz.
- Infrasonic Sounds (below 20 Hz): Produced by vibrating pendulums, rhinoceroses (5 Hz), elephants, whales, and earthquakes.
- Ultrasonic Waves (above 20 kHz): Heard by dogs, dolphins, bats, and rats. Bats and rats also produce ultrasonic sounds.
Hearing Aid:- It is battery operated electronic device used by persons who are hard of hearing. Microphone convert sound into electrical signals, than those are amplified by amplifier. Amplified signals are send to the speaker of hearing aid. The speaker converts the amplified signal to sound and sends to ear for clear hearing.
Applications of Ultrasound
- Used to detect cracks in metal blocks without damage.
- Used to clean hard-to-reach parts of objects in industries.
- Helps in examining internal organs like liver, kidneys, and heart.
- Ecocardiography: Used to create heart images by reflecting sound waves.
- Ultrasonography: A method to get pictures of internal organs using ultrasound echoes.
- Helps break kidney stones into fine particles.
SONAR
- SONAR stands for Sound Navigation And Ranging.
- Used to find distance, direction, and speed of underwater objects.
- Consists of a transmitter and detector installed on a ship.
- Transmitter sends ultrasonic waves, which reflect off objects and return to the detector.
- Detector converts reflected waves into electric signals.
Structure of Human Ear
- The ear consists of three parts: outer ear, middle ear, and inner ear.
- The ears are the sense organs which help us in hearing sound.
- The outer ear is called the pinna. It collects sound from surroundings.
- This sound passes through the auditory canal.
- At the end of the auditory canal, there is a thin elastic membrane called the ear drum or tympanic membrane.
- The middle ear contains three bones: hammer, anvil, and stirrup, linked with one another. The free end of the hammer touches the ear drum, and the stirrup is linked with the membrane of the oval window of the inner ear.
- The lower part of the middle ear has a narrow 'Eustachian tube'.
- The inner ear has a coiled tube called the cochlea, which is connected with the oval window. The cochlea is filled with a liquid containing nerve cells.
- The other side of the cochlea is connected to the auditory nerve, which goes to the brain.
Working of the Human Ear
➯ Working of Human ear:
Pinna → Ear canal → Ear drum → Hammer → Anvil → Stirrup → Oval window → Cochlea → Auditory nerve → Brain
- When compression of sound waves strikes the ear drum, the pressure on the outside of the ear drum increases and pushes the ear drum inwards. During rarefaction, the ear drum moves outwards. Thus, the ear drum starts vibrating back and forth.
- These vibrations are amplified by the three bones in the middle ear, which transmit the amplified pressure variations received from sound waves to the inner ear.
- In the inner ear, the pressure variations are turned into electric signals by the cochlea.
- These electric signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.
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