1. What is Sound? Sound is a form of energy which produces a sensation of hearing in our ears. Sound is produced by vibrating objects. Examples include vibrating tuning fork, vocal cords in humans, bells, musical instruments, etc. Vibration: Rapid to-and-fro (back and forth) motion of an object. Short Question: Can sound be produced without vibration? Answer: No, sound cannot be produced without vibration. 2. Medium of Sound The substance through which sound travels is called a medium . Sound can travel through solids, liquids, and gases but cannot travel through vacuum . Process of Sound Propagation: When an object vibrates, it makes the air particles around it vibrate. These air particles move back and forth from their original (rest) position. The vibrating air particles push nearby air particles. This causes the nearby particles to also start vibrating. In this way, vibrations pass from one particle to another. This process continues through the medi...
1. What is Sound?
- Sound is a form of energy which produces a sensation of hearing in our ears.
- Sound is produced by vibrating objects. Examples include vibrating tuning fork, vocal cords in humans, bells, musical instruments, etc.
Vibration: Rapid to-and-fro (back and forth) motion of an object.
Short Question: Can sound be produced without vibration?
Answer: No, sound cannot be produced without vibration.
2. Medium of Sound
The substance through which sound travels is called a medium.
Sound can travel through solids, liquids, and gases but cannot travel through vacuum.
Process of Sound Propagation:
- These air particles move back and forth from their original (rest) position.
- This causes the nearby particles to also start vibrating.
- In this way, vibrations pass from one particle to another.
- The disturbance moves forward through air.
- Finally, the disturbance reaches our ears and we hear sound.
- The particles of air do not travel from the source to the ear.
- Only the disturbance (energy of sound) travels through the medium.
Why can’t sound travel in vacuum?
Answer: Because there are no particles in vacuum to transmit sound vibrations.
3. Sound as a Wave
Sound travels in the form of mechanical waves. Mechanical waves require a medium.
A wave is a disturbance that travels through a medium carrying energy without transporting matter.
This mechanism allows sound to move efficiently through solids, liquids, and gases, enabling us to hear vibrations from various sources.
Why are sound waves called mechanical waves?
Answer: Because sound waves require a material medium to propagate.
4. Types of Waves
Mechanical Waves
- Require a medium
- Example: Sound waves
Electromagnetic Waves
- Do not require a medium
- Example: Light waves
5. Transverse and Longitudinal Waves
Transverse Waves
- Particles vibrate (oscillate) perpendicular to wave direction
- Have crests (high points) and troughs (low points).
- Examples: light waves, water waves and wave on a string.
- Can travel without a medium.
Longitudinal Waves
- Particles vibrate (oscillate) parallel to wave direction
- Have compressions (high pressure regions) and rarefactions (low pressure regions)
- Examples: sound waves, seismic waves
- Require medium to travel.
Why is sound a longitudinal wave?
Answer: Because particles of the medium vibrate parallel to the direction of sound propagation.
6. Compression and Rarefaction
Compression: Region of high pressure and high density.
Rarefaction: Region of low pressure and low density.
What happens to particles during compression?
Answer: Particles come closer together.
Experiment: Sound and Air Medium
Setup:
An electric bell is placed inside an airtight glass bell jar. The bell jar is connected to a vacuum pump.
Observation:
- When air is present inside the jar, the bell rings and sound is heard clearly.
- As air is slowly removed using the vacuum pump, the sound becomes fainter.
- When most of the air is removed, only a very weak sound is heard.
- When all the air is removed (complete vacuum), the bell rings but no sound is heard.
Conclusion:
Sound needs a medium like air to travel. In the absence of air (vacuum), sound cannot propagate.
7. Characteristics of a Sound Wave
(a) Frequency (ν)
Number of oscillations per second.
SI unit: Hertz (Hz)
What is pitch?
Answer: Pitch is how the brain interprets frequency of sound.
Higher frequency → Higher pitch.
Example: A violin has a higher pitch due to its higher frequency compared
to a drum.
(b) Time Period (T)
Time taken to complete one oscillation from compression (maximum density) to rarefaction (minimum density) and back to compression.
T = 1 / ν
(c) Amplitude (A)
Maximum displacement of particles from its mean position during the wave oscillation.
Determines loudness of sound.
Larger amplitude → Louder sound
What determines loudness?
Answer: Amplitude of the sound wave.
(d) Wavelength (λ)
Distance between two consecutive compressions (or crests) or rarefactions (or troughs).
SI unit: metre (m)
(e) Speed of Sound (v)
Distance travelled by sound per unit time.
SI unit: m/s
Formula:
Speed (v) = Distance (s) / Time(t) = Wavelength (λ) / Time (t)
v = λ × ν. [ν = 1 / t]
Does speed of sound depend on frequency?
Answer: No, speed depends on medium and temperature.
8. Factors Affecting Speed
- Speed of sound depends on the nature of the medium.
- Speed of sound increases with increase in temperature and density of the medium.
- Fastest in solids (eg: iron)
- Slower in liquids (eg: water)
- Slowest in gases (eg: air)
Note:
Speed of sound is lower than that of light. And thats why the lightning of thunderstorm is seen but sound is heard after sometime.
What is intensity of sound?
The amount of sound energy passing through a unit area per second is called intensity.
9. Reflection of Sound
When sound waves bounce back from a surface, it is called reflection of sound.
Laws of reflection of sound:
- The incident sound wave, the reflected sound wave and normal at the point of incidence lie in the same plane.
- Angle of reflection of sound is always equal to the angle of incidence of sound.
10. Echo
Repetition of sound due to reflection of sound.
- Minimum time interval between original and reflected sound must be atleast 0.1 s. as sound persists in our brain for about 0.1s.
- Minimum distance for echo in air should be 17.2 m. Multiple echoes can be heard due to multiple reflections.
Why echo is not heard in small rooms?
Answer: Because distance is less than 17.2 m.
11. Reverberation
- Reverberation is the persistence of sound due to multiple reflections from walls, ceilings, and other surfaces in an enclosed space.
- Excessive reverberation reduces clarity of sound.
To reduce reverberation:
- In cinema halls, the walls, roofs and seats covered with sound-absorbing materials like compressed fibre board to minimize reflection. These measures help improve sound quality and ensure clear acoustics in large spaces.
12. Uses of Multiple Reflection of Sound
Megaphones, Horns, and Musical Instruments
These devices use repeated reflections of sound inside a tube to send sound in one direction, making it louder and clearer.-
Stethoscope
It is used by doctors. In a stethoscope, sound from the heart and lungs reaches the doctor’s ears through multiple reflections inside the tubes. -
Acoustic Design of Halls
Curved ceilings and soundboards in halls reflect sound many times so that it spreads evenly and reaches the entire audience clearly.
13. Range of Hearing
Audible Range:
Normal range of human hearing covers frequencies from approximately 20 Hz – 20,000 Hz (20 kHz)
Inaudible Range:
Infrasound: Frequencies below 20 Hz are known as infrasound. Humans cannot hear infrasound. These infrasound frequencies is sometimes produced by natural events like earthquakes or by man- made sources.
Ultrasound: Frequencies above 20,000 Hz are known as ultrasound. Humans cannot hear ultrasound, but animals such as bats and certain marine mammals can hear these ultrasonic frequencies for various purposes like navigation and communication.
14. Applications of Ultrasound
- Cleaning electronic components: Ultrasonic waves detach dust, grease, and dirt in a cleaning solution.
- Detecting cracks in metals: Ultrasound reflects back from defects, indicating their presence.
- Ultrasonography:
- Generates images of internal organs (e.g., liver, kidney, gall bladder, uterus).
- Used to detect stones, tumors, and abnormalities in organs.
- Helps in examining fetal development during pregnancy.
5. Longer-wavelength sounds bend around defects and are unsuitable for precise detection.
Ultrasound provides clear, reliable results for industrial and medical uses.
15. SONAR (Sound Navigation And Ranging)
SONAR uses ultrasonic waves to find distance and depth underwater.
How SONAR Works (Components):
- Transmitter: Produces and sends ultrasonic waves.
- Detector: Receives reflected waves and converts them into electrical signals.
- Ultrasonic waves are sent through water.
- These waves hit an object like the seabed or a submarine and reflect back.
- A detector receives the reflected waves.
- By measuring the time taken, the distance of the object is calculated.
- d = Distance or depth of the object
- v = Speed of sound in water
- t = Time taken for the echo to return
- Measuring sea depth (echo-ranging method)
- Finding underwater objects like submarines, hills, valleys, icebergs, and sunken ships
- Bats produce ultrasonic sounds.
- The sound reflects from objects.
- From the echo, bats find prey and avoid obstacles in the dark.
- Porpoises use ultrasound in water.
- It helps them navigate and find food in dark underwater environments.
- Outer Ear
- Middle Ear
- Inner Ear
- Outer Ear (Pinna): Collects sound from surroundings and sends it into the ear canal.
- Auditory Canal: A tube that carries sound waves to the eardrum.
- Eardrum (Tympanic Membrane): A thin membrane that vibrates when sound waves strike it.
- Middle Ear (Three Bones): Contains hammer, anvil, and stirrup. These bones amplify the vibrations.
- Inner Ear (Cochlea): Converts vibrations into electrical signals.
- Auditory Nerve: Carries electrical signals to the brain.
- Sound waves enter the ear and strike the eardrum.
- The eardrum vibrates back and forth.
- These vibrations are amplified by three small bones in the middle ear.
- The inner ear (cochlea) converts vibrations into electrical signals.
- The auditory nerve carries these signals to the brain.
- The brain interprets them as sound.
SONAR System
Process of SONAR
Formula Used in SONAR
Total distance travelled by sound = 2d
2d = v × t
Distance of object:
d = (v × t) / 2
Where:
Applications of SONAR
Use of Ultrasound by Animals
Bats
Porpoises
Structure of Human Ear
Hearing is the process in which the ear converts sound waves into electrical signals. These signals are sent to the brain through the auditory nerve for understanding sound.
The ear has three main parts:
Parts of the Ear and Their Functions
Working of Human Ear
16. Human Ear
Parts of human ear:
- Outer Ear
- Middle Ear
- Inner Ear
The cochlea converts sound vibrations into electrical signals.
Which part converts sound into electrical signals?
Answer: Cochlea
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