Sound Waves: Important Points and Summary
Introduction
This blog gives a complete revision of the chapter Sound Waves: Characteristics and Applications. It summarises the main concepts, important terms, formulas, examples and applications of sound waves in a simple Class 9 style.
Sound is a form of energy produced by vibrating objects. It travels through a material medium as a wave and helps us hear, communicate, locate objects and use many modern technologies.
Chapter Flow Diagram
1. Production of Sound
Sound is produced by vibrations. Vibration means the periodic to and fro motion of an object.
- A stretched rubber band produces sound when it vibrates.
- A tuning fork produces sound when its prongs vibrate.
- Human sound is produced by the vibration of vocal cords.
- Musical instruments produce sound by vibrating strings, membranes or air columns.
2. Source of Sound
The object that produces sound is called the source of sound.
| Source | Vibrating Part |
|---|---|
| Human voice | Vocal cords |
| Tuning fork | Prongs |
| Flute | Air column |
| Drum | Membrane |
| String instruments | Strings |
3. Propagation of Sound
The travelling of sound from the source to the listener is called propagation of sound.
Sound needs a material medium to propagate. The medium may be a solid, liquid or gas.
- Sound travels through solids.
- Sound travels through liquids.
- Sound travels through gases.
- Sound cannot travel through vacuum.
4. Sound Needs a Medium
Sound cannot travel in vacuum because there are no particles to transfer the disturbance.
In the bell jar experiment, when air is removed from the jar, the sound of the ringing bell becomes fainter and finally almost cannot be heard. This shows that sound needs a medium to travel.
5. Sound Wave as a Disturbance
Sound travels through a medium as a disturbance. The particles of the medium do not move from the source to the listener. They only vibrate about their mean positions.
The disturbance travels forward and transfers energy from one region to another.
6. Compressions and Rarefactions
A sound wave consists of alternate compressions and rarefactions.
| Term | Meaning |
|---|---|
| Compression | Region where particles are closer and density is higher than average |
| Rarefaction | Region where particles are farther apart and density is lower than average |
7. Sound as a Longitudinal Mechanical Wave
Sound is a longitudinal mechanical wave.
- It is longitudinal because particles vibrate parallel to the direction of wave propagation.
- It is mechanical because it needs a material medium to travel.
8. Longitudinal and Transverse Waves
| Longitudinal Wave | Transverse Wave |
|---|---|
| Particles vibrate parallel to the direction of wave propagation. | Particles vibrate perpendicular to the direction of wave propagation. |
| Sound wave is an example. | Light wave is an example. |
| It may have compressions and rarefactions. | It has crests and troughs. |
9. Energy of Sound Waves
Sound is a form of energy. When sound reaches an object, it can make the object vibrate.
For example, when a loud sound is produced near a stretched sheet with grains on it, the grains jump because the sound wave transfers energy to the sheet.
In sound propagation, energy is transferred, not the particles of the medium.
10. Graphical Representation of Sound Wave
A sound wave can be represented by showing variation of density with distance or time.
- The highest point of the graph is called the crest.
- The lowest point of the graph is called the trough.
- Compression corresponds to density above average.
- Rarefaction corresponds to density below average.
11. Characteristics of a Sound Wave
| Characteristic | Meaning | SI Unit |
|---|---|---|
| Wavelength | Distance between two consecutive crests or two consecutive troughs | metre |
| Frequency | Number of complete density oscillations per unit time | hertz |
| Time period | Time taken for one complete oscillation | second |
| Amplitude | Maximum change in density from average density | Depends on quantity represented |
| Intensity | Sound energy passing per unit area per unit time | watt per square metre |
| Speed | Distance travelled by a point on a wave in unit time | metre per second |
12. Important Formulas
| Formula | Meaning |
|---|---|
| \(\nu = \frac{1}{T}\) | Frequency is reciprocal of time period |
| \(v = \lambda \nu\) | Speed = wavelength × frequency |
| \(d = \frac{vt}{2}\) | Distance of reflecting surface in echo or SONAR |
13. Speed of Sound
The speed of sound depends on the medium through which it travels.
Speed of sound: solids > liquids > gases
| Medium | Approximate Speed |
|---|---|
| Air | About \(340\,m\,s^{-1}\) |
| Water | About \(1500\,m\,s^{-1}\) |
| Steel | About \(5000\,m\,s^{-1}\) |
- Sound travels fastest in solids because particles are closely packed.
- Sound travels slower in liquids than in solids.
- Sound travels slowest in gases.
- The speed of sound in air increases with temperature.
- The speed of sound in air also increases with humidity.
14. Human Perception of Sound
Physical properties of sound can be measured, but how we experience sound depends on our hearing. Human perception of sound is mainly described by pitch and loudness.
| Perception | Related Physical Property | Meaning |
|---|---|---|
| Pitch | Frequency | High frequency sounds are generally high-pitched; low frequency sounds are low-pitched. |
| Loudness | Amplitude | Larger amplitude sounds are heard louder; smaller amplitude sounds are softer. |
15. Audible, Infrasonic and Ultrasonic Sounds
Humans can hear sounds only in a limited frequency range.
Human audible range = 20 Hz to 20 kHz
| Type of Sound | Frequency Range | Human Hearing |
|---|---|---|
| Infrasonic sound | Below 20 Hz | Cannot be heard by humans |
| Audible sound | 20 Hz to 20 kHz | Can be heard by humans |
| Ultrasonic sound | Above 20 kHz | Cannot be heard by humans |
- Dogs, cats, bats and dolphins can detect ultrasound.
- Elephants can detect infrasound.
16. Human Ear and Hearing
When sound enters the ear, it makes the eardrum vibrate. Tiny bones amplify these vibrations. The cochlea converts them into electrical signals, which are sent to the brain. The brain perceives these signals as sound.
Having two ears helps us find the direction of the sound source because the brain compares which ear receives the sound first.
17. Tone, Musical Note, Overtones and Timbre
| Term | Meaning |
|---|---|
| Tone | A sound of a single frequency |
| Musical note | A combination of fundamental frequency and overtones |
| Fundamental frequency | The lowest frequency present in a musical note |
| Overtones | Higher frequencies present along with the fundamental frequency |
| Timbre | Quality of sound that helps us distinguish between different instruments |
| Octave | Interval between two notes where one has double the frequency of the other |
18. Reflection of Sound
The bouncing back of sound waves from a surface is called reflection of sound.
Sound follows the same laws of reflection as light.
- The angle of incidence is equal to the angle of reflection.
- The incident sound wave, reflected sound wave and normal lie in the same plane.
19. Echo
An echo is the repetition of sound heard after reflection from a distant hard surface.
- The time gap between the original sound and reflected sound should be at least \(0.1\,s\).
- If the speed of sound is \(340\,m\,s^{-1}\), the minimum distance of the reflecting surface should be about \(17\,m\).
- Hard and smooth surfaces produce clearer echoes.
- Soft surfaces absorb sound and reduce echo.
20. Reverberation
Reverberation is the persistence of sound due to multiple reflections after the sound source has stopped emitting sound.
- It occurs in large halls and auditoriums.
- Too much reverberation makes sound unclear or garbled.
- Sound absorbing panels, upholstered chairs, curtains and soft porous surfaces reduce reverberation.
21. Applications of Infrasonic and Ultrasonic Waves
| Wave Type | Applications |
|---|---|
| Infrasonic waves | Detection of earthquakes, volcanic eruptions and severe storms |
| Ultrasonic waves | Ultrasonography, kidney stone treatment, ultrasonic welding, cleaning delicate parts and detecting defects in metal blocks |
22. Echolocation
Echolocation is the ability to locate objects using reflected sound waves.
- Most bats use ultrasonic waves to locate prey and obstacles in the dark.
- Dolphins and whales also use reflected sound waves underwater.
- Echolocation works on the principle of reflection of sound.
23. SONAR
SONAR stands for Sound Navigation and Ranging.
SONAR uses reflected sound waves to locate underwater objects such as submarines and shipwrecks. It can also be used to measure ocean depth.
\(d = \frac{vt}{2}\)
Here, \(d\) is the distance of the object, \(v\) is the speed of sound in water and \(t\) is the total time taken by sound to go to the object and return.
24. Other Applications of Sound Waves
- A microphone converts sound energy into an electrical signal.
- A speaker converts an electrical signal into sound.
- Sound apps can produce and identify frequencies.
- Audio surveillance can detect drones and aircraft using characteristic sounds.
- Sound can be used in scientific exploration and studying hidden natural activities.
25. Important Terms at a Glance
| Term | Short Meaning |
|---|---|
| Vibration | Periodic to and fro motion of an object |
| Medium | Material through which sound travels |
| Compression | High density region of a sound wave |
| Rarefaction | Low density region of a sound wave |
| Wavelength | Distance between two consecutive crests or troughs |
| Frequency | Number of oscillations per unit time |
| Time period | Time taken for one complete oscillation |
| Amplitude | Maximum change in density from average density |
| Intensity | Sound energy passing per unit area per unit time |
| Pitch | Perception of frequency |
| Loudness | Perception of amplitude |
| Echo | Repeated sound heard after reflection |
| Reverberation | Persistence of sound due to multiple reflections |
| Ultrasound | Sound above 20 kHz |
| Infrasound | Sound below 20 Hz |
| SONAR | Sound Navigation and Ranging |
26. Very Important Points
- Sound is produced by vibrating objects.
- Sound is a form of energy.
- Sound needs a material medium to propagate.
- Sound cannot travel through vacuum.
- Sound can travel through solids, liquids and gases.
- Sound is a longitudinal mechanical wave.
- Particles of the medium do not travel with the wave; they only vibrate about their mean positions.
- Sound wave consists of compressions and rarefactions.
- Sound transfers energy from one place to another.
- Wavelength is the distance between two consecutive crests or troughs.
- Frequency is the number of oscillations per unit time.
- Time period and frequency are inversely related.
- Amplitude is related to the energy carried by the sound wave.
- Intensity decreases as sound spreads away from the source.
- Speed of sound is given by \(v = \lambda \nu\).
- Sound travels fastest in solids and slowest in gases.
- The speed of sound in air increases with temperature and humidity.
- Pitch is related to frequency.
- Loudness is related to amplitude.
- The human audible range is 20 Hz to 20 kHz.
- Sound below 20 Hz is infrasonic sound.
- Sound above 20 kHz is ultrasonic sound.
- Reflection of sound produces echo and reverberation.
- SONAR uses reflected sound waves to locate underwater objects.
- Ultrasonic waves are useful in medicine and industries.
27. Common Mistakes to Avoid
| Mistake | Correct Idea |
|---|---|
| Thinking that air particles travel from source to ear | Only the disturbance travels; particles vibrate about their mean positions. |
| Thinking sound can travel in vacuum | Sound needs a material medium and cannot travel in vacuum. |
| Confusing loudness with intensity | Intensity is measurable; loudness depends on perception. |
| Confusing pitch with loudness | Pitch is related to frequency, while loudness is related to amplitude. |
| Forgetting the factor 2 in echo or SONAR problems | Sound travels to the object and returns, so total distance is \(2d\). |
28. Quick Revision Questions
- What produces sound?
- Why can sound not travel through vacuum?
- What are compressions and rarefactions?
- Why is sound called a longitudinal mechanical wave?
- What is the relation between frequency and time period?
- What is the relation between speed, wavelength and frequency?
- Why does sound travel faster in solids than in gases?
- What is the human audible range?
- What is the difference between pitch and loudness?
- What is the difference between echo and reverberation?
- What is SONAR used for?
Conclusion
Sound is produced by vibrating objects and travels through a material medium as a longitudinal mechanical wave. It propagates through compressions and rarefactions and transfers energy without carrying the particles of the medium along with it. Important characteristics of sound include wavelength, frequency, time period, amplitude, intensity and speed. Humans perceive sound mainly as pitch and loudness. Sound also shows reflection, which leads to echo and reverberation. Infrasonic and ultrasonic waves are beyond the human hearing range but have many useful applications. Sound waves are used in microphones, speakers, ultrasonography, SONAR, echolocation and audio surveillance. Thus, sound waves are important in daily life, science and technology.
English 
हिन्दी