Characteristics & Applications (ध्वनि)
Sound is produced by vibrating objects. When an object vibrates, it moves back and forth rapidly, disturbing the particles of the medium around it. These disturbances travel as waves and reach our ears, where we perceive them as sound.
Every sound you hear — a guitar string, a drum, your voice, a bell — comes from something vibrating. Stop the vibration, and the sound stops too.
Watch how a vibrating object pushes air particles, creating compressions (C) and rarefactions (R):
Aim: To show that sound is produced by vibration.
Do this: Place your hand on your throat. Now speak or hum a song. Feel your throat with your fingers.
Observation: You can feel vibrations in your throat when you speak or hum.
Conclusion: The vocal cords in our throat vibrate to produce sound.
Aim: To show that a vibrating object can cause effects in the surrounding medium.
Do this: Strike a tuning fork on a rubber pad. Quickly touch the prongs to the surface of water in a beaker.
Observation: Water splashes when the vibrating prongs touch the surface.
Conclusion: The vibrating tuning fork transfers energy to the water, causing splashes. This proves the fork is vibrating.
Aim: To show that a vibrating tuning fork can push a light object.
Do this: Suspend a table-tennis ball from a stand. Touch a vibrating tuning fork to the ball.
Observation: The ball bounces away from the vibrating tuning fork.
Conclusion: The vibrating fork pushes the ball, proving that vibration produces a force that can displace nearby objects.
Do this: Hold a ruler firmly at the edge of a table so part of it extends beyond the edge. Press the free end down and release.
Observation: The free end vibrates up and down, producing a humming sound. Shorter free length → faster vibration → higher pitch.
Sound waves are mechanical waves — they need a material medium (solid, liquid, or gas) to propagate. Unlike light, sound cannot travel through a vacuum.
The particles of the medium vibrate and pass the disturbance along to their neighbours. No particles = no sound transmission.
Do this: Place an electric bell inside a glass bell jar connected to a vacuum pump. Switch on the bell. Now gradually pump out the air.
Observation: As air is pumped out, the sound becomes fainter and fainter. In a perfect vacuum, no sound is heard at all. When air is let back in, the sound returns.
Conclusion: Sound needs a material medium to travel. It cannot travel through a vacuum.
Sound travels as a longitudinal wave. In a longitudinal wave, the particles of the medium vibrate parallel (back and forth) to the direction of wave propagation.
This creates alternating regions of:
Do this: Stretch a slinky on a table. Push one end forward and pull it back. Watch the compression travel along the slinky.
Observation: A compression (bunch of coils close together) moves along the slinky, followed by rarefactions (coils far apart). Each particle oscillates back and forth — it doesn't travel with the wave.
Conclusion: This is a longitudinal wave — the coils (particles) vibrate back and forth in the same direction as the wave travels. Sound in air behaves exactly like this.
Do this: Stretch the slinky. Move one end up and down (sideways). Watch the wave travel along.
Observation: Crests (peaks) and troughs (valleys) move along the slinky.
Conclusion: This is a transverse wave — the coils vibrate perpendicular to the direction of wave travel. Light waves are transverse; sound waves are NOT.
Watch how a vibrating speaker pushes air molecules, creating compressions (C) and rarefactions (R). Particles don’t travel — only energy does!
Compare how particles move in each type. Both waves carry energy rightward, but particle motion differs!
| Property | Longitudinal Wave | Transverse Wave |
|---|---|---|
| Particle motion | Parallel to wave direction | Perpendicular to wave direction |
| Features | Compressions & rarefactions | Crests & troughs |
| Example | Sound in air, slinky push-pull | Light, water waves, slinky up-down |
| Needs medium? | Yes (mechanical) | Not always (EM waves don’t) |
A sound wave can be described by these key properties:
Watch how a vibrating particle creates waves. Frequency = how many cycles per second. Time Period = time for one complete cycle.
Drag the sliders to see how frequency and amplitude change the wave shape:
The speed of sound depends on the properties of the medium. It is fastest in solids, slower in liquids, and slowest in gases.
| Medium | State | Speed (m/s) at 25°C |
|---|---|---|
| Air (0°C) | Gas | 331 |
| Air (20°C) | Gas | 344 |
| Water (25°C) | Liquid | 1498 |
| Sea water | Liquid | 1531 |
| Iron | Solid | 5130 |
| Aluminium | Solid | 6420 |
| Sound Source | Loudness (dB) |
|---|---|
| Normal breathing | 10 dB |
| Whisper | 20 dB |
| Normal conversation | 60 dB |
| Busy traffic | 70 dB |
| Factory / loud music | 90–100 dB |
| Jet plane at 30 m | 150 dB |
Sound waves obey the laws of reflection, just like light:
Sound reflects best off hard, smooth surfaces (walls, cliffs, polished metal). Soft, rough surfaces absorb sound.
Setup: Place two long tubes at angles against a smooth wall. Put a ticking clock at the end of one tube. Listen through the other tube.
Observation: The sound is heard clearly when the angle of the listening tube equals the angle of the source tube from the wall (∠i = ∠r).
Conclusion: Sound follows the laws of reflection, similar to light.
An echo is the repetition of a sound caused by the reflection of sound waves from a hard surface (like a cliff or building).
For an echo to be heard distinctly, the reflected sound must reach the ear at least 0.1 seconds after the original sound (this is the persistence of hearing).
A person shouts towards a cliff. The sound travels, hits the wall, reflects back, and is heard as an echo after a delay.
Reverberation is the persistence of sound in an enclosed space after the original sound has stopped. It is caused by repeated reflections of sound from walls, ceiling, and floor.
Excessive reverberation makes speech unclear. Concert halls and auditoriums must be carefully designed to reduce it.
Click "Clap" to produce a sound pulse. Watch it reflect off walls multiple times, creating reverberation.
The human ear can hear sounds in the frequency range of 20 Hz to 20,000 Hz. This is called the audible range.
Ultrasound (frequency > 20,000 Hz) has many practical uses because it can be directed in well-defined beams and has high energy:
SONAR stands for Sound Navigation And Ranging. It uses ultrasound waves to measure distances, find objects underwater, and map the ocean floor.
1. A transmitter on the ship sends ultrasound pulses towards the ocean floor.
2. The pulses reflect off the ocean floor (or any object) and return to the ship.
3. A receiver on the ship detects the reflected signal.
4. The time taken is used to calculate the depth.
The human ear is a remarkable organ that converts sound waves (mechanical energy) into electrical signals that the brain interprets as sound. It has three main parts:
1. Pinna collects sound waves → 2. Waves travel through ear canal → 3. Hit eardrum, making it vibrate → 4. Ossicles amplify vibrations (~20x) → 5. Vibrations reach cochlea, where fluid waves bend hair cells → 6. Hair cells convert vibrations to electrical signals → 7. Auditory nerve sends signals to brain → 8. Brain interprets signals as sound!
Complete end-of-chapter exercises with step-by-step solutions. Try answering first, then click to check!