Sound Waves: Characteristics and Applications
Complete virtual-teacher learning module covering production, propagation, compressions, rarefactions, wave characteristics, human perception, reflection, echo, reverberation, ultrasound, infrasound, echolocation and SONAR.
Chapter Navigation
1. Production of Sound
When a stretched string is plucked, a membrane is struck, a metal object is hit or air is blown through a flute, some part of the system begins moving repeatedly about its normal position. This repeated motion produces sound.
Vibration and Oscillation
A vibration is a periodic to-and-fro motion or oscillation of an object about its mean position.
Vibrating Strings
Rubber bands, sitar strings, veena strings and tanpura strings vibrate.
Vibrating Membranes
Tabla and mridangam membranes vibrate when struck.
Vibrating Air Columns
In a bansuri or flute, the air column inside the hollow tube vibrates.
Vibrating Solids
Tuning forks and metallic objects can vibrate when struck.
The curved positions represent successive positions of the vibrating band.
Activity 10.1 — Vibrating Rubber Band
Aim: To show that sound is produced by vibration.
Materials: Cardboard box and rubber band.
- Stretch a rubber band across an open side of a cardboard box.
- Pluck the rubber band.
- Listen to the sound and observe its motion.
- Wait until the vibration stops.
- Change the tension and pluck it again.
- Remove the band from the box and pluck it while stretched between the fingers.
Observation: Sound is produced while the rubber band vibrates. When vibration stops, the sound stops. Changing tension changes the sound produced.
Conclusion: Sound is produced by vibrating objects.
Sound Production in Humans
Humans produce sound using the vocal cords, which are muscular flaps located inside the larynx or voice box.
The tongue, lips, mouth and nasal cavity help convert the sound into speech and music.
Tuning Fork
A tuning fork is a U-shaped metal bar with a stem. It is usually made of steel or aluminium. The two arms are called prongs or tines.
Activity 10.2 — Tuning Fork
- Hold a tuning fork by its stem.
- Strike one prong gently on a soft rubber pad.
- Bring the vibrating fork near the ear.
- Touch one vibrating prong gently to a water surface.
- Observe waves appearing on the water.
Conclusion: The formation of water waves proves that the tuning fork prongs are vibrating while producing sound.
Kongthong — The Whistling Village
2. Propagation of Sound
Sound Through Solids
When one student knocks a classroom desk and another places an ear against the desk, sound can be heard through the solid.
Sound Through Liquids
When metal spoons are struck together beneath water, their sound can still reach the listener. This proves that sound can travel through liquids.
Sound Through Gases
Ordinary conversation demonstrates sound travelling through air, which is a gas.
Sound Needs a Medium
A region containing no matter is called a vacuum. Sound cannot travel through vacuum because there are no material particles available to transfer the mechanical disturbance.
Vacuum Bell-Jar Experiment
As air is removed from the bell jar, the sound becomes progressively fainter even though the bell continues to vibrate. When air is allowed back into the jar, the sound becomes louder again.
Astronauts in Space
Outer space is nearly a vacuum. Astronauts performing a spacewalk therefore cannot hear each other directly through surrounding space and must use communication devices.
3. Sound Waves
Sound propagates as a travelling disturbance in a medium. The medium particles vibrate about their mean positions, while the disturbance itself moves forward.
Activity 10.5 — Slinky Model
Regions where the turns are closer together model compressions. Regions where they are farther apart model rarefactions.
Compression
Rarefaction
Piston Model
Spherical Propagation
Longitudinal Waves
In sound waves the particles vibrate parallel to the direction in which the disturbance propagates.
Longitudinal vs Transverse Waves
4. Energy of Sound Waves
Activity 10.6 — Sound Moving Grains
A stretched rubber or cellophane sheet is placed over a container and small grains are sprinkled over it. A loud sound near the sheet makes the sheet vibrate, causing the grains to move or jump.
This demonstrates that sound carries energy.
Microphone and Speaker
Microphone
Sound energy → diaphragm vibration → electrical signal
Speaker
Electrical signal → diaphragm/cone vibration → sound
5. Graphical Representation of Sound
A sound wave may be represented by plotting density of the medium against distance at a particular instant or against time at a fixed location.
6. Characteristics of a Sound Wave
Wavelength
The distance between consecutive equivalent points such as two crests or two troughs is called the wavelength.
Symbol: λ SI unit: metre (m)
Frequency
The number of density oscillations at a fixed position per unit time is called frequency.
Unit: hertz (Hz)
Time Period
The time required for one complete oscillation is called the time period.
Amplitude
Amplitude is the maximum change in density in a compression or rarefaction relative to the average density.
Intensity
Sound intensity is the amount of sound energy passing through unit area perpendicular to the direction of propagation per unit time.
As sound spreads over a larger area with increasing distance, its intensity decreases.
Speed of Sound
In one time period, a wave travels one wavelength:
| Medium | Approximate speed at 15 °C |
|---|---|
| Steel | 5000 m s−1 |
| Water | 1500 m s−1 |
| Air | 340 m s−1 |
Interactive Sound Wave Explorer
7. Human Perception of Sound
Pitch
Pitch is how frequency is perceived by humans. Higher frequency generally produces higher pitch.
Human Audible Range
The approximate human hearing range is 20 Hz to 20,000 Hz or 20 kHz.
Loudness
Greater amplitude is generally perceived as a louder sound, while smaller amplitude produces softer sound.
Intensity is a measurable physical quantity, whereas loudness depends on the listener.
Human Ear
Tone and Musical Note
8. Reflection of Sound
Sound waves can bounce from surfaces. This is called reflection of sound.
- Angle of incidence = angle of reflection.
- Incident sound, reflected sound and the normal lie in the same plane.
Echo
An echo is a reflected sound heard separately from the original sound.
At a speed of approximately 340 m/s, a distinct echo requiring about 0.1 s corresponds to a minimum reflector distance of approximately 17 m.
Reverberation
Multiple reflections that make sound persist after the source stops are called reverberation.
9. Infrasonic and Ultrasonic Waves
Infrasonic
<20 Hz
Audible
20 Hz–20 kHz
Ultrasonic
>20 kHz
Echolocation
Bats emit ultrasonic waves and analyse the returning echoes to locate prey and obstacles.
SONAR
SONAR = Sound Navigation and Ranging.
Ultrasonic waves are transmitted through water. Reflected waves from underwater objects are detected and used to calculate distance.
10. Formula Sheet
Frequency
ν = N/tTime Period
T = 1/νWave Speed
v = λνWavelength
λ = v/νDistance
d = vtEcho / SONAR
d = vt/211. Solved Numerical Practice
1 A source completes 240 oscillations in 2 s. Find its frequency.
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2 Find the time period of a 25-Hz wave.
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3 A sound wave travels at 340 m/s and has frequency 170 Hz. Find wavelength.
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4 Thunder is heard 3 s after lightning. Use speed = 340 m/s.
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5 An echo returns in 0.4 s. Speed of sound = 340 m/s. Find wall distance.
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12. Question Bank
MCQs
1 Sound is produced by:
A. colour B. vibration C. vacuum D. reflection only
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2 Which region has higher density?
A. Rarefaction B. Compression C. Vacuum D. Trough
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3 The SI unit of frequency is:
A. metre B. second C. hertz D. metre per second
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Graph-Based Practice
Graph A
Graph B
Graph C — numerical scale
Graph D
4 Graph C shows successive crests 4 m apart. What is its wavelength?
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5 If Graph C travels at 340 m/s and λ = 4 m, find its frequency.
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ν = 340/4
ν = 85 Hz.
13. Last-Minute Revision
Must Remember
- Sound is produced by vibration.
- Sound needs a material medium.
- Sound is longitudinal and mechanical.
- Particles vibrate locally.
- Energy is transferred.
Important Values
- Audible range: 20 Hz–20 kHz
- Infrasonic: below 20 Hz
- Ultrasonic: above 20 kHz
- Clear echo interval: about 0.1 s
- Approximate minimum echo distance: 17 m at 340 m/s
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