CBSE Class 9 > Class 9 Notes > Science New NCERT 2026-27 (New Syllabus) > Cheat Sheet: Characteristics And Applications
Cheat Sheet: Characteristics And Applications
1. Production of Sound
1.1 Definitions and Key Terms
| Term | Definition |
|---|---|
| Sound | Energy produced by vibrating objects that propagates through a medium |
| Vibration | Periodic to-and-fro motion of an object |
| Source | Object that produces sound |
| Vocal cords | Tightly stretched muscular flaps in the larynx that vibrate to produce human sound |
| Larynx | Voice box in the throat containing the vocal cords |
1.2 Tuning Fork
| Term | Observation |
|---|---|
| Tuning fork | U-shaped metal bar with two prongs (tines) and a stem that vibrates when struck |
| Prong touching water | Vibrating prong produces waves on water surface |
| Orientation | Sound heard regardless of fork orientation indicates propagation in multiple directions |
2. Propagation of Sound
2.1 Medium and Vacuum
| Term | Definition / Fact |
|---|---|
| Propagation | Sound travels through solids, liquids, and gases |
| Medium | Material (solid, liquid, or gas) through which sound propagates |
| Vacuum | Space with no medium; sound cannot travel in vacuum |
- Vacuum bell jar experiment: removing air makes a ringing bell become inaudible despite visible vibration.
- Sound in outer space: near vacuum prevents direct hearing; communication requires special devices in spacesuits.
3. Sound Waves
3.1 Compressional Structure and Wave Type
| Term | Definition |
|---|---|
| Compression (C) | Region of increased medium density |
| Rarefaction (R) | Region of decreased medium density |
| Sound wave | Series of alternating compressions and rarefactions that propagate through a medium without net flow of particles |
| Longitudinal wave | Wave in which particles vibrate parallel to direction of propagation |
| Mechanical wave | Wave that requires a material medium for propagation |
| Direction of propagation | Direction in which the disturbance (wave) travels |
4. Energy of Sound Waves
4.1 Energy Transfer and Devices
| Term | Definition / Fact |
|---|---|
| Sound as energy | Vibrating source transfers energy to the surrounding medium; energy propagates via sound waves |
| Particle motion | Particles vibrate about mean positions; energy transfers, particles do not travel with wave |
| Microphone | Device that converts sound energy to electrical energy via a vibrating diaphragm |
| Speaker | Device that converts electrical signals into corresponding sound by vibrating a diaphragm |
5. Graphical Representation of a Sound Wave
5.1 Density vs Distance Graph
| Feature | Representation |
|---|---|
| Compressions | Appear as crests (peaks) on density vs distance graph |
| Rarefactions | Appear as troughs (valleys) on density vs distance graph |
| Axes | Y-axis: density; X-axis: distance |
| Average density | Marked by a horizontal dashed line on the graph |
6. Characteristics of a Sound Wave
6.1 Wavelength, Frequency, and Time Period
| Quantity | Definition, Symbol, Unit |
|---|---|
| Wavelength | Distance between two consecutive crests or troughs; symbol λ; unit metre (m) |
| Frequency | Number of density oscillations per unit time at a point; symbol ν; unit hertz (Hz or s⁻¹) |
| Time period | Time for one complete density oscillation at a point; symbol T; unit second (s) |
| Relation | ν = 1/T |
6.2 Amplitude and Intensity
| Quantity | Definition / Fact |
|---|---|
| Amplitude | Maximum change in medium density in a compression or rarefaction relative to average density |
| Amplitude vs energy | Larger amplitude corresponds to greater energy carried by the wave |
| Intensity | Sound energy passing through unit area perpendicular to propagation per unit time |
| Intensity vs distance | Intensity decreases with distance from the source as energy spreads over larger area |
6.3 Speed of Sound
| Term | Definition / Formula / Values |
|---|---|
| Speed of sound | Distance traveled by a point on the wave per unit time |
| Wave relation | v = λ × ν |
| Dependence on medium | Sound speed: solids > liquids > gases |
| Typical speeds at 15°C | Steel ≈ 5000 m s⁻¹; Water ≈ 1500 m s⁻¹; Air ≈ 340 m s⁻¹ |
| Temperature dependence | 331 m s⁻¹ at 0°C; nearly 344 m s⁻¹ at 22°C |
| Relative factors | ≈4-5 times faster in water than air; ≈15-20 times faster in solids than air |
6.4 Human Perception
| Aspect | Definition / Fact |
|---|---|
| Pitch | Perception of frequency; higher frequency → higher pitch |
| Loudness | Perception of amplitude; measured in decibels (dB) |
| Audible range | Human hearing: 20 Hz to 20,000 Hz |
| Timbre | Quality that distinguishes sounds of same pitch and loudness; depends on overtones and instrument characteristics |
| Tone | Sound of a single frequency |
| Musical note | Combination of a fundamental frequency and overtones |
| Octave | Interval between two notes where one has double the frequency of the other |
7. Reflection of Sound
7.1 Laws and Echo
| Law / Term | Definition / Condition |
|---|---|
| Laws of reflection | Incident and reflected sound make equal angles with the normal; all three lie in same plane |
| Echo | Reflected sound heard distinctly after the original sound |
| Minimum time gap for echo | At least 0.1 s between original and reflected sound for distinction by brain |
| Minimum reflecting distance | 17 m (for speed of sound = 340 m s⁻¹ and 0.1 s requirement) |
| Reflecting surfaces | Hard, smooth surfaces produce stronger echoes; soft surfaces absorb sound |
7.2 Reverberation
| Term | Definition / Fact |
|---|---|
| Reverberation | Persistence of sound due to multiple reflections after source stops |
| Time-difference condition | Reverberation occurs when reflections arrive within less than 0.05 s |
| Auditorium design | Use of sound-absorbing panels and soft surfaces reduces unwanted reverberation |
8. Ultrasonic and Infrasonic Waves and Applications
8.1 Definitions and General Uses
| Term | Definition / Application |
|---|---|
| Infrasonic waves | Frequencies below 20 Hz |
| Ultrasonic waves | Frequencies above 20 kHz |
8.2 Applications
- Infrasound: detecting earthquakes and volcanic eruptions
- Infrasound: detecting severe storms across long distances
- Ultrasound: medical imaging of internal organs (ultrasonography)
- Ultrasound: breaking kidney stones into smaller pieces
- Ultrasound: ultrasonic welding in industries
- Ultrasound: ultrasonic cleaning of delicate machine parts
- Ultrasound: detecting internal defects in metal blocks during testing
- Ultrasound/echolocation: locating objects by analysing reflected sound
- Echolocation (biological)
- Echolocation: bats emit ultrasonic bursts and use echoes to locate obstacles and prey
- Sonar
- Sonar: sending ultrasonic waves in water and analysing reflections to find distance, direction, and speed of underwater objects
- Audio surveillance
- Audio surveillance: detecting characteristic low-frequency sounds of drones and aircraft using sensitive sound sensors
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