New NCERT Exploration · Class 9 Science · Chapter 10: Sound Waves: Characteristics and Applications
This chapter explains how sound is produced by vibrations and how it travels through a medium as longitudinal waves. It covers the characteristics of sound waves such as wavelength, frequency, time period, amplitude, intensity and speed. It also covers pitch, loudness, the human ear, echo, infrasonic and ultrasonic waves, SONAR and timbre.
From the Class 9 Science Short Notes, made as per the new NCERT book Exploration · Pages 74–88 of the book.
Key points
- Sound is a form of energy produced by vibrations, the rapid to-and-fro motion of an object; when vibration stops, sound stops.
- Humans produce sound with vocal cords in the larynx; birds use the syrinx, frogs a vocal sac, and bees their fast-moving wings.
- Sound needs a material medium (solid, liquid or gas) and cannot travel through a vacuum, as shown by the bell jar experiment.
- Sound travels as longitudinal waves of compressions (high pressure, high density) and rarefactions (low pressure, low density).
- Particles of the medium only vibrate about their positions; the energy moves forward.
- Wavelength (λ) is the distance between two consecutive compressions or rarefactions; its SI unit is metre.
- Frequency (ν) is the number of complete vibrations per second, measured in hertz (Hz); time period (T) is the time for one vibration.
- Frequency and time period are inversely proportional; higher frequency means higher pitch.
- Amplitude decides loudness and energy; intensity decreases as sound spreads away from the source.
- Sound travels fastest in solids, slower in liquids and slowest in gases: about 340 m/s in air, 1500 m/s in water and 5000 m/s in steel.
- Humans hear from 20 Hz to 20,000 Hz; below 20 Hz is infrasonic and above 20,000 Hz is ultrasonic.
- Reflection of sound causes echoes and is used in echolocation by bats and in SONAR to find underwater objects.
Formulas
| Speed of sound | v = distance (λ) ÷ time (t) |
| Intensity and distance | I ∝ 1 ÷ distance² Intensity falls as sound spreads over a larger area |
| Frequency and time period | Frequency ∝ 1 ÷ Time period They are inversely proportional |
| Octave | Frequency of higher note = 2 × frequency of lower note e.g. 250 Hz → 500 Hz |
Key terms
- Vibration: The repeated to-and-fro movement of an object about its resting position.
- Compression: Region of a sound wave where particles are close together, with high pressure and density.
- Rarefaction: Region of a sound wave where particles are far apart, with low pressure and density.
- Longitudinal wave: A wave in which particles vibrate parallel to the direction the wave travels.
- Frequency: Number of complete vibrations produced in one second; SI unit hertz (Hz).
- Amplitude: Maximum displacement of particles from the mean position; decides loudness.
- Pitch: The characteristic that tells whether a sound is high (shrill) or low (deep); it depends on frequency.
- Echo: Repetition of a sound caused by reflection of sound waves from a distant surface.
- Timbre: The unique quality that makes the same note sound different on different instruments.
Diagrams to learn
- Rubber band on a box producing sound
- Sources of sound and the human larynx
- Tuning fork touching water
- Sound travelling through solid, liquid and gas
- Bell jar experiment
- Compressions and rarefactions from a speaker and a slinky
- Density-distance graph with crest and trough
- Long and short wavelength; low and high frequency waves
- Low and high amplitude waves
- Intensity falling with distance from a speaker
- Table of speed of sound in air, water and steel
- Structure of the human ear
- Echo from a mountain, echolocation by bats and SONAR
- In a sound wave, energy travels forward but the particles only move to and fro around their original positions.
- Pitch depends on frequency, while loudness depends on amplitude.
- Sound travels slightly faster in humid air than in dry air, and faster at higher temperature.
A real page from the book
Get the full chapter in the book
This page is a summary. The Class 9 Science Short Notes has the complete chapter in colour, with every diagram and worked example.
Frequently asked questions
Why can't sound travel through a vacuum?
Sound needs a material medium whose particles carry the vibrations. A vacuum has no particles, so sound cannot travel through it, as shown by the bell jar experiment.
What is the audible range of human hearing?
A healthy human can hear sounds from 20 Hz to 20,000 Hz. Sounds below 20 Hz are infrasonic and sounds above 20,000 Hz are ultrasonic.
How does SONAR work?
A ship sends ultrasonic waves into the water. They strike underwater objects and return as echoes, and by measuring the time taken, scientists calculate the distance, depth and direction of the objects.
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