Sound
About 6 min · Science · English
Time worked out from the lesson's 420 words at 150 words a minute, plus a minute for each question.
Sound is a mechanical wave that travels through a medium by creating vibrations. Students must understand how sound is produced, how it travels, what makes sounds different from each other (pitch, loudness, quality), and phenomena like reflection and echo.
Key ideas
- Sound is a mechanical wave — It requires a material medium (solid, liquid or gas) to travel. Sound cannot propagate through vacuum because there are no particles to vibrate.
- Production by vibration — All sound originates from vibrating objects. When an object vibrates, it creates compressions (high pressure regions) and rarefactions (low pressure regions) in the surrounding medium, forming a longitudinal wave.
- Speed varies with medium — Sound travels fastest in solids (particles are closely packed), slower in liquids, and slowest in gases. In air at 20°C, speed ≈ 344 m/s; in water ≈ 1500 m/s; in steel ≈ 5000 m/s. Speed increases with temperature in gases.
- Pitch depends on frequency — High frequency sounds have high pitch (like a whistle), low frequency sounds have low pitch (like a drum). Human hearing range: 20 Hz to 20,000 Hz. Sounds below 20 Hz are infrasonic; above 20,000 Hz are ultrasonic.
- Loudness depends on amplitude — Greater amplitude means more energy and louder sound. Loudness is measured in decibels (dB). Normal conversation ≈ 60 dB; threshold of pain ≈ 120 dB.
Quick reference
- Sound = longitudinal mechanical wave; needs medium; travels as compressions and rarefactions
- Speed: Solid > Liquid > Gas; in air ≈ 340 m/s
- Pitch ∝ frequency; Loudness ∝ amplitude; Quality = waveform/harmonics
- Audible range: 20–20,000 Hz; infrasonic < 20 Hz; ultrasonic > 20,000 Hz
- Echo: Distance to surface = (Speed × Time) / 2; minimum 17 m for distinct echo
Check yourself: 3 questions
1. A person standing at a distance from a cliff claps his hands and hears the echo after 3 seconds. If the speed of sound in air is 340 m/s, what is the distance of the cliff from the person?
2. Which of the following statements is correct regarding the propagation of sound waves?
3. A tuning fork produces sound waves with a frequency of 256 Hz. If the speed of sound in air is 344 m/s, what is the wavelength of the sound wave produced?
This lesson is cut from the topic's note, not rewritten. Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain. The questions are from Shishya's validated question bank.
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