Question 35
Describe an experiment to estimate the speed of sound in air.
Estimation of speed of sound in air:
- Suppose we choose two hills A and B about a kilometre apart.
- A person at hill A fires a gun.
- Another person at hill B starts a stop watch as he sees the flash of fire and stops it on hearing the sound.
He measures the time interval between the sight of flash and hearing the sound. Let it be t second.
Measure the distance between the hills A and B. Let it be d metre.
So speed of sound V = = m s-1.
Experimentally it is found that speed of sound in air is 330 m s-1.
ICSE Class 7 Physics — Sound: Complete Study Guide
Sound is an engaging chapter in ICSE Class 7 Physics, worth approximately 12 marks. It covers the production, propagation, speed, characteristics, and reflection of sound, along with important environmental topics like noise pollution. This chapter connects directly to students' daily experiences — from music and speech to echoes and thunder.
The key concept is that sound is produced by vibrating objects and requires a material medium (solid, liquid, or gas) to travel. Unlike light, sound cannot travel through vacuum. Students learn that sound travels fastest in solids and slowest in gases — a counterintuitive fact that requires clear explanation. The three characteristics of sound (pitch, loudness, and quality) help students understand why a flute and a drum sound different even when playing the same note.
Echo calculations are a common exam question and require understanding that sound travels to a reflecting surface and back, so the actual distance is half the total distance covered. The chapter concludes with noise pollution — its causes, effects on health, and prevention measures — connecting physics to environmental awareness and civic responsibility.
| Concept | Key Facts |
|---|---|
| Speed in air | ~340 m/s at 20°C |
| Speed in water | ~1500 m/s |
| Speed in steel | ~5000-6000 m/s |
| Minimum echo distance | ~17 m |
| Echo formula | Distance = (Speed x Time) / 2 |
| Audible range | 20 Hz to 20,000 Hz |
Must-Know Concepts
- Sound is produced by vibration — no vibration means no sound
- Sound needs a medium; cannot travel through vacuum
- Speed order: Solids > Liquids > Gases
- Pitch depends on frequency; loudness depends on amplitude; quality on waveform
- Echo requires minimum 17 m distance; formula: d = (v x t) / 2
Common Mistakes
- Saying sound travels fastest in air — it travels SLOWEST in gases
- Forgetting to divide by 2 in echo calculations (sound goes to wall AND back)
- Confusing pitch (frequency) with loudness (amplitude) — they are independent
- Believing sound can travel through vacuum (movies show explosions in space — this is wrong)
Exam Tips
- In echo numericals, clearly write: "Total distance = 2 x distance to wall" before solving
- For the bell jar experiment, describe setup, observation, AND conclusion
- Compare musical sound vs noise in a table with at least 4 points
- For noise pollution, provide at least 3 causes, 3 effects, and 3 prevention measures
Why do we see lightning before hearing thunder?
Light travels at approximately 300,000 km/s while sound travels at only 340 m/s in air. Lightning and thunder occur simultaneously, but light reaches us almost instantly while sound takes much longer. The delay in hearing thunder tells us how far away the lightning is: multiply the seconds by 340 to get the distance in metres.
How does SONAR work?
SONAR (Sound Navigation and Ranging) sends ultrasonic waves from a ship toward the ocean floor. The waves travel through water, reflect off the seabed, and return to the ship. By measuring the time taken for the echo to return and knowing the speed of sound in water (~1500 m/s), the depth can be calculated: Depth = (Speed x Time) / 2.
What is the minimum distance required to hear an echo?
The minimum distance is approximately 17 metres. The human ear needs a time gap of at least 0.1 seconds to distinguish an echo from the original sound. In 0.1 seconds, sound in air (340 m/s) travels 34 metres — this is the round trip, so the wall must be at least 34/2 = 17 metres away.