Complete Concept Guide (100% Curriculum Coverage)
1. The Mechanical Nature of Sound
Sound is produced by vibrating bodies and is a mechanical wave that CANNOT travel through a vacuum (proved by the electric bell jar experiment: as air is vacuumed out, the bell's sound fades to silence). Sound travels fastest in solids, slower in liquids, and slowest in gases.
2. Longitudinal Waves: Compressions & Rarefactions
Sound waves travel as longitudinal pressure waves: particles oscillate parallel to wave direction, creating alternating zones of high density (Compressions) and low density (Rarefactions).
• Speed Formula: $$\mathbf{v = \nu \lambda} \quad (\text{Speed} = \text{Frequency} \times \text{Wavelength})$$
3. Reflection, Echo & Ultrasound (SONAR)
Sound reflects off hard surfaces. To hear a distinct Echo, the reflecting barrier must be at least $17.2\text{ meters}$ away (since persistence of hearing is $0.1\text{ s}$). Ultrasound ($> 20,000\text{ Hz}$) is used in medicine to image internal organs without surgery, and in naval SONAR to map ocean depths ($2d = v \times t$).
Check Your Understanding (Diagnostic Practice Questions)
Diagnostic questions testing core conceptual clarity. Answers are hidden initially — solve each problem first, then click to reveal the step-by-step verified solution.
1
Why can two astronauts on the Moon not hear each other speak directly without radio headsets?
Reveal Answer & Explanation
Answer: Because sound is a mechanical wave requiring a material medium to propagate; since the Moon has no atmosphere (vacuum), sound waves cannot travel.
Vacuum on Moon blocks sound.
2
A sound wave has a frequency of 2 kHz and wavelength 35 cm. How long will it take to travel 1.5 km?
Reveal Answer & Explanation
Answer: $\nu = 2000\text{ Hz}, \lambda = 0.35\text{ m}$. Speed $v = \nu\lambda = 2000 \times 0.35 = 700\text{ m/s}$. Distance $= 1500\text{ m}$. $\text{Time} = \frac{1500}{700} = 2.14\text{ seconds}$.
v = freq * wavelength, then t = d/v.
3
What is the minimum distance required from an obstacle to hear a distinct echo in air at $22^\circ\text{C}$?
Reveal Answer & Explanation
Answer: 17.2 meters (calculated from speed of sound $344\text{ m/s} \times 0.1\text{ s} / 2$).
17.2 meters.
4
What is 'Reverberation' and how is it reduced in cinema halls and auditoriums?
Reveal Answer & Explanation
Answer: Reverberation is the persistence of repeated sound reflections creating an echoey blur. It is minimized by covering walls and ceilings with sound-absorbing materials like acoustic tiles, heavy draperies, and compressed fiberboard.
Sound-absorbing materials on walls.
5
How does a submarine use SONAR to measure the depth of the ocean bed?
Reveal Answer & Explanation
Answer: The submarine transmitter emits an ultrasonic pulse towards the seabed and detects the reflected echo with a receiver; depth is calculated using $d = \frac{v \times t}{2}$.
d = (v * t) / 2.
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