Chapter 5 — Acoustics: Multiple Choice Questions
Instructions: Each question carries 1 mark. Choose the best answer.
1. Sound is a ______ wave.
Answer
(a) Transverse (b) Electromagnetic (c) Longitudinal (d) Both transverse and longitudinal
✅ Answer: (c) Longitudinal
ExplanationSound is a longitudinal wave — the particles of the medium vibrate parallel to the direction of wave propagation. It travels as compressions (high pressure regions) and rarefactions (low pressure regions). Light is transverse; sound requires a material medium to travel.
2. The speed of sound is maximum in ______.
Answer
(a) Air (b) Water (c) Vacuum (d) Steel
✅ Answer: (d) Steel
ExplanationSound travels fastest in solids because molecules are closest together and vibrations are transmitted faster. Speed in steel ≈ 5100 m/s; in water ≈ 1500 m/s; in air ≈ 340 m/s. Sound cannot travel in vacuum (no medium).
3. The frequency range of human hearing is ______.
Answer
(a) 0 Hz to 20 Hz (b) 20 Hz to 20,000 Hz (c) 20,000 Hz to 40,000 Hz (d) 0 Hz to 100 Hz
✅ Answer: (b) 20 Hz to 20,000 Hz
ExplanationHuman ears can detect sounds between 20 Hz and 20 kHz (20,000 Hz). This is called the audible range. Sounds below 20 Hz are infrasound; sounds above 20 kHz are ultrasound.
4. Sound with frequency above 20,000 Hz is called ______.
Answer
(a) Infrasound (b) Audible sound (c) Ultrasound (d) Supersound
✅ Answer: (c) Ultrasound
ExplanationUltrasound has frequency > 20,000 Hz (20 kHz). Uses: sonography (medical imaging), SONAR, cleaning industrial equipment, detecting flaws in metals, bats' navigation. Infrasound is < 20 Hz — produced by earthquakes, used by elephants and whales.
5. The minimum distance from a wall needed to hear an echo is ______.
Answer
(a) 10 m (b) 17 m (c) 25 m (d) 340 m
✅ Answer: (b) 17 m
ExplanationFor a separate echo, the sound must travel to the wall and back (total 2 × 17 = 34 m) in at least 0.1 s (the persistence of human hearing). Speed of sound ≈ 340 m/s. Distance = speed × time / 2 = 340 × 0.1 / 2 = 17 m.
6. SONAR uses ______.
Answer
(a) Infrasound (b) Ultrasound (c) Radio waves (d) Visible light
✅ Answer: (b) Ultrasound
ExplanationSONAR (Sound Navigation And Ranging) uses ultrasound to detect underwater objects (submarines, fish shoals, ocean floor depth). Ultrasound pulses are sent out; the time taken for the echo to return is used to calculate distance.
7. The pitch of a sound depends on its ______.
Answer
(a) Amplitude (b) Wavelength (c) Frequency (d) Speed
✅ Answer: (c) Frequency
Explanation
- Pitch (how high or low a sound seems) depends on frequency — higher frequency = higher pitch (shriller sound)
- Loudness depends on amplitude — larger amplitude = louder sound
- Timbre/quality depends on the waveform (harmonic content)
8. A ship uses SONAR and receives the echo after 4 seconds. If the speed of sound in water is 1500 m/s, the depth of the ocean is ______.
Answer
(a) 6000 m (b) 3000 m (c) 1500 m (d) 750 m
✅ Answer: (b) 3000 m
ExplanationTime for echo = 4 s; sound travels to floor AND back. One-way time = 4/2 = 2 s Depth = speed × time = 1500 × 2 = 3000 m
9. Reverberation is caused by ______.
Answer
(a) Multiple reflections of sound (b) Diffraction of sound (c) Absorption of sound (d) Refraction of sound
✅ Answer: (a) Multiple reflections of sound
ExplanationReverberation is the persistence of sound due to multiple reflections from surfaces (walls, ceiling, floor) in an enclosed space. If reverberation is too long, it causes echoes and muddiness. Concert halls are designed with sound-absorbing materials to control reverberation time (Sabine's formula).
10. Bats navigate using ______.
Answer
(a) Visible light (b) Infrared radiation (c) Echolocation (ultrasound) (d) Magnetic field
✅ Answer: (c) Echolocation (ultrasound)
ExplanationBats emit high-frequency ultrasound pulses and listen for the echoes. The returning echoes tell them the distance, size, and speed of objects ahead — this is called echolocation. This allows them to fly and hunt insects in complete darkness. Dolphins also use echolocation.
Quick Revision — Key Acoustics Concepts
| Concept | Key Fact |
|---|
| Nature of sound | Longitudinal mechanical wave |
| Sound in vacuum | Cannot travel |
| Speed of sound in air | ~340 m/s (at 25°C) |
| Speed of sound in steel | ~5100 m/s |
| Audible range | 20 Hz – 20,000 Hz |
| Infrasound | < 20 Hz |
| Ultrasound | > 20,000 Hz |
| Echo distance | Minimum 17 m from wall |
| Persistence of hearing | 0.1 second |
| SONAR depth formula | d = v × t / 2 |
Common MCQ Traps
- Pitch vs Loudness: Pitch = frequency; Loudness = amplitude. Don't confuse.
- Echo formula: Total distance = v × t; depth or half-distance = v × t / 2
- Sound in vacuum: Zero — sound CANNOT travel without a medium
- Ultrasound not infrasound for SONAR/bats: Both are above 20,000 Hz (ultrasound)
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