Sound Waves
Key Concepts
| # | Concept | One-line Explanation |
|---|---|---|
| 1 | Longitudinal wave | Particle vibration // to wave travel; creates C&R. |
| 2 | Speed formula | v = f λ ; remember v_air ≈ 330 m s⁻¹ at 25 °C. |
| 3 | Factors affecting speed | v ∝ √T (Kelvin) in gas; v_solids > v_liquids > v_gases. |
| 4 | Loudness ∝ (Amplitude)² | 20 dB jump = 10× intensity; 0 dB = human threshold. |
| 5 | Pitch ∝ frequency | Shrill = high f; flat = low f. |
| 6 | Quality/timbre | Depends on number & relative intensity of harmonics. |
| 7 | Reflection | Echo: d_min = 17 m ⇒ t ≥ 0.1 s for distinct hearing. |
| 8 | Resonance | Forced f = natural f ⇒ big amplitude; breaking glass, swing. |
15 Practice MCQs
(Use in tag for self-quiz)
- The speed of sound is maximum in which medium at 25 °C?
A. Dry air
B. Hydrogen gas
C. Steel rail
D. Rain water
Answer: C. Steel rail
Solids have highest modulus ⇒ highest v (≈ 5000 m s⁻¹).
Shortcut: “Solid > Liquid > Gas” – tick the solid option.
Concept tag: Speed dependence on medium.
- A 1 kHz wave has λ = 66 cm in air. Temperature of air is about
A. −6 °C
B. 11 °C
C. 26 °C
D. 41 °C
Answer: C. 26 °C
v = f λ = 1000 × 0.66 = 330 m s⁻¹ ⇒ matches 25–26 °C.
Trick: 330 m s⁻¹ ≈ room temperature; match table.
Tag: v ∝ √T.
- If amplitude doubles, by what factor does loudness increase?
A. 2
B. 4
C. 6
D. 10
Answer: B. 4
Loudness ∝ (Amplitude)².
Tag: Loudness vs amplitude.
- Echo is not heard in a 10 m hall because
A. Absorption is high
B. t < 0.1 s
C. Frequency is low
D. Reflection is weak
Answer: B. t < 0.1 s
2d = 20 m ⇒ t = 20/330 ≈ 0.06 s < 0.1 s; brain merges sound.
Trick: 17 m minimum ⇒ hall needs ≥17 m.
Tag: Reflection & echo.
- The voice of ladies is shrill because of
A. Large amplitude
B. High frequency
C. High speed
D. Large wavelength
Answer: B. High frequency
Pitch ∝ f.
Tag: Pitch.
- SONAR uses which characteristic of sound?
A. Interference
B. Reflection
C. Resonance
D. Diffraction
Answer: B. Reflection
Echo-ranging principle.
Tag: Applications.
- A railway platform announcement spreads around a pillar due to
A. Reflection
B. Refraction
C. Diffraction
D. Polarization
Answer: C. Diffraction
Bending round obstacle.
Tag: Wave behaviour.
- The human ear can normally hear up to
A. 2 kHz
B. 20 kHz
C. 200 kHz
D. 2 MHz
Answer: B. 20 kHz
Upper audible limit.
Tag: Range of hearing.
- Quality of musical note depends on
A. Wavelength
B. Number of overtones
C. Speed
D. Phase
Answer: B. Number of overtones
Timbre decided by harmonic content.
Tag: Quality.
- A 60 dB sound has how many times the intensity of 30 dB sound?
A. 2
B. 10
C. 100
D. 1000
Answer: D. 1000
30 dB → 10³ intensity ratio (10 dB = 10×).
Trick: Difference 30 dB ⇒ 10^(30/10) = 1000.
Tag: Decibel scale.
- In a gas, if temperature is raised 4 times (Kelvin), speed of sound
A. Doubles
B. Triples
C. Quadruples
D. Unchanged
Answer: A. Doubles
v ∝ √T ⇒ √4 = 2.
Tag: Speed vs temperature.
- A tuning fork produces 512 Hz. If prongs are loaded with wax, frequency
A. Increases
B. Decreases
C. No change
D. Becomes zero
Answer: B. Decreases
Added mass ↓ natural f.
Tag: Resonance & natural frequency.
- Ultrasonic waves are used for cleaning by virtue of
A. High speed
B. Cavitation
C. Interference
D. Polarization
Answer: B. Cavitation
Micro-bubble collapse scrubs surface.
Tag: Applications.
- Sound cannot travel through
A. Iron
B. Water
C. Vacuum
D. Air
Answer: C. Vacuum
Needs material medium.
Tag: Medium requirement.
- A thunder is heard 5 s after lightning. Distance of strike is about
A. 0.33 km
B. 1.65 km
C. 3.3 km
D. 16.5 km
Answer: B. 1.65 km
d = v t ≈ 330 × 5 = 1650 m = 1.65 km.
Trick: 3 s ≈ 1 km ⇒ 5 s ≈ 1.65 km.
Tag: Real-life estimation.
Speed Tricks
| Situation | Shortcut | Example |
|---|---|---|
| Convert °C → v_air | Add 0.6 m s⁻¹ per 1 °C above 0 °C | 35 °C ⇒ 330 + 35×0.6 ≈ 351 m s⁻¹ |
| Echo minimum distance | d_min = 17 m (remember “1–7”) | Hall 15 m → no echo |
| dB → intensity ratio | 10^(ΔdB/10) | 40 dB excess → 10⁴ = 10 000× |
| f_unknown with resonance | f = v/4L (closed pipe, fundamental) | 0.2 m pipe ⇒ f ≈ 330/(4×0.2) ≈ 412 Hz |
| Order of speed | “S-L-G” (Solid-Liquid-Gas) | Pick solid when asked “maximum speed” |
Quick Revision
| Point | Detail |
|---|---|
| 1 | Sound = mechanical, longitudinal, needs medium. |
| 2 | v = f λ constant for fixed medium; f fixed by source. |
| 3 | In air v ≈ 332 + 0.6 T(°C) m s⁻¹. |
| 4 | Loudness in dB; 0 dB threshold, 120 dB pain. |
| 5 | Pitch depends on f; quality on harmonics. |
| 6 | Echo condition: 2d ≥ 34 m or t ≥ 0.1 s. |
| 7 | SONAR, Bats, DOLPHIN → ultrasonic reflection. |
| 8 | Ultrasonic > 20 kHz; infrasonic < 20 Hz. |
| 9 | Resonance: forced freq = natural freq → amplitude ↑. |
| 10 | Railway application: Rail-wheel flaw detection uses ultrasonic waves. |
Keep this sheet on your phone; revise daily + solve 15 Q in 6 min → 85 %ile guaranteed.
BETA
