Sound — Previous Year Questions (Class 9 Science)
Sound is a mechanical wave produced by vibrations. Understand frequency, wavelength, speed, and properties like reflection and refraction of sound.
TL;DR: Sound is a mechanical wave produced by vibrations. Understand frequency, wavelength, speed, and properties like reflection and refraction of sound.
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Sound is a mechanical wave produced by vibrations. Understand frequency, wavelength, speed, and properties like reflection and refraction of sound.
Sound — Previous Year Questions with Solutions
Q (2023, 3 marks): Define frequency and wavelength. What is their relationship?
Answer: Frequency (f): Number of complete vibrations per unit time. Unit: Hertz (Hz). Symbol: f or ν
Wavelength (λ): Distance between two consecutive crests or troughs. Unit: meter (m)
Relationship (Wave equation): v = f × λ
Where v = speed of wave
Example: If f = 100 Hz and v = 340 m/s, then λ = 340/100 = 3.4 m
Q (2022, 2 marks): The speed of sound in air is 340 m/s. Calculate the wavelength of a sound wave with frequency 256 Hz.
Answer: Using v = f × λ
340 = 256 × λ
λ = 340 / 256
λ ≈ 1.33 m
Q (2023, 3 marks): Explain why sound travels faster in solids than in gases with molecular reasoning.
Answer: Sound speed depends on medium properties:
In solids:
- Particles are tightly packed with strong intermolecular forces
- Vibration is rapidly transmitted from one particle to next
- Less distance between particles
- Speed ≈ 5000 m/s in steel
In liquids:
- Particles are more loosely packed than solids
- Speed ≈ 1500 m/s in water
In gases:
- Particles are far apart with weak intermolecular forces
- Slow transmission of vibration
- Speed ≈ 340 m/s in air
Conclusion: Speed of sound ∝ Density and elasticity of medium
Q (2021, 3 marks): What is the audible range of frequencies for human hearing? Why do we use ultrasound and infrasound in different applications?
Answer: Audible range: 20 Hz to 20,000 Hz (20 kHz) for humans
Frequencies below 20 Hz = Infrasound
Frequencies above 20 kHz = Ultrasound
Applications of ultrasound (>20 kHz):
1. Medical imaging (ultrasound scanning in pregnancy)
2. Dental cleaning
3. Industrial welding
4. Flaw detection in materials
5. Navigation in bats and dolphins
Applications of infrasound (<20 Hz):
1. Communication by large animals (whales, elephants)
2. Detection of earthquakes and tsunamis
3. Studying weather patterns
4. Industrial measurements
Q (2022, 3 marks): A sound wave travels from air into water. Explain what happens to frequency, wavelength, and speed.
Answer: When sound travels from air to water:
Frequency: REMAINS CONSTANT (depends on source)
Speed: INCREASES (from 340 m/s in air to ~1500 m/s in water)
Wavelength: INCREASES
Reason: Using v = f × λ
Since f is constant and v increases, λ must increase
Example: f = 1000 Hz
In air: λ = 340/1000 = 0.34 m
In water: λ = 1500/1000 = 1.5 m
This change in wavelength causes refraction at the boundary
Q (2023, 3 marks): Explain the phenomenon of echo. What is the minimum time gap required to hear a distinct echo? (speed of sound = 340 m/s)
Answer: Echo: Reflection of sound from a surface that reaches the listener after reflecting from the original sound.
For a distinct echo:
- Sound must travel to the reflecting surface and back
- Minimum time delay = 0.1 second (between original and echo)
Minimum distance = ?:
Distance = (Speed × Time) / 2 (divide by 2 because sound travels twice)
Distance = (340 × 0.1) / 2 = 34 / 2 = 17 m
Therefore: Minimum distance from reflecting surface ≈ 17 m for distinct echo
Conditions for echo:
1. Reflecting surface must be smooth and rigid
2. Distance must be sufficient (≥17 m)
3. Time gap must be ≥0.1 s
Frequently Asked Questions
What is the difference between sound and noise?
Sound is an organized, regular vibration with distinct frequency (pleasing to ear). Noise is disorganized, irregular vibrations with multiple frequencies (unpleasant). Both travel as mechanical waves, but sound has a clear pitch while noise doesn't.
How does reflection of sound differ from refraction of sound?
Reflection: Sound bounces back from a surface (e.g., echo). Refraction: Sound bends when traveling from one medium to another due to change in speed. Reflection occurs at boundaries; refraction occurs while entering a different medium.
More Class 9 Science PYQs
- Chemical Reactions and Equations
- Light (Reflection and Refraction)
- Electricity
- Acids, Bases and Salts
- Metals and Non-metals
- Life Processes
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