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CHAPTER 10 — SOUND
Class IX Science — Sound Waves
Preeti Kushwah Classes — Unit Test (Set 3: Advanced)
General Instructions:
1. All questions are compulsory.
2. Section A has 6 questions of 1 mark each.
3. Section B has 5 questions of 2 marks each.
4. Section C has 4 questions of 3 marks each.
5. Section D has 2 questions of 5 marks each.
6. Draw neat figures wherever required. Show all calculations clearly.
1. All questions are compulsory.
2. Section A has 6 questions of 1 mark each.
3. Section B has 5 questions of 2 marks each.
4. Section C has 4 questions of 3 marks each.
5. Section D has 2 questions of 5 marks each.
6. Draw neat figures wherever required. Show all calculations clearly.
Section A — (1 Mark Each) [6 × 1 = 6]
Q1.1
Two children are at opposite ends of an aluminium rod. One strikes the end with a stone. If the speed of sound in air is 346 m/s and in aluminium is 6420 m/s, what is the ratio of times taken by sound in air to that in aluminium to reach the other child?
Q2.1
A source of sound vibrates 100 times in one second. How many times does it vibrate in one minute?
Q3.1
Name the type of waves that cannot travel through vacuum.
Q4.1
What is the minimum time gap between the original sound and the echo for a distinct echo to be heard?
Q5.1
How does temperature affect the speed of sound in air?
Q6.1
Name the medical technique that uses ultrasound to break kidney stones.
Section B — (2 Marks Each) [5 × 2 = 10]
Q7.2
In which of the three media — air, water, or iron — does sound travel the fastest at a particular temperature? Explain why.
Q8.2
A sound wave has frequency 2 kHz and wavelength 35 cm. How long will it take to travel 1.5 km?
Q9.2
At 0°C, the speed of sound in air is 331 m/s. Calculate the minimum distance required to hear an echo at this temperature. How does it compare with the value at 20°C?
Q10.2
Distinguish between infrasound, audible sound and ultrasound with one example of each.
Q11.2
State the two laws of reflection of sound. How can they be verified experimentally?
Section C — (3 Marks Each) [4 × 3 = 12]
Q12.3
The audible range for a person is 20 Hz to 20 kHz. Taking speed of sound in air as 344 m/s, calculate:
(a) Wavelength for 20 Hz [1]
(b) Wavelength for 20,000 Hz [1]
(c) Which frequency has a longer wavelength and by how much (ratio)? [1]
(a) Wavelength for 20 Hz [1]
(b) Wavelength for 20,000 Hz [1]
(c) Which frequency has a longer wavelength and by how much (ratio)? [1]
Q13.3
Compare the three characteristics of sound — pitch, loudness, and quality — in a tabular form with columns: characteristic, depends on, unit (if any), and one example.
Q14.3
Explain how ultrasound is used for:
(a) Detecting flaws in metal blocks — with diagram showing wave path in defective vs. non-defective piece [2]
(b) Medical imaging (ultrasonography) — state why it is preferred over X-rays [1]
(a) Detecting flaws in metal blocks — with diagram showing wave path in defective vs. non-defective piece [2]
(b) Medical imaging (ultrasonography) — state why it is preferred over X-rays [1]
Q15.3
Describe an experiment to show that sound follows the laws of reflection. Draw a neat diagram and state the two laws.
Section D — (5 Marks Each) [2 × 5 = 10]
Q16.5
(a) Draw a neat labelled diagram of the human ear showing outer ear, middle ear and inner ear. [2]
(b) Describe the step-by-step process of how we hear a sound — from sound entering the pinna to the brain interpreting it. [2]
(c) What is the role of the three ossicles (hammer, anvil, stirrup)? By how much do they amplify vibrations? [1]
(b) Describe the step-by-step process of how we hear a sound — from sound entering the pinna to the brain interpreting it. [2]
(c) What is the role of the three ossicles (hammer, anvil, stirrup)? By how much do they amplify vibrations? [1]
Q17.5
(a) A sonar device on a ship sends ultrasonic signals towards the seabed. The signal returns after 3.62 seconds. If speed of sound in sea water is 1531 m/s, calculate the depth of the sea at that point. [2]
(b) Explain why sound cannot travel through vacuum. Describe an experiment that proves this. [2]
(c) What type of wave is a sound wave? Why is it classified so? [1]
(b) Explain why sound cannot travel through vacuum. Describe an experiment that proves this. [2]
(c) What type of wave is a sound wave? Why is it classified so? [1]
Bonus Question (Optional) [2 Marks]
Q18.2
★ A girl standing in front of a cliff claps her hands and hears the echo after 1.5 seconds. Speed of sound = 340 m/s.
(a) How far is the cliff? [1]
(b) If she moves 85 m closer to the cliff, will she still hear a distinct echo? Justify with calculation. [1]
(a) How far is the cliff? [1]
(b) If she moves 85 m closer to the cliff, will she still hear a distinct echo? Justify with calculation. [1]
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Answer Key & Detailed Solutions
Q1. [1 Mark]
Ratio = tair / taluminium = (d / vair) / (d / valu) = valu / vair = 6420 / 346 = 18.55 : 1.
Sound takes about 18.55 times longer in air than in aluminium.
Sound takes about 18.55 times longer in air than in aluminium.
Q2. [1 Mark]
100 × 60 = 6000 vibrations.
Q3. [1 Mark]
Mechanical waves (sound waves). Electromagnetic waves can travel through vacuum, but mechanical waves cannot.
Q4. [1 Mark]
0.1 second (1/10 of a second) — this is the persistence of hearing.
Q5. [1 Mark]
Speed of sound increases with increase in temperature. At 0°C it is 331 m/s, at 20°C it is 344 m/s.
Q6. [1 Mark]
Lithotripsy (Extracorporeal Shock Wave Lithotripsy — ESWL).
Q7. [2 Marks]
Iron (solid). In solids, particles are closely packed and strongly bonded. Vibrations transfer quickly from one particle to the next.
Speed in iron: 5130 m/s vs water: 1498 m/s vs air: 344 m/s.
[1 mark for answer, 1 mark for explanation]
Speed in iron: 5130 m/s vs water: 1498 m/s vs air: 344 m/s.
[1 mark for answer, 1 mark for explanation]
Q8. [2 Marks]
v = f × λ = 2000 × 0.35 = 700 m/s.
Time = distance / speed = 1500 / 700 = 2.14 seconds (approximately).
[1 mark for speed, 1 mark for time]
Time = distance / speed = 1500 / 700 = 2.14 seconds (approximately).
[1 mark for speed, 1 mark for time]
Q9. [2 Marks]
dmin = v × t / 2 = 331 × 0.1 / 2 = 16.55 m.
At 20°C: dmin = 344 × 0.1 / 2 = 17.2 m.
The minimum distance is slightly less at 0°C because sound is slower.
[1 mark for calculation at 0°C, 1 mark for comparison with 20°C]
At 20°C: dmin = 344 × 0.1 / 2 = 17.2 m.
The minimum distance is slightly less at 0°C because sound is slower.
[1 mark for calculation at 0°C, 1 mark for comparison with 20°C]
Q10. [2 Marks]
Infrasound: frequency below 20 Hz. Example: earthquakes, elephant communication.
Audible sound: 20 Hz to 20,000 Hz. Example: human speech (500–5000 Hz).
Ultrasound: above 20,000 Hz. Example: bat echolocation, dog whistles.
[½ mark for each definition, ½ mark for examples]
Audible sound: 20 Hz to 20,000 Hz. Example: human speech (500–5000 Hz).
Ultrasound: above 20,000 Hz. Example: bat echolocation, dog whistles.
[½ mark for each definition, ½ mark for examples]
Q11. [2 Marks]
Laws:
(i) Angle of incidence = Angle of reflection (∠i = ∠r).
(ii) Incident wave, reflected wave, and normal all lie in the same plane.
Experiment: Two long tubes angled towards a smooth wall. Clock at one end, ear at other. Sound heard clearest when tube angles are equal from wall — verifying ∠i = ∠r.
[1 mark for laws, 1 mark for experiment]
(i) Angle of incidence = Angle of reflection (∠i = ∠r).
(ii) Incident wave, reflected wave, and normal all lie in the same plane.
Experiment: Two long tubes angled towards a smooth wall. Clock at one end, ear at other. Sound heard clearest when tube angles are equal from wall — verifying ∠i = ∠r.
[1 mark for laws, 1 mark for experiment]
Q12. [3 Marks]
(a) λ = v / f = 344 / 20 = 17.2 m. [1 mark]
(b) λ = 344 / 20000 = 0.0172 m = 1.72 cm. [1 mark]
(c) 20 Hz has the longer wavelength. Ratio = 17.2 / 0.0172 = 1000 : 1. [1 mark]
(b) λ = 344 / 20000 = 0.0172 m = 1.72 cm. [1 mark]
(c) 20 Hz has the longer wavelength. Ratio = 17.2 / 0.0172 = 1000 : 1. [1 mark]
Q13. [3 Marks]
Pitch — depends on frequency — no separate unit — Example: whistle (high pitch) vs drum (low pitch).
Loudness — depends on amplitude — unit: decibel (dB) — Example: loud music vs whisper.
Quality / Timbre — depends on waveform — no unit — Example: sitar vs flute playing the same note.
[1 mark per characteristic]
Loudness — depends on amplitude — unit: decibel (dB) — Example: loud music vs whisper.
Quality / Timbre — depends on waveform — no unit — Example: sitar vs flute playing the same note.
[1 mark per characteristic]
Q14. [3 Marks]
(a) Flaw detection: Ultrasound waves are sent through the metal block. In a non-defective block, waves pass through and are detected on the other side. In a defective block (with crack), waves reflect back from the crack and are detected — showing smaller amplitude on the far side and an extra peak in the reflected signal graph. [2 marks]
(b) Ultrasonography: Ultrasound waves reflected from organs/tissues create images. Preferred over X-rays because: ultrasound is safe, non-ionizing, no harmful radiation. Especially safe for examining foetus during pregnancy. [1 mark]
(b) Ultrasonography: Ultrasound waves reflected from organs/tissues create images. Preferred over X-rays because: ultrasound is safe, non-ionizing, no harmful radiation. Especially safe for examining foetus during pregnancy. [1 mark]
Q15. [3 Marks]
Experiment: Two long pipes/tubes arranged at angles to a smooth hard wall. A ticking clock placed at the open end of one tube. Listener places ear at the other tube. By adjusting angles, sound is heard loudest when angle of incidence equals angle of reflection.
Diagram: Two tubes, wall with normal line, angles i and r marked equal.
Laws:
(i) ∠i = ∠r (angle of incidence equals angle of reflection).
(ii) Incident wave, reflected wave, and normal lie in the same plane.
[1 mark for experiment description, 1 mark for diagram, 1 mark for laws]
Diagram: Two tubes, wall with normal line, angles i and r marked equal.
Laws:
(i) ∠i = ∠r (angle of incidence equals angle of reflection).
(ii) Incident wave, reflected wave, and normal lie in the same plane.
[1 mark for experiment description, 1 mark for diagram, 1 mark for laws]
Q16. [5 Marks]
(a) Diagram: Pinna → Ear Canal → Eardrum → Hammer → Anvil → Stirrup → Cochlea → Auditory Nerve → Brain. Three sections labelled: Outer Ear, Middle Ear, Inner Ear. [2 marks]
(b) Steps:
1. Pinna collects sound waves.
2. Waves travel through the ear canal.
3. Strike the eardrum, causing it to vibrate.
4. Ossicles amplify the vibrations.
5. Vibrations reach the cochlea where fluid waves bend hair cells.
6. Hair cells generate electrical signals.
7. Auditory nerve sends signals to the brain.
8. Brain interprets the signals as sound. [2 marks]
(c) The three ossicles (hammer, anvil, stirrup) are the smallest bones in the body. They amplify vibrations from the eardrum by about 20 times before passing them to the inner ear. [1 mark]
(b) Steps:
1. Pinna collects sound waves.
2. Waves travel through the ear canal.
3. Strike the eardrum, causing it to vibrate.
4. Ossicles amplify the vibrations.
5. Vibrations reach the cochlea where fluid waves bend hair cells.
6. Hair cells generate electrical signals.
7. Auditory nerve sends signals to the brain.
8. Brain interprets the signals as sound. [2 marks]
(c) The three ossicles (hammer, anvil, stirrup) are the smallest bones in the body. They amplify vibrations from the eardrum by about 20 times before passing them to the inner ear. [1 mark]
Q17. [5 Marks]
(a) d = v × t / 2 = 1531 × 3.62 / 2 = 5542.22 / 2 = 2771.11 m.
Depth ≈ 2771 m. [2 marks]
(b) Sound needs a material medium. Bell jar experiment: Bell ringing inside a jar; pump out air; sound fades; in vacuum, no sound is heard. Let air back in — sound returns. This proves sound cannot travel through vacuum. [2 marks]
(c) Sound is a longitudinal mechanical wave. Longitudinal because particles vibrate parallel to the direction of propagation (creating compressions and rarefactions). Mechanical because it requires a material medium. [1 mark]
Depth ≈ 2771 m. [2 marks]
(b) Sound needs a material medium. Bell jar experiment: Bell ringing inside a jar; pump out air; sound fades; in vacuum, no sound is heard. Let air back in — sound returns. This proves sound cannot travel through vacuum. [2 marks]
(c) Sound is a longitudinal mechanical wave. Longitudinal because particles vibrate parallel to the direction of propagation (creating compressions and rarefactions). Mechanical because it requires a material medium. [1 mark]
Q18. Bonus [2 Marks]
(a) d = v × t / 2 = 340 × 1.5 / 2 = 255 m. The cliff is 255 m away. [1 mark]
(b) New distance = 255 − 85 = 170 m.
Time for echo = 2d / v = 2 × 170 / 340 = 1.0 s.
Minimum time needed = 0.1 s. Since 1.0 s > 0.1 s, yes, she will still hear a distinct echo. [1 mark]
(b) New distance = 255 − 85 = 170 m.
Time for echo = 2d / v = 2 × 170 / 340 = 1.0 s.
Minimum time needed = 0.1 s. Since 1.0 s > 0.1 s, yes, she will still hear a distinct echo. [1 mark]