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CHAPTER 10 — SOUND
Class IX Science — Sound Waves
Preeti Kushwah Classes — Unit Test (Set 2: Intermediate)
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.
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.
Section A — (1 Mark Each) [6 × 1 = 6]
Q1.1
A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source. What is the time interval between successive compressions from the source?
Q2.1
Which characteristic of sound helps you identify your friend by his voice while sitting in a dark room?
Q3.1
The frequency of ultrasound is:
(a) below 20 Hz (b) between 20 Hz and 20 kHz (c) above 20,000 Hz
Choose the correct option.
(a) below 20 Hz (b) between 20 Hz and 20 kHz (c) above 20,000 Hz
Choose the correct option.
Q4.1
What is the speed of sound in air at 0°C?
Q5.1
Does sound travel faster in humid air or dry air?
Q6.1
Which part of the human ear converts sound vibrations into electrical signals?
Section B — (2 Marks Each) [5 × 2 = 10]
Q7.2
A sound wave has frequency 220 Hz and speed 440 m/s in a given medium. Find its wavelength.
Q8.2
Explain why the ceilings of concert halls are curved.
Q9.2
What is reverberation? Give two methods to reduce it.
Q10.2
Distinguish between pitch and loudness of sound.
Q11.2
How do bats use ultrasound to navigate in the dark?
Section C — (3 Marks Each) [4 × 3 = 12]
Q12.3
Describe with the help of a diagram how compressions and rarefactions are produced in air near a source of sound. How does this sound reach your ear?
Q13.3
Calculate the wavelength of sound waves corresponding to the extreme frequencies of the audible range (20 Hz and 20,000 Hz). Speed of sound in air = 344 m/s. Which sound has a longer wavelength?
Q14.3
Draw a neat labelled diagram of the human ear. Explain the function of:
(i) Pinna (ii) Eardrum (iii) Cochlea
(i) Pinna (ii) Eardrum (iii) Cochlea
Q15.3
Explain how sound reflection is used in:
(i) a stethoscope (ii) a megaphone
(i) a stethoscope (ii) a megaphone
Section D — (5 Marks Each) [2 × 5 = 10]
Q16.5
(a) Define the following characteristics of a sound wave: wavelength, frequency, time period, amplitude. [4]
(b) A sound wave has frequency 2 kHz and wavelength 35 cm. How long will it take to travel 1.5 km? [1]
(b) A sound wave has frequency 2 kHz and wavelength 35 cm. How long will it take to travel 1.5 km? [1]
Q17.5
(a) Describe any four applications of ultrasound. [4]
(b) Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen. Explain why. [1]
(b) Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen. Explain why. [1]
Bonus Question (Optional) [2 Marks]
Q18.2
★ Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend? Explain with reason. Can you think of any alternative way to communicate on the moon?
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Answer Key & Detailed Solutions
Q1. [1 Mark]
T = 1/f = 1/500 = 0.002 s.
Distance does not affect the time period — it depends only on the frequency of the source.
Distance does not affect the time period — it depends only on the frequency of the source.
Q2. [1 Mark]
Quality (Timbre). Each person’s voice produces a unique waveform due to different harmonics.
Q3. [1 Mark]
(c) above 20,000 Hz. Ultrasound refers to sound waves with frequency greater than 20 kHz, which is above the upper limit of human hearing.
Q4. [1 Mark]
331 m/s.
Q5. [1 Mark]
Humid air. Water vapour molecules are lighter than nitrogen and oxygen molecules, so the density of humid air is lower and sound travels faster in it.
Q6. [1 Mark]
Cochlea (the hair cells inside the cochlea convert mechanical vibrations into electrical signals sent to the brain via the auditory nerve).
Q7. [2 Marks]
λ = v / f = 440 / 220 = 2 m.
[1 mark for formula, 1 mark for correct calculation]
[1 mark for formula, 1 mark for correct calculation]
Q8. [2 Marks]
Curved ceilings reflect sound uniformly towards the audience. This ensures every seat gets clear sound and reduces excessive reverberation and echoes.
[1 mark for reflection concept, 1 mark for explanation]
[1 mark for reflection concept, 1 mark for explanation]
Q9. [2 Marks]
Reverberation: The persistence of sound in an enclosed space due to repeated reflections from walls, ceiling, and floor.
Methods to reduce:
(i) Using sound-absorbing materials like curtains, carpets, and seat cushions.
(ii) Covering walls with acoustic panels or compressed fibreboard.
[1 mark for definition, 1 mark for two methods]
Methods to reduce:
(i) Using sound-absorbing materials like curtains, carpets, and seat cushions.
(ii) Covering walls with acoustic panels or compressed fibreboard.
[1 mark for definition, 1 mark for two methods]
Q10. [2 Marks]
Pitch: How high or low a sound appears; depends on frequency; higher frequency = higher pitch.
Loudness: How loud or soft a sound appears; depends on amplitude; greater amplitude = louder sound; measured in decibels (dB).
[1 mark for pitch, 1 mark for loudness]
Loudness: How loud or soft a sound appears; depends on amplitude; greater amplitude = louder sound; measured in decibels (dB).
[1 mark for pitch, 1 mark for loudness]
Q11. [2 Marks]
Bats emit high-frequency ultrasound pulses. These reflect off objects (insects, obstacles) and return to the bat. By analysing the time delay and direction of the echo, bats judge the position, size, and distance of objects. This is called echolocation.
[1 mark for process description, 1 mark for naming echolocation]
[1 mark for process description, 1 mark for naming echolocation]
Q12. [3 Marks]
When a vibrating object (e.g., tuning fork prong) moves forward, it pushes air particles together, creating a compression (high-pressure region). When it moves back, a rarefaction (low-pressure region) forms. These alternate C–R–C–R pattern spreads outward as a longitudinal wave through the medium until it reaches the ear, making the eardrum vibrate.
[Diagram: Tuning fork with alternating C and R regions spreading outward. 1 mark for diagram, 1 mark for compression/rarefaction explanation, 1 mark for how sound reaches the ear]
[Diagram: Tuning fork with alternating C and R regions spreading outward. 1 mark for diagram, 1 mark for compression/rarefaction explanation, 1 mark for how sound reaches the ear]
Q13. [3 Marks]
For 20 Hz: λ = v / f = 344 / 20 = 17.2 m.
For 20,000 Hz: λ = 344 / 20000 = 0.0172 m = 1.72 cm.
The sound of 20 Hz has a much longer wavelength.
[1 mark each for two calculations, 1 mark for comparison]
For 20,000 Hz: λ = 344 / 20000 = 0.0172 m = 1.72 cm.
The sound of 20 Hz has a much longer wavelength.
[1 mark each for two calculations, 1 mark for comparison]
Q14. [3 Marks]
[Diagram with labelled parts: Pinna, Ear Canal, Eardrum, Ossicles (Hammer, Anvil, Stirrup), Cochlea, Auditory Nerve]
(i) Pinna: Funnel-shaped outer part that collects sound waves from the surroundings and directs them into the ear canal.
(ii) Eardrum: Thin membrane that vibrates when sound waves strike it, converting sound energy into mechanical vibrations.
(iii) Cochlea: Coiled, fluid-filled tube with tiny hair cells. Vibrations create waves in the fluid, bending the hair cells, which generate electrical signals sent to the brain via the auditory nerve.
[1 mark for diagram, ½ mark each for three functions]
(i) Pinna: Funnel-shaped outer part that collects sound waves from the surroundings and directs them into the ear canal.
(ii) Eardrum: Thin membrane that vibrates when sound waves strike it, converting sound energy into mechanical vibrations.
(iii) Cochlea: Coiled, fluid-filled tube with tiny hair cells. Vibrations create waves in the fluid, bending the hair cells, which generate electrical signals sent to the brain via the auditory nerve.
[1 mark for diagram, ½ mark each for three functions]
Q15. [3 Marks]
(i) Stethoscope: Sound of the heartbeat enters the chest piece, undergoes multiple reflections inside the rubber tube, and reaches the doctor’s ears without energy loss. The tube prevents sound from spreading out.
(ii) Megaphone: The cone shape causes sound waves to reflect inward repeatedly, focusing them in one direction so sound travels farther and reaches a larger audience.
[1½ marks each]
(ii) Megaphone: The cone shape causes sound waves to reflect inward repeatedly, focusing them in one direction so sound travels farther and reaches a larger audience.
[1½ marks each]
Q16. [5 Marks]
(a)
Wavelength (λ): The distance between two consecutive compressions or two consecutive rarefactions. Unit: metre (m).
Frequency (f): The number of complete vibrations (oscillations) per second. Unit: hertz (Hz).
Time Period (T): The time taken for one complete vibration. T = 1/f. Unit: second (s).
Amplitude (A): The maximum displacement of the medium particles from their mean position. It determines the loudness of the sound.
[1 mark each]
(b)
v = f × λ = 2000 × 0.35 = 700 m/s.
Time = distance / speed = 1500 / 700 = 2.14 s (approx.).
[1 mark]
Wavelength (λ): The distance between two consecutive compressions or two consecutive rarefactions. Unit: metre (m).
Frequency (f): The number of complete vibrations (oscillations) per second. Unit: hertz (Hz).
Time Period (T): The time taken for one complete vibration. T = 1/f. Unit: second (s).
Amplitude (A): The maximum displacement of the medium particles from their mean position. It determines the loudness of the sound.
[1 mark each]
(b)
v = f × λ = 2000 × 0.35 = 700 m/s.
Time = distance / speed = 1500 / 700 = 2.14 s (approx.).
[1 mark]
Q17. [5 Marks]
(a) Four applications of ultrasound:
(i) Cleaning: Ultrasonic waves dislodge dirt and grease from jewellery, surgical instruments, and electronic components.
(ii) Detecting flaws: Ultrasound passed through metal parts detects internal cracks by analysing reflected wave patterns (ultrasonic flaw detection).
(iii) Medical imaging (Ultrasonography): Ultrasound creates images of internal organs and is safe for examining a foetus during pregnancy.
(iv) Lithotripsy: High-energy ultrasound waves break kidney stones into tiny pieces that can be flushed out naturally.
[1 mark each]
(b) The speed of light (3 × 10⁸ m/s) is much greater than the speed of sound (~344 m/s). Light reaches us almost instantly, but sound takes an appreciable time to travel the same distance. So we see the flash first and hear thunder a few seconds later.
[1 mark]
(i) Cleaning: Ultrasonic waves dislodge dirt and grease from jewellery, surgical instruments, and electronic components.
(ii) Detecting flaws: Ultrasound passed through metal parts detects internal cracks by analysing reflected wave patterns (ultrasonic flaw detection).
(iii) Medical imaging (Ultrasonography): Ultrasound creates images of internal organs and is safe for examining a foetus during pregnancy.
(iv) Lithotripsy: High-energy ultrasound waves break kidney stones into tiny pieces that can be flushed out naturally.
[1 mark each]
(b) The speed of light (3 × 10⁸ m/s) is much greater than the speed of sound (~344 m/s). Light reaches us almost instantly, but sound takes an appreciable time to travel the same distance. So we see the flash first and hear thunder a few seconds later.
[1 mark]
Q18. Bonus [2 Marks]
No. The moon has no atmosphere (vacuum). Sound is a mechanical wave that requires a material medium (solid, liquid, or gas) to propagate. Without air or any medium, sound cannot travel between two people on the moon.
Alternative: They could use radio waves (electromagnetic waves) which do not need a medium and can travel through vacuum. Communication devices like walkie-talkies or wireless radio sets would work. They could also use wired communication devices or sign language.
[1 mark for explanation, 1 mark for alternative]
Alternative: They could use radio waves (electromagnetic waves) which do not need a medium and can travel through vacuum. Communication devices like walkie-talkies or wireless radio sets would work. They could also use wired communication devices or sign language.
[1 mark for explanation, 1 mark for alternative]