15 extra multiple-choice questions for Acoustics (10th Standard Science, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
Which of the following statements correctly describes the propagation of sound waves?
- A. Sound waves travel as longitudinal waves in gases and transverse waves in solids.
- B. Sound waves can propagate through all states of matter as longitudinal waves.Correct
- C. Sound waves do not require a material medium and can travel through vacuum.
- D. Sound waves travel fastest in gaseous media due to their high elasticity.
Explanation. Sound waves are longitudinal mechanical waves that require a solid, liquid, or gaseous material medium to propagate. They cannot travel through a vacuum.
Q2
If the atmospheric pressure is increased by four times while the temperature of the air is kept constant, what happens to the velocity of sound?
- A. It becomes four times its original value.
- B. It is halved.
- C. It remains unchanged.Correct
- D. It is doubled.
Explanation. For a gas, the velocity of sound is independent of pressure when the temperature is constant. This is because any change in pressure is compensated by a corresponding change in density.
Q3
When a longitudinal sound wave traveling in air strikes a rigid wall, how is the compression of the wave reflected?
- A. It is reflected back as a rarefaction.
- B. It is reflected back as a compression.Correct
- C. It is completely absorbed by the wall.
- D. It is converted into a transverse crest.
Explanation. When a compression strikes a rigid wall (denser medium), the wall exerts an equal and opposite reaction force, causing the compression to be reflected back as a compression.
Q4
When a longitudinal wave traveling in a solid strikes the interface of a rarer medium like air, what happens to the compression?
- A. It is reflected back as a compression.
- B. It is reflected back as a rarefaction.Correct
- C. It is refracted without reflection.
- D. It is converted into a transverse trough.
Explanation. At the boundary of a rarer medium, the particles are free to move. This produces a rarefaction at the interface, so the compression is reflected as a rarefaction.
Q5
Why does the speed of sound in air increase as the relative humidity of the air increases?
- A. Humid air is denser than dry air, which increases the speed of sound.
- B. Humid air is less density than dry air, which increases the speed of sound.Correct
- C. Water molecules in humid air act as resonators to amplify the wave.
- D. The presence of humidity always increases the temperature of the air.
Explanation. The density of water vapor is less than that of dry air. Therefore, when humidity increases, the effective density of air decreases, causing the speed of sound to increase.
Q6
If the velocity of sound in a planet's atmosphere is \(800\text{ ms}^{-1}\), what is the minimum distance between the source of sound and an obstacle to hear a distinct echo?
- A. 40.0 mCorrect
- B. 20.0 m
- C. 80.0 m
- D. 17.2 m
Explanation. Using the persistence of hearing of \(0.1\text{ s}\), the minimum distance is calculated using \(d = \frac{v \times t}{2}\). Substituting \(v = 800\text{ ms}^{-1}\) and \(t = 0.1\text{ s}\) yields \(40.0\text{ m}\).
Q7
What is the specific geometric shape of the walls in a whispering gallery, such as the one in St. Paul's Cathedral in London, that allows sound to travel long distances through multiple reflections?
- A. Parabolic
- B. Spherical
- C. EllipticalCorrect
- D. Hyperbolic
Explanation. Whispering galleries are built with elliptically shaped walls. Sound waves produced at one focus of the ellipse are continuously reflected toward the other focus, allowing whispering to be heard clearly.
Q8
A stationary sound source is positioned at the center of a circular path. If an observer moves along this circular path at a constant speed, what is the apparent frequency heard by the observer?
- A. It increases continuously as the observer moves.
- B. It remains equal to the actual frequency of the source.Correct
- C. It decreases continuously as the observer moves.
- D. It fluctuates periodically above and below the actual frequency.
Explanation. Since the observer moves in a circle around the source at the center, the distance between them remains constant. There is no relative motion along the line of propagation, so no Doppler shift occurs.
Q9
Under which of the following circumstances will there be no Doppler effect, meaning the apparent frequency heard by a listener is exactly equal to the source frequency?
- A. When the source is at rest and the listener moves toward it.
- B. When the listener is at rest and the source moves away from it.
- C. When the source and the listener move in mutually perpendicular directions.Correct
- D. When both the source and the listener move toward each other.
Explanation. According to the conditions for no Doppler effect, no frequency shift is observed when the source and the listener are moving in mutually perpendicular directions.
Q10
Where is a speaker positioned relative to a curved concave sound board in an auditorium to ensure the sound is reflected parallelly toward the audience?
- A. At the center of curvature of the concave board.
- B. At the focus of the concave board.Correct
- C. Directly in contact with the concave board.
- D. At any arbitrary position behind the board.
Explanation. The speaker is placed at the focus of the concave sound board. The sound waves reflecting off the concave surface are directed as parallel beams toward the audience, improving sound quality.
Q11
Which of the following devices is a hearing aid designed with one wide end and one narrow end to concentrate sound entering the ear canal through multiple reflections?
- A. Megaphone
- B. Ear trumpetCorrect
- C. Sound board
- D. Stethoscope
Explanation. An ear trumpet is a hearing aid with a wide end to receive sound and a narrow end that fits in the ear. Multiple reflections concentrate the sound waves, increasing their intensity.
Q12
Which of the following types of sound waves cannot be heard by the human ear because their frequency is below the audible range?
- A. Audible waves
- B. Ultrasonic waves
- C. Infrasonic wavesCorrect
- D. Radio waves
Explanation. Infrasonic waves are sound waves with frequencies below \(20\text{ Hz}\), which is below the lower limit of the human audible range (\(20\text{ Hz}\) to \(20,000\text{ Hz}\)).
Q13
Using the speed of sound values given in the textbook, arrange copper, water, and air (at 20 °C) in decreasing order of their sound velocities.
- A. Water > Copper > Air
- B. Copper > Water > AirCorrect
- C. Air > Water > Copper
- D. Copper > Air > Water
Explanation. Sound travels fastest in solids, then liquids, and slowest in gases. According to the textbook, the speed of sound in copper is \(5010\text{ ms}^{-1}\), in water is \(1493\text{ ms}^{-1}\), and in air at \(20\,^\circ\text{C}\) is \(343\text{ ms}^{-1}\).
Q14
For a gaseous medium, how does the velocity of sound waves vary with its absolute temperature (\(T\))?
- A. It is directly proportional to \(T\).
- B. It is directly proportional to the square root of \(T\).Correct
- C. It is inversely proportional to \(T\).
- D. It is independent of temperature.
Explanation. In a gaseous medium, the velocity of sound is directly proportional to the square root of its absolute temperature (\(v \propto \sqrt{T}\)).
Q15
In marine navigation, how is the Doppler effect utilized in SONAR systems?
- A. To measure the salinity of seawater.
- B. To determine the speed of marine animals and submarines.Correct
- C. To capture high-resolution images of the seabed.
- D. To measure the temperature of seawater at different depths.
Explanation. In SONAR, the Doppler effect is used to determine the speed of submarines and marine animals by measuring the shift in frequency between transmitted and received signals.
