NDA (I) 2025 General Ability Test · Question 149 of 150

Correct — B, Reflection of sound waves. An echo is the repetition of a sound heard after the original, produced when sound waves bounce back (reflect) from a distant hard surface such as a cliff or wall. To be heard as a separate echo the reflected sound must return after at least about 0.1 second, which needs the reflecting surface to be roughly 17 metres or more away (sound travels about 344 m/s in air).

• (a)Refraction of sound waves — Refraction is the bending of sound as it passes between layers of different temperature or density — it does not send the sound back to produce an echo.
• (c)Diffraction of sound waves — Diffraction is the spreading of sound around obstacles or through openings; it explains why we hear around corners, not the return of an echo.
• (d)Resonance due to sound waves — Resonance is the large-amplitude vibration of a body driven at its natural frequency; it amplifies sound but does not create the delayed repeat that defines an echo.

Sound waves obey the same reflection rules as other waves. When they strike a large, hard surface they bounce back; if the reflected wave reaches the ear at least about 0.1 s after the direct sound, the brain perceives it as a separate echo. Multiple reflections merging together instead produce reverberation. The same principle underlies SONAR and echocardiography.

All four options are wave behaviours of sound, but only reflection sends the sound back to the listener. The delay and the need for a distant hard surface are the fingerprints of an echo, distinguishing it from refraction, diffraction and resonance.

• An echo is heard when sound reflects off a distant surface and returns after roughly 0.1 s or more.
• The minimum distance to hear a distinct echo in air is about 17 m (speed of sound ≈ 344 m/s at 22 °C).
• Many reflections merging together give reverberation rather than a clear echo.
• Echo (reflection of sound) is the basis of SONAR for measuring ocean depth and detecting objects.

• Confusing reverberation (many overlapping reflections) with a distinct echo.
• Picking resonance or diffraction because they are also sound phenomena.

Asked directly about the cause of an echo, or through numericals on echo time, minimum distance, or SONAR depth-finding.

No directly related past PYQ was found.

• practice — not a real PYQ

  The technique that uses reflection of sound (ultrasound) to measure the depth of the sea is

  • (a)RADAR
  • (b)SONAR
  • (c)LASER
  • (d)MRI

  Answer(b) SONAR — it times echoes of ultrasound reflected from the seabed to find the depth.

• practice — not a real PYQ

  The persistence of sound due to repeated reflections in a large hall is called

  • (a)Echo
  • (b)Reverberation
  • (c)Resonance
  • (d)Refraction

  Answer(b) Reverberation — overlapping reflections that prolong the sound rather than give a single clear echo.