NDA (I) 2021 General Ability Test · Question 79 of 149

Correct — A, Reverberation. Reverberation is the persistence of sound in a large enclosed space produced by repeated (multiple) reflections of the sound waves from the walls, ceiling and floor. These many reflected waves reach the listener again and again and overlap before dying away, which is exactly the lingering sound the question describes.

• (b)Dispersion — Dispersion is the splitting of a wave into its component frequencies or colours (as white light through a prism); it has nothing to do with sound persisting by reflection.
• (c)Refraction — Refraction is the bending of a wave when it passes from one medium to another because its speed changes; no repeated reflection is involved.
• (d)Diffraction — Diffraction is the bending or spreading of waves around obstacles or through openings; it does not describe the prolonging of sound in a hall.

When sound is produced in a large closed hall, the waves bounce repeatedly off the surfaces. Because these successive reflections arrive with only tiny time gaps, the ear cannot separate them and hears one continuous, gradually fading sound. This overlapping repeated reflection is called reverberation.

A single reflected sound heard as a separate repetition is an echo, which needs the reflecting surface to be far enough away. Reverberation is the overlapping many-reflection version. Options refraction, dispersion and diffraction are all genuine wave phenomena added to tempt students who spot 'wave' vocabulary.

• Reverberation is the persistence of sound caused by repeated reflections inside an enclosure.
• A single, distinctly heard reflected sound is an echo, which needs the reflector to be roughly 17 m or more away.
• Excessive reverberation is reduced with sound-absorbing materials such as curtains, carpets, seat cushions and acoustic panels.
• Reverberation time is the time taken for the sound intensity to fall to one-millionth of its original value.

• Confusing reverberation (overlapping repeated reflections) with a single, separate echo.
• Choosing refraction or diffraction just because they are wave terms.

Asked directly ('persistence of sound due to repeated reflection is called…') or by contrasting an echo with reverberation.

No directly related past PYQ was found.

• practice — not a real PYQ

  The persistence of sound in a large hall due to repeated reflection is best reduced by

  • (a)polishing the walls smooth
  • (b)covering the walls with sound-absorbing material
  • (c)increasing the size of the hall
  • (d)removing all the seats

  Answer(b) covering the walls with sound-absorbing material — absorbers cut down the repeated reflections that cause reverberation.

• practice — not a real PYQ

  A single distinct sound heard after reflection from a distant surface is called

  • (a)reverberation
  • (b)an echo
  • (c)resonance
  • (d)a beat

  Answer(b) an echo — a separately heard reflected sound, distinct from overlapping reverberation.