NDA (II) 2022 General Ability Test · Question 145 of 150

Correct — C, ultrasonic waves. Sonar works by echo ranging — a transducer on the ship sends out a short pulse of sound into the water, the pulse reflects off the sea bed or an object, and the receiver times the returning echo. If the pulse takes time t to go and come back and the speed of sound in sea water is about 1500 metres per second, the distance is vt/2. The pulse is made ultrasonic, above the twenty thousand hertz limit of human hearing, for two practical reasons. Ultrasound has a short wavelength, so it can be shaped into a narrow directional beam instead of spreading in all directions, and that short wavelength lets it resolve small targets. It also keeps the ship's own working sound out of the audible band. Electromagnetic waves cannot do this job because they are absorbed within a short distance in sea water, which is why ships use sound rather than radar underwater.

• (a)Infrasonic waves — Infrasound lies below about twenty hertz and has wavelengths of tens of metres in water. Such long waves cannot be focused into a usable beam and cannot resolve a target of ordinary size, so they are no good for ranging.
• (b)Sound waves in audible range for human beings — Audible frequencies between about twenty hertz and twenty thousand hertz are not what the syllabus assigns to sonar. Their wavelengths are still too long for sharp directional beaming and fine resolution compared with ultrasound.
• (d)All of the above — An all-of-the-above option only wins if every individual option is separately correct, and here two of them are not. The item is asking which type sonar uses, and the answer required is the single ultrasonic band.

Sonar stands for sound navigation and ranging. Active sonar transmits a pulse and listens for the echo; passive sonar only listens. The quantity actually measured is time, and distance is recovered from it because the speed of sound in the medium is known — about 1500 metres per second in sea water against roughly 340 metres per second in air. The same echo principle is used to map the sea bed, to locate shoals of fish, to detect submarines and wrecks, and, with the same physics at much shorter range, in medical ultrasonography. Bats and dolphins navigate by their own biological version of it.

The important habit here is to divide the frequency line into three named bands and attach a use to each — infrasound below about 20 Hz, the audible range from about 20 Hz to 20 kHz, and ultrasound above 20 kHz. Ultrasound is the band of the working instruments: sonar, ultrasonography, industrial flaw detection and ultrasonic cleaning. One honest refinement is worth carrying without letting it disturb the answer. Real naval sonar sets have historically operated across a wide span of acoustic frequencies, some of them well inside the audible range, because low frequencies travel further in water. The exam, and this official key, use the textbook statement that sonar sends ultrasonic pulses, which is the case that explains why short wavelengths give a directed beam and fine resolution.

• Sonar stands for sound navigation and ranging, and it works by timing the echo of a transmitted pulse.
• Ultrasound is sound above about 20,000 Hz, beyond the upper limit of human hearing; infrasound lies below about 20 Hz.
• The distance to a target is d = vt/2, where t is the total to-and-fro time of the pulse.
• The speed of sound in sea water is roughly 1500 m s⁻¹, against about 340 m s⁻¹ in air.
• Ultrasound is preferred for ranging because its short wavelength allows a narrow directional beam and finer resolution.

• Forgetting the factor of two in d = vt/2 — the pulse makes a round trip.
• Choosing an all-of-the-above option when only one of the listed bands is being asked for.
• Using the speed of sound in air, about 340 m s⁻¹, in an underwater calculation.

As a direct recall of the wave type used by sonar, or as a numerical asking for depth from a measured echo time.

• practice — not a real PYQ

  A sonar pulse sent from a ship returns after 4 s. Taking the speed of sound in sea water as 1500 m s⁻¹, the depth of the sea bed at that point is

  • (a)1500 m
  • (b)3000 m
  • (c)6000 m
  • (d)750 m

  Answer(b) 3000 m — the pulse covers 1500 × 4 = 6000 m in the round trip, so the depth is half of that.

• practice — not a real PYQ

  Sound waves of frequency below about 20 Hz are known as

  • (a)ultrasonic waves
  • (b)infrasonic waves
  • (c)audible waves
  • (d)electromagnetic waves

  Answer(b) infrasonic waves — below the lower limit of human hearing, while ultrasonic waves lie above the upper limit of about 20,000 Hz.