NDA (II) 2018 General Ability Test · Question 67 of 146

Correct — D, greater than 20 kHz. Sound is classified by whether the human ear can register it, and the audible range for a healthy young person runs from about 20 Hz at the low end to about 20,000 Hz — that is 20 kHz — at the high end. Anything below 20 Hz is infrasound; anything above 20 kHz is ultrasound. So ultrasonic waves are, by definition, those of frequency greater than 20 kHz. In every other respect they are ordinary sound: longitudinal, mechanical, needing a medium, travelling at the same speed as audible sound in the same material. Their short wavelength is what makes them useful, because it lets them be sent out as a narrow beam and reflected off small objects and internal flaws.

• (a)less than 20 Hz — That is infrasound, the band below the lower limit of hearing. It is produced by earthquakes, volcanoes, heavy machinery and by animals such as elephants and whales that use it to communicate over long distances.
• (b)between 20 Hz and 2 kHz — Well inside the audible band. Most of human speech and the fundamental notes of most musical instruments lie in this range, which is precisely why we can hear them.
• (c)between 2 kHz and 20 kHz — Still audible — this is the upper part of the range the ear responds to, and it is where the ear is in fact most sensitive around 2 to 5 kHz. Ultrasound begins where this band ends.

The frequency spectrum of sound is divided at the two limits of human hearing. Below 20 Hz lies infrasound, from 20 Hz to 20 kHz the audible range, and above 20 kHz ultrasound. The upper limit falls with age, so an adult may not hear beyond 15 or 16 kHz while a child hears close to 20 kHz. Many animals hear well past the human limit: bats emit and hear calls up to well over 100 kHz, and dogs, dolphins and porpoises all exceed our range.

The mark here is really for holding two numbers, 20 Hz and 20 kHz, and for noticing that the options are laid out as a ladder across them — one band below, two inside, one above. Read the units carefully, because the paper mixes Hz and kHz within a single option set and a student who reads (c) as 2 kHz to 20 kHz but (d) as 20 Hz can pick the wrong one while reasoning correctly. The applications follow from the physics: a short wavelength can be beamed narrowly and reflects off small targets, which is why ultrasound is used for echolocation, SONAR, flaw detection in castings, and medical imaging.

• Infrasound is below 20 Hz, the audible range runs from about 20 Hz to 20 kHz, and ultrasound is above 20 kHz.
• Ultrasonic waves travel at the same speed as audible sound in the same medium and, like all sound, cannot cross a vacuum.
• Bats and dolphins navigate and hunt by echolocation using ultrasonic pulses.
• SONAR finds the depth of the sea bed and the distance of underwater objects by timing ultrasonic echoes.
• Ultrasound is used to detect cracks in metal castings, to clean delicate parts and for medical imaging.

• Reading the units carelessly and confusing 20 Hz with 20 kHz.
• Believing ultrasonic waves travel faster than audible sound; the speed depends on the medium, not the frequency.
• Assuming ultrasound can travel through a vacuum because it is used in advanced instruments; it is still sound.

NDA asks either for the numerical boundary, as here, or for a property or application of ultrasound — most often echolocation by bats or the working of SONAR.

• practice — not a real PYQ

  Waves of frequency below 20 Hz are called

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

  Answer(b) infrasonic waves — the band below the lower limit of human hearing, produced by sources such as earthquakes and large machinery.

• practice — not a real PYQ

  Ultrasonic waves are preferred to audible sound in SONAR mainly because they

  • (a)travel much faster in water
  • (b)can be sent out in a narrow beam and reflect off small objects
  • (c)do not need a medium to travel
  • (d)are not absorbed by sea water at all

  Answer(b) can be sent out in a narrow beam and reflect off small objects — their short wavelength gives the directionality that echo ranging needs.