NDA (I) 2018 General Ability Test · Question 62 of 150

Correct — B, 20 – 20,000 Hz. The range of hearing for a healthy young human ear runs from about twenty hertz at the low end to about twenty thousand hertz at the high end, and that is the band called audible sound. Below twenty hertz the vibrations are infrasonic and are felt rather than heard; above twenty thousand hertz they are ultrasonic and pass us by entirely, though bats, dolphins and dogs can hear well into that region. Option (b) is the only one that gives both limits correctly.

• (a)0 – 200 Hz — Both ends are wrong. The lower limit is twenty hertz, not zero — anything below that is infrasonic. And the upper limit of two hundred hertz would cut out almost the whole of speech and music; a woman's speaking voice alone runs well above it.
• (c)200 – 20,000 Hz only — The upper limit is right but the lower one is ten times too high. Sounds between twenty and two hundred hertz are perfectly audible — they are the bass notes of a drum, a tuba or a male speaking voice.
• (d)2,000 – 20,000 Hz only — This throws away almost the entire useful band. Middle C on a piano is about 262 hertz and the fundamental of ordinary speech lies between roughly 85 and 255 hertz, all of it far below two thousand hertz and all of it plainly audible.

Sound is a longitudinal mechanical wave, and its frequency is the number of complete vibrations passing a point each second, measured in hertz. Frequency is what the ear interprets as pitch. The human range of hearing, roughly 20 Hz to 20 kHz, divides the spectrum into three bands — infrasonic below it, audible within it, and ultrasonic above it. The upper limit falls with age and with exposure to loud noise, so most adults hear rather less than the textbook twenty kilohertz.

Three of the four options here are built by shifting one limit of the correct range, so the reliable method is to check both ends separately rather than to recognise a familiar-looking pair of numbers. It also helps to attach the boundaries to something real. Twenty hertz is around the lowest note of a large pipe organ; twenty thousand hertz is beyond what most adults can hear at all. Ultrasound above the upper limit is what bats use to navigate, what a sonar set uses to find objects underwater, and what a hospital scanner uses to image a patient. Infrasound below the lower limit is produced by earthquakes and by volcanic eruptions and is thought to be one of the cues some animals respond to before a tremor.

• The audible range for humans is about 20 Hz to 20,000 Hz.
• Frequencies below 20 Hz are infrasonic; those above 20,000 Hz are ultrasonic.
• Frequency determines pitch, while amplitude determines loudness.
• Bats and dolphins use ultrasound to navigate and to locate prey by echolocation.
• The upper limit of hearing declines with age, a change called presbycusis.

• Recognising the upper limit and accepting an option whose lower limit has been shifted.
• Confusing the audible range with the range of a musical instrument.
• Mixing up frequency, which sets pitch, with amplitude, which sets loudness.

NDA asks either for the audible range itself or for the band that lies just outside it, most often by asking where ultrasound begins.

• practice — not a real PYQ

  Sound waves of frequency below 20 Hz are called

  • (a)ultrasonic
  • (b)infrasonic
  • (c)supersonic
  • (d)audible

  Answer(b) infrasonic — 'supersonic' describes a speed greater than that of sound, not a frequency, and is a common decoy here.

• practice — not a real PYQ

  Bats avoid obstacles in the dark chiefly by using

  • (a)infrasound
  • (b)ultrasound
  • (c)visible light
  • (d)radio waves

  Answer(b) ultrasound — they emit high-frequency pulses above the human range and listen for the echoes.