Which one of the following is the audible range of hearing for humans?

Correct — B, 20 Hz – 20 kHz. The healthy young human ear responds to sound between roughly 20 hertz and 20 kilohertz. Below 20 hertz the vibration is infrasonic and is felt rather than heard; above 20 kilohertz it is ultrasonic and passes unheard, which is the range bats and dolphins use for echolocation and which medical ultrasound scanners exploit at frequencies of a few megahertz. The upper limit falls steadily with age and with exposure to loud noise, so most adults lose the top of the band well before it matters clinically, but 20 hertz to 20 kilohertz is the figure quoted as the standard range.

• (a)20 kHz – 200 kHz — 20 kHz to 200 kHz is entirely above the audible band. This is the ultrasonic region used in sonar, echolocation and industrial cleaning, and no part of it can be heard.
• (c)20 Hz – 35 kHz — 20 Hz to 35 kHz overstates the upper limit by 15 kilohertz. Nothing above about 20 kilohertz registers on the human ear at any age.
• (d)20 Hz – 40 kHz — 20 Hz to 40 kHz doubles the true upper limit. Forty kilohertz is a frequency used by ultrasonic cleaning baths and by many bat species, not one humans can hear.

Sound is a longitudinal pressure wave that needs a medium, and its frequency is what the ear perceives as pitch. The audible band is bounded at both ends by the mechanics of the ear: the eardrum and ossicles cannot follow very rapid pressure changes, and very slow ones do not excite the hair cells of the cochlea. Sound below the band is infrasonic, produced by earthquakes, volcanoes and large machinery; sound above it is ultrasonic and is used in echolocation, sonar, non-destructive testing and diagnostic imaging.

Three options share the same lower limit and differ only in the ceiling, which tells you the examiner is testing the upper figure. The convenient symmetry of 20 hertz to 20 kilohertz is what makes it easy to hold: a thousand-fold range with the same digits at both ends. It is worth knowing that infrasound generated by an approaching storm or earthquake is detected by some animals, which is one explanation offered for reports of animals reacting before a tremor is felt.

• The human audible range is about 20 hertz to 20 kilohertz.
• Frequencies below 20 hertz are infrasonic; those above 20 kilohertz are ultrasonic.
• Bats and dolphins echolocate using ultrasound; medical ultrasound imaging runs at a few megahertz.
• The upper limit of hearing falls with age and with exposure to loud sound.
• Sound is a longitudinal wave and cannot travel through vacuum.

• Confusing the audible range with the ultrasonic range used by bats.
• Remembering the lower limit correctly and stretching the upper one.
• Assuming the range is the same at every age.

A single-figure recall item where three of the four options share a correct lower bound, so the ceiling is what is being tested.

• practice — not a real PYQ

  Sound of frequency below 20 Hz is described as

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

  Answer(b) infrasonic — below the lower limit of human hearing.

• practice — not a real PYQ

  Ultrasound used for medical imaging typically has a frequency of the order of

  • (a)a few hertz
  • (b)a few kilohertz
  • (c)a few megahertz
  • (d)a few gigahertz

  Answer(c) a few megahertz — high enough to resolve fine internal detail.