Students should know that the range of normal human hearing is from 20 Hz…

The idea

These two figures must be recalled. 20 Hz means twenty complete oscillations every second and is the lowest frequency a person with normal hearing can detect; 20 kHz is 20 000 Hz and is the highest. Keep the units straight, because kHz means a thousand hertz, so the audible range spans a factor of one thousand. The numbers matter because they define ultrasound, which is any sound above 20 kHz, and because the wave equation turns them into wavelengths: with the speed of sound in air about 330 m/s, 20 Hz gives a wavelength of about 16.5 m while 20 kHz gives about 1.65 cm. The range is for normal hearing; individuals vary, and the upper limit falls with age and with exposure to loud noise.

What earns the marks

• The lower limit given as 20 Hz.
• The upper limit given as 20 kHz, or as 20 000 Hz.
• A correct conversion from kHz to Hz before using the wave equation.
• Defining ultrasound as sound with a frequency above the upper limit of human hearing.

Common mistakes and how to put them right

• Writing the upper limit as 20 kHz but then substituting 20 into a calculation that needs hertz.

• Giving the range as 20 Hz to 20 MHz, confusing kilo with mega.

• Treating the limits as exact for every person rather than as normal hearing.

• Quoting the range in decibels, which measures sound level and not frequency.

Exam tips

• Convert kHz to Hz on the first line of any calculation and show the conversion so it can be credited.
• Recall questions on these values are worth one mark; write the number with its unit and move on.
• If asked whether a given frequency is ultrasound, compare it with 20 000 Hz explicitly.