Sound
Waves
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 7% of your exam marks.
Speed of sound calculations, echo timing and ultrasound uses are standard shorter questions.
The audible range
- A healthy young human ear responds to sound waves with frequencies roughly 20 Hz to 20 000 Hz (20 kHz)
- This range narrows with age; older adults often cannot hear above 12–15 kHz, while children may still hear close to the full upper limit
- Many other animals have very different ranges:
- dogs hear up to about 45 kHz (which is why high-frequency dog whistles work)
- bats hear and emit sounds up to roughly 120 kHz for echolocation
- elephants and whales can produce and detect frequencies well below 20 Hz
Below the range: infrasound
- Infrasound is sound with a frequency below 20 Hz, too low for the human ear
- Sources of infrasound include earthquakes, volcanic activity, large explosions, and slow-rotating industrial machinery
- The body sometimes detects infrasound as a feeling of vibration in the chest rather than as a heard tone
Above the range: ultrasound
- Ultrasound is sound with a frequency above 20 000 Hz, too high for the human ear
- Common uses of ultrasound include medical scanning (prenatal scans, scans of internal organs), industrial cleaning of delicate parts, sonar in ships and submarines, and animal-distance sensors
- Ultrasound is covered in more detail in later topics
State the frequency range for human hearing
What comes up: "state the frequency range for human hearing" (2 marks: one for the lower limit, one for the upper).
Write (two marks): (1) 20 Hz; (2) 20 000 Hz (equivalently, 20 kHz). Both figures must be given as frequencies — writing a non-frequency unit loses both marks.
Watch out: both values must be stated as frequencies — the mark scheme awards one mark per correct limit and penalises a response that gives a non-frequency unit (e.g. a wavelength). Writing the upper limit as "20 kHz" is accepted as equivalent to 20 000 Hz. Omitting either limit loses that mark.
Echo-distance calculation (sonar / reflection)
A sonar emitter on a boat sends out a pulse of sound. The pulse travels down to the seabed and the reflected echo is detected 0.40 s after the pulse was sent. The speed of sound in sea water is 1500 m/s. Calculate the depth of the water.
Solution:
- Total distance travelled by the pulse (there and back): d = speed × time = 1500 × 0.40 = 600 m
- The pulse travels down and back, so depth = 600 ÷ 2 = 300 m
Describe a sound wave as longitudinal
What comes up: "describe the difference between a transverse wave and a longitudinal wave" (2–3 marks).
Write (two marks): (1) state that the vibrations (oscillations/particle movement) are parallel to the direction of wave travel/energy transfer for a longitudinal wave; (2) state that the vibrations are perpendicular to the direction of travel/energy transfer for a transverse wave. A clearly labelled diagram can earn both marks.
Watch out: you must correctly match each direction relationship to the correct wave type. Swapping the descriptions (parallel to transverse, perpendicular to longitudinal) scores zero. Saying particles "move along the wave" without the word "parallel" or an equivalent is accepted, but "move with the wave" is ambiguous — use "parallel to the direction of travel" for clarity.