Define ultrasound using the frequency boundary
Use frequencies strictly above 20 kHz and retain sound's mechanical medium requirement.
Section P3.3 · Sound
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Recall the approximate audible range.
- Convert Hz and kHz.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does 25 kHz equal?
What you will learn
- Define ultrasound as sound above 20 kHz.
- Distinguish ultrasound from electromagnetic radiation and merely louder sound.
Use strictly above the reference
Ultrasound is sound with frequency higher than 20 kHz, or 20 000 Hz. In the stated definition, exactly 20 kHz is the boundary and not strictly higher. A 25 kHz sound is ultrasound; a 15 kHz sound is below the ultrasound threshold.
Ultrasound remains a mechanical sound disturbance and needs a suitable material medium. Its frequency lies beyond ordinary human hearing, but an appropriate detector can still receive it. Being unheard by a person does not make it absent.
Do not substitute amplitude or a different wave family
A large-amplitude 10 kHz sound is not ultrasound simply because it is loud. A small-amplitude 30 kHz disturbance is still ultrasound by frequency. Frequency classification is independent of the separately stated amplitude.
Ultraviolet is electromagnetic radiation, while ultrasound is high-frequency sound. Their similar prefixes do not make them the same phenomenon. Unlike electromagnetic radiation, ultrasound cannot propagate through an ideal empty vacuum.
Sources for this lesson
Pause and explain
Which is ultrasound under the strict stated definition?
Worked example
Classify sound at 15 kHz, exactly 20 kHz and 25 kHz under the stated ultrasound definition.
Show the worked solution
- 15 kHz is below the boundary.
- 20 kHz is exactly the boundary, not strictly above.
- 25 kHz is higher than 20 kHz.
Answer Only the 25 kHz example is ultrasound under the strict definition.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define ultrasound in Hz and kHz.
Give me a hint
Include the inequality rather than only a number.
Compare my reasoning
- The frequency must be above 20 kHz.
- 20 kHz equals 20 000 Hz.
- It is high-frequency sound.
Sound with frequency greater than 20 kHz, or 20 000 Hz.
Look for these in your work
- Use strictly greater.
- Keep it a sound definition.
Classify sound at 19 000, 20 000 and 21 000 Hz.
Give me a hint
Compare each to the strict threshold.
Compare my reasoning
- 19 000 is below.
- 20 000 equals the boundary.
- 21 000 is above.
Only 21 000 Hz is ultrasound under the stated definition.
Look for these in your work
- Respect equality.
- Use frequency rather than amplitude.
Can ultrasound cross an empty space segment merely because people cannot hear it?
Give me a hint
Compare hearing sensitivity with the mechanical transmission requirement.
Compare my reasoning
- Ultrasound is still sound.
- Mechanical sound needs a material medium.
- Being outside human hearing does not supply that medium.
No; ultrasound still requires a suitable material path.
Look for these in your work
- Name the wave family.
- Separate audibility from propagation.
Common mistakes
- Use higher than, not automatically equal to, 20 kHz.
- Ultrasound still requires a medium.
- Ultrasound and ultraviolet are different wave families.
What can you explain now?
Does louder 10 kHz sound become ultrasound without a frequency change?
Compare with the explanation
No.
The ultrasound definition depends on frequency above 20 kHz rather than amplitude.
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Make it stick
Define ultrasound precisely and distinguish frequency, amplitude, hearing and medium requirements.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.