GCSE · Physics · Edexcel · Spec 1PH0
Ultrasound (>20 000 Hz)
A note climbs higher and higher until it seems to vanish. It has not stopped — the air is still vibrating. Your ears just cannot keep up.
Turn up the frequency. When does sound stop being sound?
This graph shows how the air pressure at one spot rises and falls as a sound wave passes. Drag the frequency up from 5 000 Hz, past 20 000 Hz, and keep your eye on the shape. The peaks crowd closer together — more vibrations squeezed into every second — but the wave never turns into a different kind of wave.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Sound that vibrates too fast for human ears — more than 20 000 times a second.
What you need to know
- Frequency is the number of vibrations each second, measured in hertz (Hz).
- Ultrasound is sound with a frequency greater than 20 000 Hz.
- 20 000 Hz is the same as 20 kHz, because 1 kHz = 1000 Hz.
- Ultrasound is still a sound wave. Only its frequency is different, not the type of wave.
The big picture
Ultrasound is sound with a frequency greater than 20 000 Hz. It is still a sound wave, the same kind as the sounds you hear; it just vibrates more than 20 000 times a second, which is too fast for human ears to detect.
Key points
Worked example
Problem
A signal generator plays a sound through a speaker. Its frequency is slowly turned up from 15 000 Hz to 30 000 Hz, and the loudness setting is not changed. Describe what happens to the sound wave, and to what a person listening can hear.
⚠ Watch out
Counting exactly 20 000 Hz as ultrasound. Ultrasound is sound with a frequency GREATER than 20 000 Hz, so a sound of exactly 20 000 Hz sits on the boundary and does not count.
Memory hook
"Ultra" means "beyond". Ultrasound is sound beyond 20 000 Hz — beyond the top of human hearing.
Check yourself
Cover the page: which are ultrasound — 18 kHz, 20 000 Hz, 20 500 Hz, 45 kHz? (Answer: 20 500 Hz and 45 kHz. The other two are not greater than 20 000 Hz.)
Flashcards
(6)What is ultrasound?
What does a frequency in hertz (Hz) tell you?
Write 20 000 Hz in kHz.
Ultrasound: sound wave, or some other kind of wave?
Why can't humans hear ultrasound?
Does turning a sound up louder make it ultrasound?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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