What we can hear
Some whistles make a sound that a dog hears and you cannot. Why?
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Such a whistle has a frequency above 20 000 Hz. People cannot hear that high. A dog can.
Rule
People hear sounds from about 20 Hz to 20 000 Hz. A frequency below 20 Hz is infrasound. A frequency above 20 000 Hz is ultrasound.
- Infrasound — sound with a frequency below 20 Hz.
- Ultrasound — sound with a frequency above 20 000 Hz.
Your ear cannot hear infrasound or ultrasound. They are still sound. Something shakes, and a wave travels.
Which of these can you hear?
- 10 Hz is below 20 Hz. It is infrasound.
- 500 Hz is between 20 Hz and 20 000 Hz. You hear it.
- 30 000 Hz is above 20 000 Hz. It is ultrasound.
A sound has a time period of 0.01 s. Frequency = 1 ÷ 0.01 = 100 Hz. That is between 20 Hz and 20 000 Hz. So you can hear it.
Watch out!
A louder sound does not become ultrasound. Frequency decides the kind of sound, not loudness.