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Pitch

Waves, sound and light · level 3, lesson 4 of 6

A question to start

A dog whistle makes a sound that dogs can hear but people cannot. How can a sound be too high to hear?

The idea

The number of vibrations each second is the frequency. It is measured in hertz (Hz): 1 Hz is one vibration each second.

  • High frequency → high pitch.
  • Low frequency → low pitch.

Young people can usually hear from about 20 Hz to 20,000 Hz. Above that is ultrasound, too high for us but not for dogs and bats. Below it is infrasound, too low for us, but used by elephants.

Sound Frequency, about
Lowest note we can hear 20 Hz
The A that orchestras tune to 440 Hz
Highest note we can hear 20,000 Hz

On Mars, Perseverance's microphone heard the Ingenuity helicopter hum at about 84 Hz.

low pitch: low frequencyhigh pitch: high frequency

Worked examples

1. A guitar string vibrating 110 times a second makes a 110 Hz note, the low A on a guitar.

2. Tightening a string makes it vibrate faster, so its pitch goes up.

3. Ingenuity's hum. Each blade passing overhead gives a little push to the air. On Mars, Perseverance's microphone recorded the helicopter's hum at about 84 Hz, a low note.

Try it

  1. What is frequency?
  2. What unit is frequency measured in?
  3. Can people hear a sound at 30,000 Hz?
  4. A string vibrates 220 times a second. What is its frequency?
Answers
  1. The number of vibrations each second.
  2. Hertz (Hz).
  3. No. It is ultrasound, above the top of our hearing.
  4. 220 Hz.

Common mistake

"A higher frequency means a faster sound." All sounds in the same air travel at the same speed. A higher frequency means more vibrations each second, not a faster wave.

Where it leads