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High notes fade

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

A question to start

When thunder is far away, you hear a low rumble, not a sharp crack. On Mars, even sounds that start sharp turn dull over a short distance. Why?

The idea

As sound travels, some of its energy is turned into heat in the air. This is called absorption, and it is always worse for high frequencies: high notes fade first, and low notes carry furthest.

On Mars this effect is much stronger. The scientists found that high-frequency sound is strongly absorbed in thin carbon dioxide. So, over a short distance, a sharp sound loses its high notes and turns dull and muffled.

Fading of sound Mars Earth
Low notes carry well carry well
High notes, over a short distance fade fast carry well
A sharp crack from a distance dull and muffled still fairly sharp, until a long way off
low note: carries farhigh note: fades fast on Mars

Worked examples

1. A laser zap. Perseverance's laser makes a sharp crack on a rock a few metres away. Up close, the microphone hears it clearly.

2. Ingenuity's hum. The helicopter's hum was low, about 84 Hz, so it carried far enough for the rover to record it while the helicopter flew.

3. Thunder on Earth. Close by, thunder cracks; far away, it rumbles. The high notes faded on the way.

Try it

  1. What happens to sound energy that is absorbed?
  2. Which fade faster, high notes or low notes?
  3. Why would a distant sound on Mars be muffled?
  4. Why could the rover record Ingenuity's hum from a distance?
Answers
  1. It is turned into heat in the air.
  2. High notes.
  3. Its high notes are strongly absorbed, leaving only the low notes.
  4. The hum was low-pitched, and low notes carry well.

Common mistake

"All of a sound fades at the same rate." High and low parts fade at different rates. That is why a distant sound changes its character, not just its loudness.

Where it leads