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Loudness

Waves, sound and light · level 3, lesson 5 of 6

A question to start

A whisper and a jet engine are both sounds. A jet engine can damage your hearing. How do we measure how loud a sound is?

The idea

A louder sound comes from a bigger vibration. The air is squashed and stretched more, and the wave carries more energy. The size of the vibration is the amplitude.

We measure loudness in decibels (dB). The scale is unusual: every 10 dB more means the sound carries 10 times as much energy.

Sound Loudness, about
Rustling leaves 20 dB
A whisper 30 dB
Talking 60 dB
A busy road 80 dB
A rock concert 110 dB
A jet engine close by 140 dB: painful

Long times above about 85 dB can damage your hearing.

quiet: small amplitudeloud: big amplitude

Worked examples

1. 10 dB more. A sound of 70 dB carries 10 times the energy of a sound of 60 dB.

2. 20 dB more. Two steps of 10 dB: \(10 \times 10 = 100\) times the energy.

3. On Mars. Scientists found that the same source makes sounds about 20 dB weaker on Mars than on Earth: about a hundredth of the energy reaching your ear.

Try it

  1. What is the amplitude of a sound?
  2. What unit is loudness measured in?
  3. How many times more energy does a 90 dB sound carry than a 60 dB sound?
  4. Why should you be careful with loud music in headphones?
Answers
  1. The size of its vibration.
  2. Decibels (dB).
  3. Three steps of 10 dB: \(10 \times 10 \times 10 = 1{,}000\) times.
  4. Long times above about 85 dB can damage your hearing.

Common mistake

"Twice the decibels, twice as loud." Decibels go up by 10 for every 10 times the energy. 120 dB is not twice 60 dB: it carries a million times the energy.

Where it leads