Fading with distance

The idea

Moving away from the source, the same energy spreads over a larger area, so intensity decreases with distance; sounds of larger initial amplitude travel farther before their intensity falls to zero.

Near and far

A loudspeaker plays in a playground. You stand near it, then walk far away.

What do you think happens to the intensity of the sound as you walk away?

• It decreases
• It increases
• It stays the same

Walk away

Move the listener away from the source. Then try a larger starting amplitude. The picture is a model.

Spread thinner

As the sound travels away from its source, it spreads over a larger area. The same energy is spread over more area, so the intensity decreases. A sound that starts with a larger amplitude carries more energy and can travel farther before its intensity falls to zero.

Moving away from the source, the same energy is spread over a larger area, so intensity decreases with distance. Sounds with a larger initial amplitude carry more energy and travel farther before their intensity falls to zero.

Notes

Away from the source the same energy spreads over a larger area, so intensity decreases with distance. A larger amplitude lets sound travel farther.

Check yourself

Why does the intensity of a sound decrease as you move away from the source?

What happens to the intensity as the distance from the source increases?

Two sounds start from the same place. One has a larger initial amplitude. Which one can travel farther before its intensity falls to zero?

Energy is conserved as sound spreads out. What does this mean for the energy in the wave?

• The energy is destroyed as the sound travels. Energy must be conserved. The same energy is spread over a larger area, so the intensity decreases.
• The source gets quieter as you walk away. The source is unchanged. The intensity decreases because the same energy spreads over a larger area.
• The same energy is spread over a larger area — correct. Yes!
• It decreases — correct. Yes!
• It increases. As the wave travels away, the same energy spreads over a larger area, so the intensity decreases.
• It stays constant. The intensity decreases with distance from the source.
• The one with the smaller initial amplitude. A sound with a larger initial amplitude carries more energy, so it can travel a larger distance.
• The one with the larger initial amplitude — correct. Yes!
• Both travel the same distance. Sounds produced with a larger amplitude carry more energy and can travel a larger distance.
• The energy grows as the area grows. Energy is conserved. It is the same amount, spread over a larger area.
• The energy turns into a vacuum. Energy is conserved as the sound spreads. The same energy is spread over a larger area.
• The same amount of energy is now spread over a larger area — correct. Yes!