Sound goes everywhere
Spherical waves.
A clap in an open field
You clap once in the middle of an open field. Your friends stand in front of you, behind you and to your side.
Who do you think will hear the clap?
What this lesson covers
The idea
In an unconfined medium vibrating particles collide with surrounding particles in all directions, so sound from a small source spreads out in all directions as spherical waves.
A clap in an open field
You clap once in the middle of an open field. Your friends stand in front of you, behind you and to your side.
Who do you think will hear the clap?
- Friends in all directions
- The friend in front of you, but not the others
- The friend at your side, but not the others
How sound spreads
Press Next step.
Out in all directions
Nothing blocks the particles in the open, so the disturbance spreads out round the source like the surface of a growing ball. That is why friends all around you can hear you.
In an unconfined medium, vibrating particles collide with surrounding particles in all directions. So sound from a small source spreads out in all directions as spherical waves.
Notes
From a small source in an unconfined medium, sound spreads out in all directions as spherical waves.
Check yourself
A small source makes sound in open air. In which directions does the sound spread?
What are the waves called that spread from a small source in all directions?
Why does the sound spread out in all directions in open air?
Books often draw sound moving in only one direction. Why?
- In one direction, straight ahead of the source. In an unconfined medium the vibrating particles collide with surrounding particles in all directions, so sound spreads in all directions.
- In all directions — correct. Yes!
- Towards the listener alone. The particles collide with surrounding particles in all directions, not just towards one listener.
- Narrow beams. From a small source, compressions and rarefactions spread in all directions as spherical waves, not as a narrow beam.
- Single pulses. A source that keeps producing sound makes many compressions and rarefactions, which spread as spherical waves.
- Spherical waves — correct. Yes!
- Vibrating particles collide with the surrounding particles in all directions — correct. Yes!
- The particles are blown outwards like a wind. The particles only vibrate about their mean positions. The disturbance spreads because of collisions.
- Walls on all sides push the sound out. In an unconfined medium there are no walls. The sound spreads because particles collide with their neighbours in all directions.
- Because sound really moves in one direction only. Sound propagates in multiple directions from a source. One direction is drawn for simplicity.
- To keep the picture simple, although sound spreads in multiple directions — correct. Yes!
- Because sound is stuck inside a tube. Sound from a source spreads out in many directions. Books draw one direction only to keep the picture simple.