Sounds getting softer

Sounds dissipating

Wrong Track: Sounds run out as they get farther away from the source, and eventually stop.

Right Lines: The movement (or the energy) from the vibrating source is spread across more and more particles.

Sound energy

Thinking about the learning

Typically pupils are quite happy to say that sounds get quieter as they travel farther away from the source and suggest they die away. Pupils are not concerned with accounting for where the initial motion (or energy) of the vibrating cone and medium ends up.

Thinking about the teaching

One of the ways in which the sound get quieter is that the energy of the vibrations is transferred to more and more particles as the vibrations spread out over a larger and larger sphere.

Imagine throwing a pebble into the middle of a pond, creating waves that spread out over the surface (just like the pattern of high and low density travelling through the air) in an ever-expanding circle. As the circle gets bigger the energy available for each centimetre of the circumference gets less.

Since a sound travels out over a three-dimensional space (rather than a circle), the energy from the source is spread out much more for each centimetre moved away from the source.

In addition, some of the sound may be absorbed by the stuff through which it travels. In this case not all of the vibration of one block of particles is passed onto the next – some of the energy gets spread around, resulting in a disordered jiggling, rather than in the organised vibration of the sound. Materials and structures that do this particularly well are good sound insulators – good at insulating source from detector.