Darrel Morgan, age 12, of Calgary, Alberta, Canada for his question:

 What makes sounds disappear?

Sound is a form of energy and energy tends to spend itself. The energy of sound is spent on traveling through air, water and various solid, liquid or gaseous mediums. One reason it grows weaker is because it balloons outward as it goes. This disperses and thins the intensity of its energy. Some of the sound energy also is lost by friction with resisting molecules. Under standard conditions, a sound diminishes its intensity to one fourth as it doubles the distance from the energy that created it.

  Sounds borrow their energy from outside sources. The whistling wind is started by moving air, brushing the. leaves and crashing around the corners of solid buildings. A throbbing drum beat borrows its energy from the muscle power of the drummer. As he strikes his sticks on the taut drumhead, it bounces with vibrations that bash the air molecules and set them in jogging motion. The energy of muscle power is transferred and converted into sound energy. To keep the sound going, the drummer must sustain the beat to supply continuous energy. Otherwise the throbbing pulse diminishes and finally fades away into silence.

The original energy that starts a sound sets molecules into jogging motion. This cannot happen in a vacuum. Sounds can pass only through mediums such as air, water and metals. The vibrating energy vibrates the molecules at its source. These molecules jog those behind them and waves of sound balloon farther and farther from the source. Molecules are bouncy particles of matter but even the air resists these punching vibrations to some extent. As the sound energy is transferred on and on from the source, some is lost by friction. But this is not the major reason that sound diminishes.

Energy can be measured in units and that which starts a sound has so much and no more to spend. Its force is strongest at the source, where naturally the sound intensity is strongest. As it balloons outward, the same amount of energy spreads through a wider region. It thins out and loses intensity. Finally the original energy is spread so thin that it grows too weak to make molecules vibrate    and the sound fades away.

Acoustic power is the time rate at which sound energy is dispersed. It can be measured in units based on watts of energy striking a solid surface of one square centimeter. Under standard conditions, echos and other interferences are eliminated and the temperature adjusted to O degrees centigrade. As the distance from the source is doubled, the area of the spreading sound bubble is four times as great. Hence the sound intensity diminishes to one fourth. At three times the distance, the wave front area is nine times greater and the sound fades to one ninth of its intensity.

  All sorts of other factors can interfere to change the pattern of the sound bubble and increase its intensity with echos and reflections. When sounds traveling through the air meet a solid wall, they tend to bounce back with a spurt of extra energy. This bounces spheres of molecules back and forth from walls and ceiling. These sound waves form intricate patterns. But when the source of the sounds stop, they too use up their borrowed energy and gradually subside into silence.