When is the energy transmitted by a sound wave considered no longer to be sound?

Answer 1

It is tempting to answer this by saying that the energy will no longer be sound when it cannot be heard. But it may be able to be detected by another non-human animal - a dog or bat, for example. Sound is a form of mechanical energy. And it can exist at levels outside our ability to hear it, or to even detect it with equipment. The answer in physics is this: the energy transmitted by a sound wave will no longer be considered to be sound when its amplitude is zero. If this is an end run at the question as to whether or not a tree falling in the forest still makes a sound if no one is there to hear it, nice try. Once more: sound is a form of mechanical energy. If something like, say, a tree falls in a forest, a portion of the energy released will appear as sound. And the sound is there whether anyone is around to hear it or not.