The loudness has to do with the sound field quantity called sound pressure or sound pressure level (SPL). The sound intensity or acoustic intensity means the sound energy quantity. Our ears and the microphone diaphragms are moved by the amplitude of sound pressure variations.

Comparison of Sound Pressure and Sound Intensity Levels. Scroll down to related links and look at "Comparison of Sound Pressure and Sound Intensity Levels".

Sound travels as a wave.

"Sound that is an imitation of another sound" can be an echo, a recording, or an imitation.

sound is measured in decibles