There’s a spike that occurs as the sound builds up, so your initial noise isn’t the loudest that it’ll get. A good example of this is when you clap or shout in medium sized room with all of the doors closed. This occurs when echoes build up in an enclosed space and cause a volume swell, which then decays as the sound slowly escapes. Delay is a customizable thing, and can alter the sound during each iteration. In audio editing terms, though, an echo is considered to be a specific type of delay, one that decays but reproduces the sound otherwise accurately.
The sound waves are emanating from your mouth, traveling some distance, bouncing off of a solid surface, and returning back to your ears after a period of time. Shouting into a canyon or wide space will give you an echo. It’s a repeated iteration of a sound that occurs with a diminished volume and after a brief period of time. Delay, echo, and reverb are all different aspects of the same process: repetition of a sound over time.
