An echo is the reflection of sound waves off distant objects, whereas flutter echoes bounce repeatedly off multiple surfaces. An example of an echo is when someone shouts into a well, or along canyon walls, and the sound of the human voice comes back to the person's ears. A flutter echo occurs in large buildings with parallel walls, such as gymnasiums, where sounds increase to create loud environments.
Echoes are human perceptions of sound waves. This effect works along reflective surfaces more than 56 feet (17 meters) away to give the sound more than 0.1 second to return to the observer. Humans' perception of sound lasts in the memory for 0.1 second, so an echo must have a delay for the effect to be observed. Some, but not all, sound waves in echoes are reflected back to the observer. Like animals that use echolocation techniques, humans measure the time it takes an echo to return to calculate distances to far-away objects.
Echoes and flutter echoes are desirable occurrences in music halls where performances require good acoustics. In places that require sound suppression, materials and shapes that absorb sound are included in the architecture of offices and restaurants.
Reverberations differ from echoes in that reverberations occur in confined spaces less than 56 feet away. This is due to sound waves reflecting within the memory threshold of the previous sound.