Echos

Echos are multiple instances of an echo. In standard English, the plural is echoes; echOS may appear as an alternate spelling in some contexts or as a proper noun. The term derives from the Greek myth figure Echo and describes a sound that is heard again after reflecting off a surface.

Physical basis: When a sound wave strikes a hard surface, part of it reflects back toward the listener. If the reflected sound arrives distinctly after the original, a separate echo is heard. The speed of sound in air is about 343 meters per second at 20°C; a distinguishable echo generally requires a round-trip path of at least about 34 meters, corresponding to a delay of roughly 0.1 second. Surfaces closer than this produce reverberation, a blend of many reflections rather than a single echo.

Echoes versus reverberation: An echo is a discrete reflection with a clear delay; reverberation is the cumulative effect of many rapid reflections creating a persistent sound after the source stops. In rooms or environments with many surfaces, multiple echoes may be heard as the sound bounces around.

Contexts and applications: In nature, echolocation uses echoes to locate objects; bats and dolphins emit sounds and interpret returning echoes. In technology, sonar, radar, and ultrasound imaging rely on echoes to measure distance and form images. In acoustics and architecture, designers manage echoes and reverberation to control sound quality in theaters, classrooms, and other spaces.

Related usage: The word appears in branding, literature, and as a surname. While echo is the common term, echos may be encountered in non-English spellings or stylized contexts. The concept is closely related to echo and echolocation.