Inaudio signal processing andacoustics, anecho is areflection of sound that arrives at the listener with a delay after the direct sound. The delay is directly proportional to the distance of the reflecting surface from the source and the listener. Typical examples are the echo produced by the bottom of a well, a building, or the walls of enclosed and empty rooms.
The wordecho derives from the Greek ἠχώ (ēchō),[1] itself from ἦχος (ēchos), 'sound'.[2]Echo inGreek mythology was a mountain nymph whose ability to speak was cursed, leaving her able only to repeat the last words spoken to her.
Echolocation organs of atoothed whale, which produce echoes and receive sounds. Arrows illustrate the outgoing and incoming path of sound.
Some animals, such ascetaceans (dolphins and whales) and bats, use echo for location sensing and navigation, a process known asecholocation. Echoes are also the basis ofsonar technology.
Walls or other hard surfaces, such as mountains and privacy fences, reflect acoustic waves. The reason for reflection may be explained as a discontinuity in thepropagationmedium. This can be heard when the reflection returns with sufficient magnitude anddelay to be perceived distinctly. When sound, or the echo itself, is reflected multiple times from multiple surfaces, it is characterized as areverberation.
This illustration depicts the principle of sediment echo sounding, which uses a narrow beam of high energy and low frequency
The human ear cannot distinguish echo from the original direct sound if the delay is less than 1/10 a second.[3] The speed of sound in dry air is approximately 341m/s at a temperature of 25°C. Therefore, the reflecting object must be more than17.2m from the sound source for the echo to be perceived by a person at the source. When a sound produces an echo in two seconds, the reflecting object is343m away. In nature, canyon walls or rock cliffs facing water are the most common natural settings for hearing echoes. The echo strength is frequently measured in sound pressure level (SPL) relative to the directly transmitted wave. Echoes may be desirable (as in systems).
Insonar, ultrasonic waves are more energetic than audible sounds. They can travel undeviated through a long distance, confined to a narrow beam, and are not easily absorbed in the medium. Hence, sound ranging and echo depth sounding usesultrasonic waves. Ultrasonic waves are sent in all directions from the ship and are received at the receiver after the reflection from an obstacle (enemy ship, iceberg, or sunken ship). The distance from the obstacle is found using the formula d = (V*t)/2.Echo depth sounding is the process of finding the depth of the sea using this process.[citation needed] In themedical field, ultrasonic waves of sound are used inultrasonography andecho cardiography.[4]
Electric echo effects have been used since the 1950s in music performance and recording. TheEchoplex is atapedelayeffect, first made in 1959, that recreates the sound of an acoustic echo. Designed by Mike Battle, the Echoplex set a standard for the effect in the 1960s and was used by most of the notable guitar players of the era; original Echoplexes are highly sought after. While Echoplexes were used heavily by guitar players (and the occasional bass player, such asChuck Rainey, or trumpeter, such asDon Ellis), manyrecording studios also used the Echoplex. Beginning in the 1970s, Market built thesolid-state Echoplex for Maestro. In the 2000s, most echoeffects units used electronic or digital circuitry to recreate the echo effect.[citation needed]
