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In audio signal processing and acoustics, echo is a reflection 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, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single reflection of the sound source.[citation needed]
The word echo derives from the Greek ἠχώ (ēchō), itself from ἦχος (ēchos), "sound". Echo in the folk story of Greek is a mountain nymph whose ability to speak was cursed, only able to repeat the last words anyone spoke to her. Some animals use echo for location sensing and navigation, such as cetaceans (dolphins and whales) and bats.
In music performance and recording, electric echo effects have been used since the 1950s. The Echoplex is a tape delayeffect, 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 as Chuck Rainey, or trumpeter, such as Don Ellis), many recording studios also used the Echoplex. Beginning in the 1970s, Market built the solid-state Echoplex for Maestro. In the 2000s, most echo effects units use electronic or digital circuitry to recreate the echo effect.

A mobile phone is a wireless handheld device that allows users to make and receive calls and to send text messages, among other features. The earliest generation of mobile phones could only make and receive calls. Today’s mobile phones, however, are packed with many additional features, such as web browsers, games, cameras, video players and even navigational systems.

A mobile phone may also be known as a cellular phone or simply a cell phone.
When the first mobile phones were introduced, their only function was to make and receive calls, and they were so bulky it was impossible to carry them in a pocket.

Later, mobile phones belonging to the Global System for Mobile Communications (GSM) network became capable of sending and receiving text messages. As these devices evolved, they became smaller and more features were added, such as multimedia messaging service (MMS), which allowed users to send and receive images. Most of these MMS-capable devices were also equipped with cameras, which allowed users to capture photos, add captions, and send them to friends and relatives who also had MMS-capable phones.

A mobile phone with advanced features similar to a computer is called a smartphone, while a regular mobile phone is known as a feature phone.

A mobile phone typically operates on a cellular network, which is composed of cell sites scattered throughout cities, countrysides and even mountainous regions. If a user happens to be located in an area where there is no signal from any cell site belonging to the cellular network provider he or she is subscribed to, calls cannot be placed or received in that location.