page_title: Echo
section_title: Acoustic phenomenon
hierarchical_section_title: Echo / Acoustic phenomenon
caption_attribution_description: Principle of sediment echo sounding. A narrow beam of high energy and low frequency (ab. 3.5 kHz) submitted by the research vessel to the sea floor is reflected by the various layers, providing information about hardness and structure of the sediments. Used by marine geologists as a pre-site survey to find suitable locations for sediment coring.
context_page_description: 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.
  The word echo derives from the Greek ἠχώ, itself from ἦχος, "sound". Echo in the Greek folk story is a mountain nymph whose ability to speak was cursed, leaving her able only to repeat the last words spoken to her. Some animals use echo for location sensing and navigation, such as cetaceans and bats.
context_section_description: Acoustic waves are reflected by walls or other hard surfaces, such as mountains and privacy fences. The reason of reflection may be explained as a discontinuity in the propagation medium. This can be heard when the reflection returns with sufficient magnitude and delay to be perceived distinctly.
  When sound, or the echo itself, is reflected multiple times from multiple surfaces, the echo is characterized as a reverberation. 
  The human ear cannot distinguish echo from the original direct sound if the delay is less than 1/10 of a second. The velocity of sound in dry air is approximately 343 m/s at a temperature of 25 °C. Therefore, the reflecting object must be more than 17.2m from the sound source for echo to be perceived by a person located at the source. When a sound produces an echo in two seconds, the reflecting object is 343m away.  In nature, canyon walls or rock cliffs facing water are the most common natural settings for hearing echoes. The strength of echo is frequently measured in dB sound pressure level (SPL) relative to the directly transmitted wave.  Echoes may be desirable (as in sonar) or undesirable (as in telephone systems).
caption: This illustration depicts the principle of sediment echo sounding, which uses a narrow beam of high energy and low frequency
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