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In audio signal processing and acoustics, an 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.
The word echo derives from the Greek ἠχώ (ēchō), [1] itself from ἦχος (ēchos), 'sound'. [2] Echo in Greek mythology was 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 (dolphins and whales) and bats in a process known as echolocation. Echoes are also the basis of sonar technology.
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. [3] 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).
In sonar, utrasonic 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 uses ultrasonic 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. Using the formula d = (V*t)/2, the distance from the obstacle is found. Echo depth sounding is the process of finding the depth of the sea using this process.[ citation needed ] In the medical field, ultrasonic waves of sound are used in ultrasonography and echo cardiography. [4]
In music performance and recording, electric echo effects have been used since the 1950s. The Echoplex is a tape delay effect, 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.[ citation needed ]
Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical engineer. The application of acoustics is present in almost all aspects of modern society with the most obvious being the audio and noise control industries.
Sonar is a technique that uses sound propagation to navigate, measure distances (ranging), communicate with or detect objects on or under the surface of the water, such as other vessels.
Ultrasound is sound with frequencies greater than 20 kilohertz. This frequency is the approximate upper audible limit of human hearing in healthy young adults. The physical principles of acoustic waves apply to any frequency range, including ultrasound. Ultrasonic devices operate with frequencies from 20 kHz up to several gigahertz.
In Greek mythology, Echo was an Oread who resided on Mount Cithaeron. Zeus loved consorting with beautiful nymphs and often visited them on Earth. Eventually, Zeus's wife, Hera, became suspicious, and came from Mount Olympus in an attempt to catch Zeus with the nymphs. Echo, by trying to protect Zeus, endured Hera's wrath, and Hera made her only able to speak the last words spoken to her. So when Echo met Narcissus and fell in love with him, she was unable to tell him how she felt and was forced to watch him as he fell in love with himself.
Echo sounding or depth sounding is the use of sonar for ranging, normally to determine the depth of water (bathymetry). It involves transmitting acoustic waves into water and recording the time interval between emission and return of a pulse; the resulting time of flight, along with knowledge of the speed of sound in water, allows determining the distance between sonar and target. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes.
The epicenter, epicentre, or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates.
Bathymetry is the study of underwater depth of ocean floors, lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water depth measurements are from Ancient Egypt over 3000 years ago. Bathymetric charts, are typically produced to support safety of surface or sub-surface navigation, and usually show seafloor relief or terrain as contour lines and selected depths (soundings), and typically also provide surface navigational information. Bathymetric maps may also use a Digital Terrain Model and artificial illumination techniques to illustrate the depths being portrayed. The global bathymetry is sometimes combined with topography data to yield a global relief model. Paleobathymetry is the study of past underwater depths.
Echolalia is the unsolicited repetition of vocalizations made by another person. In its profound form it is automatic and effortless. It is one of the echophenomena, closely related to echopraxia, the automatic repetition of movements made by another person; both are "subsets of imitative behavior" whereby sounds or actions are imitated "without explicit awareness". Echolalia may be an immediate reaction to a stimulus or may be delayed.
Sound localization is a listener's ability to identify the location or origin of a detected sound in direction and distance.
A fishfinder or sounder (Australia) is an instrument used to locate fish underwater by detecting reflected pulses of sound energy, as in sonar. A modern fishfinder displays measurements of reflected sound on a graphical display, allowing an operator to interpret information to locate schools of fish, underwater debris, and the bottom of a body of water. Fishfinder instruments are used both by sport and commercial fishermen. Modern electronics allow a high degree of integration between the fishfinder system, marine radar, compass and GPS navigation systems.
Ultrasonic testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse waves with centre frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion and erosion. Ultrasonic testing is extensively used to detect flaws in welds.
Delay is an audio signal processing technique that records an input signal to a storage medium and then plays it back after a period of time. When the delayed playback is mixed with the live audio, it creates an echo-like effect, whereby the original audio is heard followed by the delayed audio. The delayed signal may be played back multiple times, or fed back into the recording, to create the sound of a repeating, decaying echo.
The Echoplex is a tape delay effect, first made in 1959. Designed by Mike Battle, the Echoplex set a standard for the effect in the 1960s—it is still regarded as "the standard by which everything else is measured." It was used by some of the most notable guitar players of the era; original Echoplexes are highly sought after.
Parking sensors are proximity sensors for road vehicles designed to alert the driver of obstacles while parking. These systems use either electromagnetic or ultrasonic sensors.
Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water, its contents and its boundaries. The water may be in the ocean, a lake, a river or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and 1 MHz. The propagation of sound in the ocean at frequencies lower than 10 Hz is usually not possible without penetrating deep into the seabed, whereas frequencies above 1 MHz are rarely used because they are absorbed very quickly.
Ultrasonic transducers and ultrasonic sensors are devices that generate or sense ultrasound energy. They can be divided into three broad categories: transmitters, receivers and transceivers. Transmitters convert electrical signals into ultrasound, receivers convert ultrasound into electrical signals, and transceivers can both transmit and receive ultrasound.
Picosecond ultrasonics is a type of ultrasonics that uses ultra-high frequency ultrasound generated by ultrashort light pulses. It is a non-destructive technique in which picosecond acoustic pulses penetrate into thin films or nanostructures to reveal internal features such as film thickness as well as cracks, delaminations and voids. It can also be used to probe liquids. The technique is also referred to as picosecond laser ultrasonics or laser picosecond acoustics.
Repetition pitch is an unexpected sensation of tonality or pitch that often occurs in nature when a sound is reflected against a sound-reflecting surface, and both the original and the reflected sound arrive at the ear of an observer, but with a short time delay between the two arrivals.
Geophysical MASINT is a branch of Measurement and Signature Intelligence (MASINT) that involves phenomena transmitted through the earth and manmade structures including emitted or reflected sounds, pressure waves, vibrations, and magnetic field or ionosphere disturbances.
An underwater survey is a survey performed in an underwater environment or conducted remotely on an underwater object or region. Survey can have several meanings. The word originates in Medieval Latin with meanings of looking over and detailed study of a subject. One meaning is the accurate measurement of a geographical region, usually with the intention of plotting the positions of features as a scale map of the region. This meaning is often used in scientific contexts, and also in civil engineering and mineral extraction. Another meaning, often used in a civil, structural, or marine engineering context, is the inspection of a structure or vessel to compare actual condition with the specified nominal condition, usually with the purpose of reporting on the actual condition and compliance with, or deviations from, the nominal condition, for quality control, damage assessment, valuation, insurance, maintenance, and similar purposes. In other contexts it can mean inspection of a region to establish presence and distribution of specified content, such as living organisms, either to establish a baseline, or to compare with a baseline.
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