An echo (also called reverberation ) occurs when reflections of a sound wave are delayed so much that they can be perceived as a separate auditory event ; thus the echo differs from the reverberation . Concentrated and strong late reflections are heard as separate echoes. The term echo is derived from the mythological figure of the same name , whose name can be traced back to ancient Greek ἠχή ēchē "sound".
A single separately perceived echo has the same pitch as the original, but the pitch is always lower than that of the original. Echoes are used by the ear to estimate room sizes and distances. Reflections below 30 ms are particularly relevant here. Reflections that arrive later are perceived as a second tone, i.e. as an echo.
The delays that a reflected sound has to have in order to be perceived separately is called the " echo threshold " or "echo perception threshold". The echo threshold is strongly dependent on the characteristics of the sound and the level of the reflections. It can be between 20 ms (for "clicks") and several seconds (for slow orchestral music). Below the echo threshold, the reflections are perceived as reverberation . Echoes are particularly easy to perceive when there are only one or only a few reflective surfaces (bridges, rock walls, etc.); if there are a large number of reflective surfaces (e.g. in churches), the density of diffusion tends to give the impression of reverberation.
Under places refers to the determination of the location of an obstacle by actively sending out wave packets. The receiver uses a controlled transmitter, the target is passive; see depth sounder and bats . If these wave packets are reflected by obstacles, the direction and distance of the obstacle can be determined from the direction and the transit time of the reflections arriving at the transmitter location .
Since the speed of sound is constant in homogeneous propagation media, the distance can be estimated from the transit time of the signals. In order to be able to distinguish between transmitted signals and echoes, the transmitted signal must be significantly shorter than the shortest expected transit time of the reflections. The direction from which the echoes arrive can be determined with the help of several receivers at different points. The direction of the echo can then be determined from the differences in transit time between the receivers.
An alternative to using multiple receivers is to use pivotable transmitters and receivers and send bundled signals. Then only obstacles that are currently in the direction of the “transmitting lobe” are detected. The direction of transmission then corresponds to the direction of the obstacle.
In order not to have to wait until all reflections have arrived after sending a wave packet, the transmitter can also send continuously, but with a time-dependent frequency. The time at which this frequency was sent results from the frequency with which an echo arrives at the receiver and from this the transit time and thus the distance of the obstacle.
The same effect can be achieved by sending time-dependent codes. By comparing the point in time at which the code just received was sent, the recipient can determine the transit time and, from this, the distance.
Echo sounders are used on ships to determine the water depth. Further applications are among others in the medicine in sonography ( ultrasound - diagnosis ). See also echolocation .
Several groups of animals developed an echolocation system to hunt prey in low-light habitats or at night. The best-known example are the bats (Microchiroptera). Even blind people can sometimes use echolocation to orient themselves in their environment by clicking their tongues and using the reflections as secondary signals for orientation.
When recording sound in the recording studio , strong reflections and excessive reverberation are generally undesirable, mainly because they shape the sound impression on one side. Acoustic recordings sometimes sound like “garage” or “like in the bathroom”. Here, more reflective-free recordings are sought in order to be able to create the artistically desired spatial impression or effect through appropriate post-processing with artificial addition of echoes and reverberation (see effect device ). Another option is to line the recording studio with sound-absorbing materials so that reflections on the walls are prevented and the sound is absorbed instead (see insulation material ).
It should be noted that in audio engineering, determining the direction of the primary signals from which the sound is coming is referred to as localization and not localization .
Echo in literature and music
In literature and music, the term echo is often a metaphor for the distorted recurrence of tones and motifs, which then often serve as a structural model. Already in the 5th century BC In Euripides and Aristophanes the elegi echoici and the versus echoicus are documented as artistic means. The model becomes important again for setting the so-called echo poems in music from the 16th to 18th centuries, but also for setting liturgical texts in church music. The baroque emblematic sees the music itself as an echo of the divine or of divine love.
In Greek mythology, a nymph with the name Echó ( ancient Greek ᾿Ηχώ ēchō ) occurs, from which the term for the phenomenon is derived.
- Ferdinand van Ingen: Echo in the 17th century: a literary-musical phenomenon in the early modern period . Koninklijke Nederlandse Akademie van Wetenschappen, Amsterdam 2002, ISBN 978-90-6984-349-0 ( knaw.nl [PDF; 184 kB ; accessed on July 1, 2016]). Better readable here http://www.knaw.nl/publicaties/pdf/20011097.pdf ( Memento from July 13, 2007 in the Internet Archive )
- Felix Urban: DELAY. Diabolical game with the time machines. 1st edition. Scientific articles from Tectum Verlag: Medienwissenschaft, No. 37 . Tectum Verlag, Baden-Baden 2006, ISBN 978-3-8288-4395-0 , p. 276 .
- Where is the echo perception threshold for sound recordings? (pdf; 124 kB) In: website: Forum for recording studio technology. Eberhard Sengpiel and Alexander Sengpiel, 1998, accessed July 1, 2016 .
- ↑ Jörg Jochen Berns: The hunt for the nymph Echo . In: Hanno Möbius, Jörg Jochen Berns: Mechanics in the arts. Studies on the aesthetic importance of science and technology . Jonas, Marburg, 1990, pp. 67-82.
- ↑ Johannes Bolte: The echo in popular belief and poetry . In: Meeting reports of the Prussian Academy of Sciences , 1935, pp. 262–288.
- ^ Walter Vetter: The early German song. Selected chapters from the development history and aesthetics of the one- and polyphonic German art song in the 17th century. Helios, Münster, 1928, p. 289.