Echo

An echo occurs when a sound wave reflections are so severely retarded that you can perceive this sound as a separate auditory event. Concentrated and strong late reflections are heard as separate echoes. The term derives from the Greek nymph Echo Ηχώ ago.

A single separately perceived echo has the same pitch as the original, but the sound volume is always lower than that of the original. Echoes are used by the ear to appreciate room sizes and distances. This particular reflections below 30 ms are relevant. Reflections, which arrive later, are perceived as second sound, so as an echo.

Echo and reverberation

The delays that must have a reflected sound to be perceived separately, is referred to as an echo or echo threshold detection threshold. The echo threshold is strongly dependent on the characteristics of the sound and the level of reflection. It can be between 20 ms (with " clicks " ) and several seconds are ( at slow orchestral music ). Below the echo threshold, the reflections are perceived as reverberation. Echoes are particularly easily perceived when only one or a few reflective surfaces exist (bridges, rock walls, etc.), in a variety of reflective surfaces ( such as churches ) is caused by the density of the diffusivity rather the sensation of a reverb.

Technical Applications

Location of obstacles

Under varieties are meant to determine the location of an obstacle by active transmission of wave packets. The receiver uses a controlled transmitter, the target is passive; see sonar and bats. If these wave packets reflected by obstacles, the direction and distance of the obstacle can be determined from the direction and the duration of arriving at the antenna site reflections.

Since the sound speed is constant in homogeneous dispersion media, the distance can be estimated from the propagation time of the signals. Here, in order to be able to distinguish transmitted signals and echoes the transmitted signal must be much shorter than the shortest expected life of the reflections. Determining the direction of arrival of the echoes can take place using a plurality of receivers in different locations. From the time differences between the receivers, the direction of the echo can then be determined.

An alternative to using a plurality of receivers, the use of the pivotable transmitters and receivers, and the transmission of collimated signals. Then only obstacles are detected, which are located just in the direction of " transmission lobe ". The transmit direction corresponds to the direction of the obstacle.

In order not to have to wait after sending a wave packet until all reflections are received, the sender can send continuously, but with time-dependent frequency. From the frequency at which an echo arrives at the receiver, there is the time at which this frequency is sent and the duration thereof and hence the distance of the obstacle. The same effect can be achieved by the transmission of time-dependent code. By the receiver compares the time at which the currently received code was shipped, it can determine the duration and the distance from it.

On ships echo sounders are used to determine the water depth. More applications can be found among others in the medicine to the sonography ( diagnostic ultrasound ). See also echolocation.

Several groups of animals developed a echolocate to hunt prey in low light habitats or at night. The most famous example is the bats ( Microchiroptera ). Even blind people can sometimes use echolocation to orient themselves in their environment by click your tongue and it took the reflections as secondary signals for orientation.

Sound equipment

In the sound recording in the recording studio strong reflections and excessive reverberation are generally undesirable, mainly because they shape the sound impression on one side. So sound acoustic recordings partially for " garage " or " such as in the bathroom ." Here anechoic recordings are rather aimed at by appropriate post-processing with artificial addition of echo and reverberation (see effects unit ) to produce exactly the artistic desired spatial impression or effect. A further possibility is the lining of the recording studio with sound absorbing materials, so that reflections are avoided at the walls, instead of the sound is absorbed (see insulation ).

Note that, in the sound detecting the direction of the primary signals from the direction of sound, with localization is not designated location.

Echo in the literature and in music

In literature and in music, the term echo is often a metaphor for the distorted return of tones and motifs, which often serves as a structural model. As early as the 5th century BC, the elegi echoici and versus echoicus are occupied as an art medium in Euripides and Aristophanes. Again, the model is significant at the setting of the so-called echo poems in the music of the 16th to the 18th century, but also for the setting of liturgical texts in church music. The baroque emblems sees the music itself as an echo of the divine or of divine love.