Depth graduation

Depth gradation is an illusion in stereo sound recording (stereo mix) that is based on hearing from a distance . In principle, phantom sound sources are only generated one-dimensionally, on a line between the loudspeakers. Depending on the transit time and direction of the first strong reflections , the level of the signals and their correlation as well as the ratio of direct sound to reverberation, we still perceive a different distance to the sound source.

Basics

The Inter Channel - transit time differences between L and R at loudspeaker stereophony are for direction localization involved ms up to the order of at most. 3 The sensation of depth stepping but not dependent on, because this has greatly with the initial time gap (Initial Time Delay Gap = ITDG) and the early reflections to do; see images under web links.

An initial time gap of less than 15 ms allows the sound event to take effect, while a “gap” of 40 ms makes it closer. This apparent contradiction can be explained geometrically. If the distance between the sound source and the listener is greater, the angle between the waves that cause direct sound and the first reflection is flatter, which is why the “detour” that the first reflection has to make is shorter. The predelay or the transit time delay in reverb devices is only a very primitive replica of this naturally "living" initial time gap that applies to the entire reverb program. In the concert hall , the initial time gap is always different depending on the location of the sound source (and the listener). The initial time gap is not a space constant.

When describing a room for sound recordings (reverb balance), one speaks of a sound-determining R / D or D / R ratio; see direct sound and room sound . The choice of the location of the microphones largely determines the quality of the electro-acoustic transmission, in which the direct and spatial sound field structures in the original room have to be "converted" to the mostly completely different acoustic and geometric conditions of the playback room. The distance of the microphones from the sound source is very important, whereby the directional characteristics of the microphones must also be observed. A characteristic acoustic-physical reference point is the reverberation radius .

The stereo process

While runtime stereophony shows the depth gradation and spatiality quite strongly with somewhat diffuse localization , the intensity stereophony is characterized by good localization of the phantom sound sources. The disadvantage of the usual support microphone technology is that the sound sources are mapped without any distance features and thus the sound recording also remains without depth graduation. This deficiency can be alleviated by delaying the addition of the support microphones.

Large-scale works, with vocal soloists, orchestra and choir, for example, have several sound levels of spatial depth, for example the vocal soloists in front, the orchestra a little further away and the choir behind. If the distribution of the sound sources in the individual distance levels is coordinated with one another, the clarity, i.e. the transparency of a stereo sound image is promoted.

A pronounced gradation of depth does not correspond to the experience from natural hearing, at least with music recordings, but only appears as an acoustic perspective from the position of a main microphone , but also from the position of the conductor. The gradation of depth is at the same time a gradation of importance. From everyday listening experience, it is the near, that which speaks to us, which appears to be more important than the more distant. The gradation of depth generated in a sound recording can be more pronounced, the larger the occupation and the larger the perceived space. A dry "foreground" must be ensured in order to increase the depth gradation. The acoustic representation of the distance is not easy to implement. Because of the single microphone method used here, the depth gradation is of no importance in the field of light music. Support microphones that are used too much destroy any existing depth graduation.

literature

Michael Dickreiter, Volker Dittel, Wolfgang Hoeg, Martin Wöhr (eds.), "Handbuch der Tonstudiotechnik", 8th, revised and expanded edition, 2 volumes, publisher: Walter de Gruyter, Berlin / Boston, 2014, ISBN 978-3- 11-028978-7 or e- ISBN 978-3-11-031650-6
Hubert Henle: The recording studio manual. Practical introduction to professional recording technology. 5th completely revised edition. Carstensen Verlag, Munich 2001, ISBN 3-910098-19-3 ( Factfinder series ).

Web links

Individual evidence

↑ What does the important initial gap ITDG mean - pdf (46 kB)

[1] What does the important initial gap ITDG mean - pdf (46 kB)