Head-related transfer function
The head-related transfer function (mostly only HRTF , more rarely head-related or outer ear transfer function ) describes the complex filter effect of the head, outer ear ( pinna ) and trunk. This amplitude evaluation is, in addition to the transit time differences between the ears, an essential basis of our acoustic localization system .
The elevation of the sound source does not cause any transit time differences, nor does a distinction between frontal and rear sound sources via which otherwise fundamental transit time differences would be possible for our localization. In the overtone range , the transit time localization fails completely because several wavelengths of the sound fit into the ear distance and the evaluation becomes ambivalent .
Therefore we are also dependent on the evaluation of amplitude differences. These increase with the frequency , because diffraction effects on the head and trunk become weaker with increasing frequencies or shorter wavelengths and thus the sound shadow becomes more pronounced. As the frequencies continue to rise, angle-dependent resonance phenomena on the outer ear enable localization in the elevation plane.
In principle, these increases and decreases are described in the Blauert bands , but above all the position of the zero points in the resonance phenomena is very different from one individual to the other. Because the HRTF localization is mainly based on learned stimulus patterns , no generalized function can be given due to the anatomical differences. The artificial head stereophony, which is based on this filter effect, only works satisfactorily if the differences to the hearing person are not too great.
In the case of loudspeaker reproduction based on phantom sound sources , the complex filter effect of the outer ear leads to significant errors, because the angle of incidence of the wave fronts usually does not match the original sound field . This causes strong amplitude errors, which u. a. lead to the elevation of the phantom center. The only remedy could be a physical reconstruction of the original sound field, as is aimed at in the holophonic approach.
See also
literature
- Jens Blauert: Spatial hearing . S. Hirzel-Verlag, Stuttgart 1974, ISBN 3-7776-0250-7 .
- 1. Postscript. New results and trends since 1972 . 1985, ISBN 3-7776-0410-0 .
- 2. Postscript. New results and trends since 1982 . 1997, ISBN 3-7776-0738-X .