Wave field synthesis

Wave field synthesis (WFS ) is a spatial audio rendering process with the goal of creating virtual acoustic environments. The synthesis produces wavefronts emanating from a virtual point. Its acoustic localization is not dependent on the listener's position and not, as in the conventional multi-channel method of psychoacoustic effects such as the phantom source education.

Physical Basics

The synthesis is based on the Huygens principle, which describes that each wavefront can be considered as a superposition of elementary waves. Therefore, can be synthesized from such elementary waves also any wavefront.

To generate sound waves controls a computer synthesis while each of the individual speakers - arranged in rows around the audience - at the very moment at which a virtual wave front would pass through its point in space. Professor Berkhout invented this method in 1988 at the Technical University of Delft.

Mathematical basis is the Kirchhoff -Helmholtz integral, which describes that the sound pressure is determined at each point within a source-free volume when sound pressure and particle velocity are determined at all points of its surface.

Therefore, each sound field can be reconstructed if sound pressure and particle velocity are reconstructed at all points on the surface of the volume. Such a spatial approach describes the principle of Holofonie. It would require all the surface of the volume, so all boundary surfaces of the reproduction room, are equipped tightly packed with monopolies and dipoles for generating sound, all controlled individually with their respective signal. Moreover, the space would need to be anechoic to satisfy the condition of the source of free volume. This is a practical approach not to be realized, so simplified, had to be found practicable method. According to Rayleigh II of the sound pressure in any point of the half-space is determined when the sound pressure is known at any point of a plane. Because our acoustic perception is most accurate in the azimuth plane, the method is generally reduced today to a single speaker series around the listener.

The starting point of the synthesized wave front can be any point within the horizontal plane of the speakers. It represents the virtual acoustic source that is not very different from a real acoustic source in position in their behavior. She appears to be no longer mitzubewegen if the listener moves in the listening area. Convex or concave wave fronts also can be generated, the virtual sound source can also be located within the speaker array and it is even " bypassed ".

Advantages of the method

Use of level and time information stored in the impulse response of the recording room or for model-based approach in a mirror image source model, the wave field synthesis produce a sound field with a very stable position of the sound sources. Basically, it would be based on the approach of Holofonie even possible to physically largely restore the sound field of the receiving space, so that a change in the listener's position in the reproduction room would affect acoustically as well as a corresponding change of location in the recording room. Even with the speaker line is the sound field " durchschreitbar ".

From the Moving Picture Expert Group of the object-oriented transmission standard MPEG-4 was standardized, which allows separate transmission of content ( the dry recorded audio signal ) and form ( the impulse response or the mirror sound source model ). Each virtual sound source requires its own realm (mono ) audio channel.

The spatial sound field in the receiving space is comprised of the direct wave of the sound source, and a spatially distributed pattern of levels of sound sources. They arise from the fact that the direct wave is reflected by any surface of the receiving space. To reduce their spatial distribution to a few speaker positions, as is the case with all conventional channel- based procedures, must inevitably have a significant loss of space information about the episode.

Much more accurately, the sound field can be synthesized on the playback side. The individual sound sources are recorded dry. The reflections of the receiving space are then synthesized reproduction side. Receiving -side problems that are unsolvable in the conventional methods, do not arise in such an object-oriented method. In addition, not all signal components are inseparably mixed during playback; direct wave, early reflections and reverberation are on the playback side separately manipulated.

Even for conventional records, the method offers a distinct advantage: "Virtual panning spots" called virtual sound sources that reflect the signal of the associated channels can be positioned far outside the real reproduction space. This reduces the influence of the listener's position, because the relative changes are much lower in incidence angle and level, as at the nearby real loudspeakers. This expands the sweet spot considerably, he can now extend over almost the entire listening area.

Remaining problems

The most clearly noticeable difference between the original sound field is so far the reduction of the sound field to the horizontal plane of the loudspeaker rows. It is therefore particularly striking because the required acoustic attenuation of the reproduction room hardly mirror sound sources outside this plane arise. However, the condition of the source-free volume would be injured from the mathematical approach without this acoustic damping. Doppelräumigkeit playback would result.

Disturbing is also the " truncation effect". Because the wave front is not formed from a single sound source but by the superposition of all wave fronts of the individual radiators, creates a sudden pressure change if at the end of the speaker arrangement provide no further spotlight more their contribution. This " shadow wave" can be weakened, though the external speakers are reduced in level. For virtual sound sources within the loudspeaker arrangement, however, this change in pressure is ahead of the actual wavefront, making it clearly audible.

Because it is tried in the WFS to simulate than the existing one to another room, the acoustics of the listening room must be suppressed. The one way to make the walls in accordance with absorptive. The second option is to play in the near field. Here, the speakers are very close to the listening area or the membrane surface must be very large.

Another problem is the high cost to date. A large number of individual transducers must be built very close together. Otherwise, spatial aliasing effects can be heard. These arise because an unlimited number can not be generated by elementary waves, as describes the mathematical approach. Due to the discretization arise within the display area position-dependent narrow dips in the frequency response. Their frequency depends on the angle of the virtual sound source and the angle of the listener against the speaker generating front -dependent:

For a largely aliasing -free reproduction throughout the entire listening area after an interval of individual radiators under 2 cm would be necessary. But fortunately, our ear is not particularly sensitive to this effect, so that he hardly interferes even at 10 to 15 cm radiator distance. On the other hand, limits the size of the antenna array to the display area, outside its borders can no virtual sound sources are generated. Therefore, the reduction of the process on the horizontal plane to present seems justified.

Research and market maturity

The recent techniques for WFS have been developed since 1988, first at the TU Delft. As part of the EU-funded project CARROUSO ( January 2001-June 2003 ) conducted research in Europe ten institutes in this field. The research institutes involved were IRCAM, IRT and Fraunhofer, the participating universities were the Delft University of Technology, the University of Erlangen, the AU Thessaloniki and the EPFL in Lausanne ( Integrated System Laboratory), company Studer, France Telecom and Thales involved.

But are for the horizontal speaker series around the listener to acoustically dampen the reproduction room also because of the need, the acceptance problems so great that the method in the home could not prevail. For such a two-dimensional method of breakthrough in this market segment is hardly to be expected, also because Ambisonics with a similar three-dimensional solution for the home exists. Also, additional WFS converter on the ceiling or the inclusion of a 2.5 - D synthesis operator are unlikely to change this situation. A three-dimensional solution proposal that involves the acoustics of the listening room in the synthesis is known, but hardly feasible because of the high costs today. Other efforts are aimed at invisible to integrate the transducer in the reproduction room.

In the WFS usually primary sound, early reflections and the reverberation are processed separately. In one based solely on the impulse response of the synthesis, the process device today for moving sound sources to the limits of available processing power. Therefore, combinations of model-based process for the direct wave front and the early strong reflections with generation of the impulse response based on the location of the sound source is less important reverberation are used. This reduces the required computing power enormously.

Different WFS systems installed - In the past few years have been - especially in the public sector. For the Fraunhofer IDMT belonging IOSONO cinema Linden light games in Ilmenau, for the Bregenz Festival and the Mörbisch, when KI.KA in Erfurt as well as at various universities, the speaker series produce a sound field in which the sound source is incomparably more stable locatable than with conventional sound systems. The WFS sound system IOSONO was developed by the Fraunhofer Institute for Digital Media Technology ( IDMT ) in Ilmenau University of Technology environment for research and is marketed under the name of the IOSONO IOSONO GmbH. Internationally, the wave field synthesis is currently at several universities research. The rapid development in the field of digital signal processors makes the process in the future even in the home for a possible alternative to the conventional channel- based methods, especially as they are confronted with further increases in the number of channels with increasing acceptance problems.

The TU Berlin has transferred together with the WDR on 29 July 2008, a concert from the Cologne Cathedral live in WFS in their lecture room 104. The auditorium with 650 seats, is equipped with the world's biggest sound system on the principle of wave field synthesis. A consisting of a total of 2,700 speakers tape circulates to the auditorium approximately at ear level of the listeners; added FOH speakers and ceiling speakers for height information. The loudspeakers are driven by digital data lines 832 independent signals which are in turn generated by a cluster of computers 16.