loudspeaker lines. This is particularly noticeable for reproduction of ambiance. The suppression of acoustics in the rendition area does not complement playback of natural acoustic ambient sources. Aliasing There are undesirable spatial aliasing distortions caused by position-dependent narrow-band break-downs in the frequency response within the rendition range. Their frequency depends on the angle of the virtual acoustic source and on the angle of the listener to the loudspeaker arrangement: For aliasing-free rendition in the entire audio range a distance of the single emitters below 2 cm would be necessary. But fortunately, our ear is not particularly sensitive to spatial aliasing. A 10–15 cm emitter distance is generally sufficient. Truncation effect Another cause for disturbance of the spherical wavefront is the truncation effect. Because the resulting wavefront is a composite of elementary waves, a sudden change of pressure can occur if no further speakers deliver elementary waves where the speaker row ends. This causes a 'shadow-wave' effect. For virtual acoustic sources placed in front of the loudspeaker arrangement, this pressure change hurries ahead of the actual wavefront whereby it becomes clearly audible. In signal processing terms, this is spectral leakage in the spatial domain and is caused by application of a rectangular function as a window function on what would otherwise be an infinite array of speakers. The shadow wave can be reduced if the volume of the outer loudspeakers is reduced; this corresponds to using a different window function that tapers off instead of being truncated. High cost A further and resultant problem is high cost. A large number of individual transducers must be very close together. Reducing the number of transducers by increasing their spacing introduces spatial aliasing artifacts. Reducing the number of transducers at a given spacing reduces the size of the emitter field and limits the representation range; outside of its borders no virtual acoustic sources can be produced. Research and market maturity Early development of WFS began 1988 at Delft University. Further work was carried out from January 2001 to June 2003 in the context of the CARROUSO project by the European Union which included ten institutes. The WFS sound system IOSONO was developed by the Fraunhofer Institute for digital media technology (IDMT) by the Technical University of Ilmenau in 2004. The first live WFS transmission took place in July 2008, recreating an organ recital at Cologne Cathedral in lecture hall 104 of the Technical University