2. The spatial sonic field

2.1 Room depiction in channels

Since Alan Blumlein has invented the stereophony principle, we store the spatial impression as the signal difference between separate channels. Studio productions have reached a very sophisticated stage today in this matter. The playback is mostly very pleasant, often each voice or each instrument is much better audible as in the live performance. In the most cases, the goal of playback is no longer the reproduction of a live event, the recordings much more a product of art.

Basically yet, we are not able to restore the spatial impression of the live event in the conventionally audio procedures. Contemplate correctly we are playing Mono signals at such studio productions. The sound sources are recorded by near assembled microphones or derived directly from the pick up. The positions at the stereo stage are specified by pan- pot and post- editing, and reverberation then completing the spatial room impression.

As far as we are pursuit alone the goal of pleasant perception, such sort of procedure is satisfying. However, for reproducing the clarity of a voice in excellent acoustics, the hum of a bee closely at our nose, not to mention the emotional impression of the spatial sonic field in one of the famous concert halls in the world, the phantom source perception, which connected by channel based procedures, completely break down.

In previous chapter was described, by which reason isn't possible capturing the correct spatial distribution of sound waves in the recording room by a set of microphones. Thus, we have to check the question, whether the main approach, which we are pursuit since Alan Blumleins times, is the right way? In any case, those channel oriented basic approach will lead to phantom acoustic sources. Nevertheless, phantom sources always migrate with the listener's position. Correct spatial depiction of the sound event isn't possible at this reason. The spatial sonic field isn't describable as signal deviation between some dedicated points, this difference unequally at any spot in room. For authentic reproduction, we have to restore the spatial structure of the wave fronts inside a building volume. Condition for that is that a migration of the listener in the playback room has to cause the same changes in perception, as adequate migration of a listener in recording room.

2.2 The mirror source approach

This task becomes solvable at another approach, mainly differently Blumlein`s procedure. We have to constitutie, the sound source itself, any soloist or any instrument, doesn't pose a kind of spatial sonic field. If we take a closer look, indeed the spatial radiation patterns are differently for each of the sources. However, any spatial perception though caused at the reflections of the source signal in the recording room.

These reflections seem to originate from mirror sound sources. Its coordinates in the recording room are dependent from the arrangement of the reflective surfaces and from the position of the primary sound source.

The starting point's of the first reflections apparently arranged outside the recording room. These primary reflection starting points are the source for further reflections behind the opposite walls.

As visible in this simple animation the second reflections don't radiate a mirrored acoustic pattern of the recording room, as described in some publications. This fiction leads to the nebulous picture of the spatial sonic field, which is extensively discussed in endless disputes concerning the perfect rendition. However, also the second, third and all further reflections radiate no other signal as the pure audio of the primary sound source!

The level of the first reflection depends by adjustment and the directional radiation pattern of the straigth sound source. That response, including in the source signal for second reflection and, of course, the contemplative behavior of each surface in the signal way superposes its own behavior across the later signal. Additionally, the sound pressure level is decreasing according to rising distance regarding the primary source. This distance will escalate by each number of the reflections, wherefore the later signals more and more damped in upper frequency range from air damping. Even so, in any case, each single of the reflections radiates the pure, dry audio signal of the primary sound source.

As far as we would be able to describe the reflective behavior of the recording room and the alignment and position of the acoustic sources, including its directional radiation pattern, it would become possible to define all positions and sound pressure levels of the mirror sound sources therein. That data would completely describe the spatial sonic field for one single sound source.

By that stored data set, we would be able to restore the complete spatial sonic field in a reflection free volume, for example on a snowy meadow. For this purpose, we have to play in a huge number of single loudspeakers the dry recorded voice of the Tenor. Any single of the separately loudspeakers would typifiy one of the mirror source positions in the recording room. The signal content of each of those sources would be the signal of the respectively supplying mirror source, filtered by the reflective behavior according to the surface.

At growing distance regarding the primary source the number of loudspeakers will arise exponentially, because each of the mirror sources causing a number of secondary mirror sources. In Principle, infinite amount of loudspeakers would be need. But in, lets say 3000 foot distance from the real audio source, which equals more as 2,5 seconds runtime across the chilly air across the snowy meadow, weakened from 20 or 30 reflection losses and the air damping according the long distance, the signal level falls below the audible -60 dB limit, which is marking the reverberation time. Thus, we can dispense tose further loudspeakers.

Nevertheless, such assembly will not be feasable in practise, also due to the fact, we would need an own group of loudspeakers for each single primary source in the recording room, because of different mirror positions and levels would arise. Over and above, all loudspeakers must changing its position, in case the Tenor makes a step across the stage.

On the other hand, a migration of the listener across the snowy meadow is causing the same changes in perception, as according change of the listeners position in the recording room. The tenors voice will be louder and drier, if we go near the loudspeaker which is radiating its direct wave. The first reflections considerably change its angle of incident and level. Only the later reverberation from the further distanced loudspeakers is hardly changed in level and direction. The superposition of the direct wave front and early strong reflections will cause the same deep comb filter effects in the response as arising in recording room at such position. Even the initial delay time will deliver the correct gap for authentic perception of the source distance.

Connected the firmly sound source positions, congruent Doppler effects and parallaxe changes will arise. The deliberation shows, there is possible in principle, building up a virtual copy of the genuine sonic field. All we need is to let wander our minds, free of deadlocked limits of the conventionally ways. The technical transformation is possible long since. As Einstein has said, we live in a time of unlimited possibilities, but unclear goals. The way for realizing the described approach is sketched in the next chapters.

next chapter>