The blueprint of the spatial sonic field/ chapter 4

Would closely spaced loudspeakers completely cover all walls of the playback room, would be able restore sound pressure and sound velocity at any point inside that volume. That´s prove by the Kirchhoff- Helmholtz- Integral, which the mathematical base for the Wave Field Synthesis procedure. Besides the effort, in home cinema range the acceptance factor would avoid any success of such attempt.

This chapter describes different idea of the author, surmounting the horizontal limitation of the loudspeaker rows and provide excellent assimilation in home cinema by assemble the loudspeakers alone behind the silver screen. Comparably the well known sound projectors, which simulate fake loudspeakers by means of wall reflections, the WFS loudspeaker screen simulate the recording room wall reflection by means of playback room wall reflection, as shown at animation in introduction chapter.

4.1 Near field solution at“acoustic curtain”

Besides wearing headphones the only solution for avoiding the unwanted impact of living room acoustics is near field reproduction. Two possibilities exist for that. Either the speakers are placed very near at listener or the radiating diaphragm becomes very large. In normal living room environments, the distance separating speakers and listener is around three to four meter.

Dependent from loudspeaker bunching factor and playback room reverberation time the playback room reflections exceed the direct radiation in normal dwellings at less than one meter. For including a three meter distanced listener in near field we would need a anechoic living room or a diameter of the diaphragm exceeding 1, 5 meter. That's impossible for a single speaker, however using the wave field synthesis principle, the loudspeakers work altogether as unit. Such two dimensionally loudspeaker screen perform the known principle of the acoustic curtain. More as a half century bygone such solution was a dream of the inventors, at the WFS principle such approach becomes feasible today:

Each of the membrane excursions in such speaker field is simply calculable in terms of distance regarding the virtual starting point. The bending of the resulting common diaphragm is depend by virtual starting point position and frequency, per eaxmple 440 Hz , virtual source 3 meters behind the screen of 48x27 Loudspeakers:

For decreasing frequence the curvature disappear, all loudspeakers executing common plunger motion. If used two inch distance between the single transducers, that diaphragm would have a size of 2.43m (~8') x 1.38 m (~4.5'); therefore, the listener position include in near field range. Yet spatial aliasing effects would occur, if not provide sufficient amount of loudspeakers. Dependent from ingoing and outgoing angles regarding the radiating surface such effects arise above this frequency:

For instance, a 30 degree angle difference would result in aliasing above 13.5 kHz for two inch spaced speakers. That's an acceptable value; it is known that our perception isn't very sensitive regarding spatial aliasing in this frequency range. But such alignment would cause total amount of 1296 single speakers. Technically such field is feasible today; some alignments already in existence contain comparable numbers of speakers. Actually, the development goes away from the horizontal rows towards “speaking screen”. The straightening effect of such large loudspeaker screen avoid supply of the playback room mirror sources unwantedly by sound energy. That substantially degrade the disturbing influence of playback room acoustics. On the other hand, targeting supply of the first reflection sources in playback room by sound energy open a way for include playback room reflections on purpose. That describe in the next chapter.

4.2 Inclusion of the playback room acoustics in Holophony approach

There are a lot of scientific publications regarding Wave Field Synthesis principle, though by which reason the principle really is a revolution in audio reproduction hardly mentioned until today . In difference regarding all other audio reproduction principles wave field synthesis provide access at each component of the sound event. No other principle would be able for manipulate during playback direct wave, first reflections and reverberation in different manner. All conventionally procedures merge together the components inseparably already during the record.

On account of that advantage Wave Field Synthesis opens unimagined possibilities for eliminate problems of audio reproduction, which was estimated unsolvable until today. For implementing this advantage we have to abandon the prevalent system approach. Traditionally, the transmission chain start at the microphone and ends by the loudspeaker. But mostly significant signal changes arise, if the loudspeakers have done its work. For congruent perception the inclusion of the playback room properties into the system approach is indespensable. The described Holophony approach doesn't aim providing perfect signals in the loudspeakers. The main goal is the signals at the listeners ears at home match the signals on dedicated virtual listener position in the recording room. For pursuit that goal, we need to introduce such dedicated listener position in the recording room and a default listener position in the Playback room. By using this new system design, we no longer need supress the playback room acoustics. Different sound detours and sound level changes in playback room become equalize during synthesis by means of both listener positions set in congruence. The model based approach subtracting additionally detours of the first reflections in the playback room. Thereby the arrival angles and reflection factors between recording and playback room get equalize.

In very simple model we can calculate direct wave and first main wall reflection for that purpose. That's the most significant cues regarding time and direction. If lock sufficient initial time delay gap, all later reflections are less important regarding time and direction. For common model must be known the recording and the playback room geometry. Point of origin in the common geometrical system the both superimposed listener positions. The outer room 3a in the scetch is the recording room, inside depict the playback room 3b. In the centre is the common listener position in recording and playback room, 3i:

The source 3c become restored by the “acoustic curtain” of the frontal speakers nearly perfect. The first reflection mirror source in recording room resides outside that range. We cannot radiate directly. But if the position in range of the loudspeaker fields mirror source in playback room, from item of listener position, we can employ the playback wall reflection in that purpose. Thus, in order to simulate the first reflection correctly, we have to shift the recording room's mirror source position in common system in that range. This is accomplished by a simple rotation in a circular path inside the common coordinate system. Mirroring this position at playback room's geometry deliver the final starting point (4b) coordinate of the virtual source. Calculating the runtimes and levels regarding each speaker is very easy in the common model, because only the distance regarding the respective speaker determines the delay time for the corresponding audio signal..

The animation sketch shows the principle for restore the perceived altitude of recording room in playback room. Corresponding procedure regarding all main surfaces restore all room dimensions which originally perceived in recording room.

4.3 Combination of model based and data based approach

Restore all of the reflections correctly was out of reach during early stage of Wave Field Syntesis. By reason of insufficient computing power was reduced the principle at the horizontal level of the listener. Until today, effective calculation procedures for extrapolation and interpolation of the measured values regarding the given virtual source and loudspeaker positions [3], or field related solution approaches as described in [4], matter of development in the scientific institutes. The high level of effort in this matter underscores the relevance of the Principle of WFS as spatial audio solution. Though, until today the WFS isn't realised in practise in all three room dimensions.

Potentially, the combination of the model based and the data based approach would solve remaining problems: The reverberation contains important cues regarding the recording room properties. The fine structure of the surface, which determines the timbre of the room, becomes reproduced by the reverberation tail. Convolution into the impulse response of the recording room is an approved method for reconstitute the reverberation very authentic. Yet from what direction arrive the wave fronts of the reverberation tail at the listener of subordinate relevance for perception. Also in recording room, the reverberation comes from all possible directions; we cannot pinpoint the origin of second or later reflections. On the other hand, the direct wave front and its first reflections in the recording room hardly determine the timbre, but contain the most important localisation cues regarding source position. By means of optically relationships we have got the skill for determine position of those acoustic sources very accurately. Humble distinctions in arrival time or amplitude of those wave fronts permit its localisation in all three room dimensions.

The sound level of the first reflections often hardly differs regarding direct wave. Thus superposition results in deep comb filter effects. In case time relations are correct, resulting notches in frequency response are  meaningfully cues regarding room impression. But wrong detours result in different pattern regarding hills and notches. The level of the resulting misguiding cues often above 20 dB. But equalising in frequency domain cannot diminish those time domain caused faults as far as all signal components already merge together. Because of the different conditions in perception doesn't meaningful finishing direct wave and its first reflections in same manner during the wave field synthesis, like widespread practise in WFS. The completely impulse response based approach delivers perfect results, but overcharge current available computing power, especially for moved sources in three dimensionally environments.

Perception of reverberation hardly change, if source or listener change its position in the recording room. Yet level changes and direction changes of the direct wave and first reflections result in substantially alter perception. The calculations of those wave fronts feasible much more easily in the model based approach. The following screenshot show a way for combine the model based approach for first reflections with the impulse based method for reverberation: 

Combination of Imp/ Model based WFS; Please klick the picture for enlarge

The model based treatment on left side of sketch via MADI or LAN connected at the Engine part at the right side. Such breakup in computing and engine was proposed by Wolcott in [5]. The interface transmits all audio source signals and all data regarding delay time and Level for each single loudspeaker position. But for those high amount of data values – each speaker needs for each primary source and for each of its first reflections in the recording room a separate delay and level value – according [5] are eight updates per second sufficient for an smooth movement of the source. The screenshot shows those calculation results for one single loudspeaker in a 32 channel system:

according delay and level calculations for two of the speakers, click for change values in the animation

By click in the sheet open a animation. In the calculation results first the soloist change his position in the recording room, goes from stage area in direction of recording room middle. As clearly visible the change of the source coordinates doesn't change alone time delay and level regarding direct wave front. All first reflections changing in the common system from recording and playback room, as describe above. Those changes are different for each dedicated loudspeaker. The values in the upper part of the sheet show the calculation for speaker in centre of the loudspeaker screen, the lower part allocate loudspeaker in the upper right corner.

Because of directed radiation inside loudspeaker near field the sound waves hardly cause playback room reflections. Such reproduction is able for render possible sound sources very near at the listener. The divergence problem solution according 102006054961A1 already is include in calculation. The model based calculation constitutes really three dimensions. The combination with the impulse response method for the reverberation tail is very practicable in the described procedure. In the example the channels 01 and 02 are reserved for constitute the reverberation. That signal create by convolution of the summary signal from all input sources into the impulse response of the recording room. The model calculation deliver a time value, during the first frames inside those impulse response must become suppressed in order to avoid a double production of first reflections.

Another way would be simply recording the reverberation in a discrete channel with an omni- directionally microphone apart all direct sound sources. During synthesis reverberation level only depended from overall volume. The later reflections come from all directions, what match its spatial distribution in the recording room. Huge advantage of the described proceeding the dispensability of spatial impulse response, recorded by a team of highly qualified technicians in preparation of a wave field synthesis recording. Conventional impulse response, which existent for all interesting environments today sufficient. Together with a crude geometric model from recording room each dry recorded signal can render as would it recorded in most attractive acoustic environments around the globe.

4.4 Compatibility

The Holophony approach convenient for reproducing conventionally recordings. The steerable wave fronts deliver some important advantages in comparison to solitary speakers:

¦ Mono: The speaker field generates parallel wave fronts. Its high direct sound portion and the low remaining influence of the playback room acoustic ensure excellent speech intelligibility.

¦ Focus Mono: Focused wave fronts effectuate high volume nearby the focus point. For other ranges in the playback room the signal hardly audible. This allow different signal contents in common room.

¦ Stereo: The most important advantage of the described Holophony procedure the ability costomize playback room acoustic during reproduction. The directed radiation widely avoid its wrong reflections. On the other Hand, synthesized early reflections can provide large playback room impression. That allay most audible deterioration during conventionally reproduction, caused by fact, the playback room acoustic widely differs regarding acoustics of recording room.

¦ Surround: Virtual sound sources beyond real playback room enlarge sweet spot. The huge effective diaphragm ensure near field for lessen influence of the playback room acoustics. Synthetic virtual sources outside horizontal level of listener provide 3D illusion. Increasing amount of canals for future possible by software update. Faked reflections resize the playback room virtually. And the most important fact: Excelent spouse acceptance factor for the unvisible speakers.

¦ Ambisonic: Grinding in ambisonics decoder suitable approach regarding those very realistic audio reproduction procedure. The number of virtual Loudspeakers at virtual positions may equivalent the number of input canals of the wfs- processor. That makes the loudspeaker screen applicable for high order ambisonics.

¦ Wave Field Synthesis: Suitable recordings for the WFS- procedure deliver a spatial sound field, hardly distinguishable from genuine. For studio productions suitably virtual environments from stored libraries deliver adapted playback environments. The playback room acoustic subtract during playback synthesis, in acoustic matter the listener become placed in recording room.