WellenFeld H 104: Hardware
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On a circumferential length of about 86 m, a total of 832 channels are arranged. Only four short areas were left out at the back of the room (for the rear exits and to provide access to the control room). In the rear of the room, the speakers are located just above the heads of the seated listeners. Proceeding to the front, the distance between the speaker and the ear level of the listeners increases, as the line of speakers had to be led along above the front entrance doors. Here, a lower position was not possible, since neither the wide entrance doors could be obstructed, nor were gaps in the line acceptable on this scale.
For Wave Field Synthesis, the smallest possible distance between the channels is desirable. As given before, a speaker distance of 0.1 m was specified for the project presented here. The resulting large number of channels required a correspondingly cost-effective concept, leading to the choice of simple full-range speakers supported by additional (shared) woofers. For the directional pattern of the speakers, a widest possible horizontal radiation angle (ranging up to the highest reproduced frequencies) was selected and combined with a rather narrow vertical radiation angle in order to reduce reflections from the floor and the ceiling of the room.
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The speakers were conceptualized as eight-channel modules with width 0.8 m, which can be connected in series so that the line is extended with equidistant channel spacing. Each channel consists of three full range speakers driven via a simple passive crossover network so that only one speaker runs up to the highest frequencies while the other two are dynamically deactivated using a low-pass filter. Using this method, a constantly narrow vertical radiation pattern can be achieved over a wide frequency range (> 1 kHz). For constant sound levels in the lower range, signal components below 200 Hz are extracted and played back on one shared woofer for each four channels.
The electronic module of the speaker is equipped with 10 amplifier channels (8 x 40 Watts and 2 x 100 Watts), a DSP-system for signal processing and an ADAT interface for signal inputs. The programming and remote monitoring are realized through a standardized Ethernet interface with DHCP capability so that all speakers can communicate via a conventional computer network with the control unit. The integrated DSP-system allows a linear phase equalization of the individual paths above 300 Hz by means of FIR filters. Four independent limiters per channel offer a high level of protection against thermal and mechanical overload of the speakers as well as against distortion due to clipping of the power amplifiers.