## Preliminary Investigations
### Experiments  
The measurements took place in an 8 by 11 by 2.8 m classroom with only a handful of chairs and tables in it, which produced a reverberant environment like ordinary offices. The room was located in a multi-story building and had a concrete floor and ceiling and plasterboard walls. On one end, it had a wall-to-wall glass window, on the other a large door and metal whiteboard. Along one side, it had large metal containers. The floor plan with the measurement arrangement can be seen in Figure 1. Special symbols have been used to denote the objects that are explained in Figure 2.  
Only one secondary source and one error microphone have been used, instead of two pairs. No different result was expected in the latter case, but it would have required much more computation. The primary sources were emitted band limited white noise of 500 Hz. The figure is only hinting at the locations of the reference microphones, which are installed around the error microphone with radius \(r\). A detailed closeup of their arrangement can be seen in Figure 3.  
Other arrangements have also been tested that are not depicted in the figure: a regular triangle in the \(x\)-\(y\) plane for 3 microphones, a regular tetrahedron for 4 microphones and a regular hexagon in the \(x\)-\(y\) plane for 6 microphones. The error microphone is set in all cases in the reference microphones' center of gravity. The influence of the radius has also been tested.  
Figure 1: Measurement arrangement in the classroom.  
Figure 2: List of symbols used for the objects of the simulation.  
The main parts of the control equipment can be seen in Figure 4. Behringer ECM8000-type ultra-linear condenser microphones have been used for reference signal acquisition, while a Voltcraft SL-400-type sound level meter has been used for the measurement of the error signal. The probe has been connected to the meter by an extension cable; thus, the error microphone could easily be set to the right position. The primary noise has been emitted by Genius SP-HF 1250B-type loudspeakers, and an Altec ACS52 type speaker has been used as the secondary source. As the bandwidth of the primary noise is 500 Hz, the sources could be considered omnidirectional. An Analog Devices AD21262 signal processor-based hardware has been used for control, which will be presented in detail in the next section. A normalized filtered reference LMS algorithm has been implemented on the controller. Each reference microphone had its own control filter, and the sum of the outputs drove the secondary loudspeaker. The filters had an equal number of taps, and the total number of taps reached the maximum allowed by the hardware, i.e., the more reference signals have been used, the shorter the control filter that has been implemented. Thus, the tap number of the control filters was a variable. The sampling frequency was 8 kHz.  
The parameters and their settings are summarized in Table 1. This variety of the parameters resulted in numerous measurements. Below, some interesting results and the conclusion are described.  
Figure 4: Control equipment for the preliminary experiments.  
Figure 3: Arrangement of the reference microphones.  
The experiments have shown that multiple reference signals helped to increase the suppression, even if only one primary source was present. This is an advantageous feature, which can be exploited during the noise-canceling chair design.