## 5 Experimental Results and Discussion
### Discussion  
While various orientations of the two quadrupole topologies were examined, especially in CP 2, the presented ones are the most effective. The concept was to set the compound source in a location to occupy the least possible space and to control the noise frequencies at all points. By inspecting Table 3, control of different frequencies and areas can be attained by the compound source in a fixed position thanks to its radiation coupling with the room modal field. If one control position suppresses a disturbing frequency, the same can be sufficient for attenuating another frequency. This is achieved via the driving signals; thus, there is no need to relocate the source. There is also no requirement for a specific compound layout for successful performance, as the source will adapt itself to the position. In addition, the engineering perception indicates where the sources must be placed to efficiently control the noise field in combination with the limited available space in an equipped room. The presented system surpasses the space constraints as it can control different frequencies without distributing secondary sources. Furthermore, if a convenient location is no longer available, an alternative one can be chosen, and the GA produces a set of optimal solutions for the driving parameters with a similar amount of noise attenuation, as has been shown for most cases in Table 2.  
In the experiments, the longitudinal source offers a fine solution for noise control in the low-frequency range, as it prevails over the lateral type in almost all cases. Considering that these sources operate at low frequencies in a room rather than in the free field, the coupling degree of sound pressure distribution is higher for the longitudinal case. The low-frequency radiation from small sources is inefficient; therefore, the use of low-_Bl_ drivers offers a fine solution for the development of low-volume subwoofer compound sources. The maximum attenuation for the frequency of 50 Hz that is obtained in all measurement points is not as high as it is for the higher frequencies of 77, 100, and 108 Hz. This is probably due to limited power output at 50 Hz, compared to higher frequencies, as was observed when designing the low-_Bl_ loudspeakers [25]. Although the attained amount of reduction is quite sufficient for such a low frequency, more source strength may be required for possible higher attenuation by designing more powerful low-_Bl_ subwoofers. This may be done at the expense of the volume of each secondary monopole, but the concept of this work is beneficial to this direction. It is noticed that the maximum noise reduction is 20 dB when placing the longitudinal quadrupole at CP 1 to reduce the 50 Hz component at MP 3, in Table 4. However, the corresponding value is 19.5 dB in Table 2. In sound level measurements, we obtain pressure fluctuations, and therefore, the same driving parameters may produce fitness score differences within 0.5 dB in subsequent measurements, which are meaningless and not audibly distinctive to the human ear. Regarding the GA performance, it is expected that the process converges in a population dominated by the optimal solution of the fitness function. This provides the opportunity to choose the most convenient set for each case among several near-global solutions that may offer an overall reduction at the microphone, which is within 0.5-1 dB of the optimal one, as presented in Figure 12. The system robustness was tested with experiments of five consecutive genetic evolutions for the case of MP 2 at 77 Hz and the longitudinal source at CP 2, each with a different initial population of the same length. Despite the slight differences, all five cases converge in the same vicinity and attain primary field attenuation within \(\pm\)0.5 dB of the maximum reduction level of 29.5 dB, showing consistency in the optimal result. Another advantage is that there is no need for a reference signal or correlation with the error signal.