## 2 Coupled Speakers as an Omnidirectional Sound Source Representation  
Coupled speakers are a novel solution for miniature omnidirectional sound source construction. Similarly to isobaric speakers, they are connected cone to cone but excited by signals in a coherent phase. A detailed explanation of the proposed concept and its preliminary verification was described in the referenced papers [30; 31]. In the given research, the direct comparison between the coupled speakers and most common omnidirectional sound sources was conducted, employing also the cubic sound source and omnidirectional sound source from Bruel & Kjaer. The sound sources used in the current research are presented in Figure 1.  
The current version of the coupled speakers source was prepared using Visaton AL 140 loudspeakers, which shared the closed enclosure volume at a distance of 2 cm. The proposed concept of coupled speakers was formally tested in an anechoic chamber with the ISO 3382 requirements for omnidirectional sources. The sound directivity measurements were performed at 2 m from the source, and the angular resolution of 2 degrees was obtained using a rotating table. Figure 2 presents the directivity and frequency characteristics of implementing coupled loudspeakers conception. In the analyzed variant, coupled loudspeakers exhibit excellent omnidirectional quality in the range of 250-2000 Hz and a lesser quality for the last octave assessed following ISO 3382 standard. The frequency response was measured with an on-axis position to indicate the actual SPL output the coupled speaker system provided. It is essential to consider the coupled speakers' source characteristics regarding frequency response shown in Figure 3. Because of the coupled speakers' implementation (cone-to-cone placement inside the minimal volume), the source frequency response and sensitivity are significantly limited. However, the source should be helpful across the desired range of 250-2000 Hz. The present project aimed to experimentally validate coupled loudspeakers as a measurement source in room acoustics and compare them to different constructions of omnidirectional sources--dodecahedral and cubic. In addition to the directivity assessment and on-axis frequency response, some directivity polar plots are presented in Figure 4. A direct comparison between the coupled speakers and the dodecahedral sound source was performed. It is clear that in lower octaves, both sources can be called omnidirectional, but in higher frequencies, the output SPL is strongly angle dependent. For the coupled speakers, we observe the conversion from the monopole to the dipole-like directivity characteristics for the frequencies near 4000 Hz and above. This limitation should be investigated in future works to improve this kind of design. Figure 1: Omnidirectional sound sources used in the research—the coupled speakers (**left**), cubic omnidirectional sound source (**middle**), and dodecahedral omnidirectional sound source (**right**) [32]. The preliminary options for the coupled speakers' performance improvement were investigated. To improve the low-frequency range, speakers bigger than the 14 cm diameter Visaton 140 AL should be used, and for better high-frequency performance, a closer distance between the speakers should be used. However, adjusting the speaker's technical properties to the desired measurement function may be essential. The given research was designed to verify the possibility of this construction being used in room acoustics, but future works should also employ improvements in the directly coupled speakers' construction performance. In the next iteration of this project, the two-way type of coupled speaker construction should be investigated with different types of loudspeakers in two cone-to-cone distances for separated frequency ranges. Figure 4: The comparison of coupled speakers and dodecahedral sound source directivity on polar plots for 250 Hz and 4000 Hz octave band. Figure 3: On-axis frequency response of coupled speakers used in the research.