## 2 Coupled Speakers as an Omnidirectional Sound Source Representation
Figure 2: Measurement results of sound source directivity used in the research—coupled speakers (left) and B&K Type 4292 (right) [32]. Method for Evaluating the Omnidirectional Performance of Source through On-Site Measurements of Acoustic Parameters  
To verify the utility of coupled loudspeakers as an omnidirectional source and compare it with two other sources, on-site measurements of the acoustic parameters were conducted within a selected interior. Based on the previous research, a method using step rotation of the source was conducted with a resolution of 15 degrees. This type of measurement was established for the in situ omni-source verification via acoustic measurements [33]. The measurements were carried out in building D1 of the AGH University of Krakow in one of the lecture halls. The measurement situation is shown in Figure 5.  
In theory, the ideal omnidirectional sound source should provide the same result of the acoustic parameters measured with a microphone at a given point, regardless of the angular position of the source. Previous research provided that this is difficult to achieve, and a better omnidirectional source is used--a smaller dispersion of the measured parameters is observed regarding the changes in source rotation. For each source, 24 measurements were performed by rotating them every 15 degrees using a protractor, allowing for precise rotation. The room impulse responses were determined using the swept-sine measurement method. The measurements were conducted using GRAS 46 AE free-field microphones. A cubic source manufactured in the Laboratory of Technical Acoustics at AGH and a dodecahedron sound source Bruel & Kjaer type 4295 were used as alternative sound sources. Based on the acquired signals, the acoustic parameters of the room, such as reverberation time and energy coefficients (D50, C80), were determined in the B&K Dirac 5.0 program. When the directivity deviations are near the limits stated in the norm, the recommendation is to rotate the source three times and average the results to decrease the possible dispersion in measurements. A minor standard deviation determined from the conducted measurements indicates better omnidirectional source quality and qualifies it for performing measurements in room acoustics. Based on this assumption, the two stages of analysis were performed:  
1. The acoustic parameter deviation in the function of source rotation--this method will allow us to study directly how the given parameter changes regarding the source's angular position, which was recognized in the reference papers [2,4]. This analysis was performed for two points marked in Figure 4.
2. Single-value parameters analysis in the function of frequency for all measured sources, which was the comparison of standard deviation (SD) calculated from all rotation angles in measurement points (24 measurements) divided by the JND values defined in the standard ISO 3382 [14]. The analysis of this parameter was previously described [1,34], as it describes the possible audibility of the changes in room acoustic parameter estimation based on the given measurements. If the value of SD/JND is greater than 1, the changes could be audible, and the source should not be used for the measurements. This analysis was provided as an average from two measured points in the room. The most common room acoustic parameters, such as EDT, T30, D50, and C80, were used for the analysis [35,36]. The EDT (early decay time) is one of the reverberation time  
Figure 5: Measurement situation in the examined room. O1, O2—receiver positions, S—source position. parameters used in acoustics. It is calculated considering the time in which the sound pressure level decays by 10 dB from its original value, and then, it is extrapolated to \(-60\) dB decay time as RT60 [36]. Based on its definition, EDT is strongly connected with the subjective perception of reverberation.