## 4 Discussion
### Justification of the Results  
One possible explanation for the relative satisfactory results may be due to the fact that requirements for an omnidirectional sound source acquired from ISO 3382-1 and ISO 18233-2006 are met to a certain extent by using a directional speaker. The requirements from ISO 3382-1 are:  
1. A maximum deviation of directivity of source in decibels for excitation with octave bands of pink noise and measured in free field is expected (details of the maximum deviation of directivity are presented in the ISO 3382-1). 2. The dodecahedron speaker shall produce a sound pressure level sufficient to provide decay curves with the required minimum dynamic range, without contamination by background noise. 3. Synchronous averaging is possible. Requirement 2 is sufficiently fulfilled by the proposed method since a common directional loudspeaker can provide sufficient levels of sound pressure without distortion in the whole frequency range. Requirement 3 is also met since synchronous averaging is possible if excitation signals such as ESS and MLS are applied for the measurement of impulse response. So if the requirement for directivity is covered to a satisfactory degree, then the results are justified to a certain extent. The dodecahedron speaker in essence is a 'dodecahedron arrangement of drivers to approximate the omnidirectional sound radiation characteristics of a monopole' [47]. The method that we propose (12 positions method) mimics the sound field created by a dodecahedron speaker by breaking it down in 12 different sound fields created from a single speaker following the placements presented in the methodology. As a next step these sound fields are summed to mimic the sound field created by the dodecahedron speaker. Hence the directivity of the proposed method as a source will resemble that of the dodecahedron speaker. However, two of the speakers (Behringer, Willich, Germany and Genelec, Iisalmi, Finland) that were used in this study are a two-way design. This means that above a certain frequency (cross-over frequency) the sound is emitted by the tweeter (or treble) speaker that is designed to produce high audio frequencies, typically from 2000 Hz to 20,000 Hz. Below the cross-over frequency, the sound is emitted from a driver speaker. The dodecahedron speaker is usually comprised from twelve driver speakers. In general, tweeter speakers have higher directivity compared with the typical driver speakers. Hence, differences of the sound field emitted by the dodecahedron speaker and by the proposed method are to be expected. This probably has a profound effect in the measurements of C\({}_{80}\) and D\({}_{50}\) with the proposed method, since these parameters are an indication of early-to-late arriving sound energy ratio. In Figure 9 there is a deviation for C\({}_{80}\) and D\({}_{50}\) parameters in the high frequency range. These results are likely to be related to the different directivity in the high frequency range of the proposed method compared with the directivity of the dodecahedral speaker. Concerning the synchronization of the measurements we have noted in the introduction that the impulse response is the temporal evolution of the sound pressure observed at a point in a room as a result of the emission of a Dirac impulse at another point in the room [1].The dodecahedron speaker produces an approximation of a Dirac impulse since the sound filed is not produced in a point in space but rather from 12 drivers emitting sound in synchronization in the same moment in time. Again, the proposed method mimics this approximation by breaking it down in 12 different sound fields created from a single speaker in different moments in time. As a next step the impulses responses are summed in the time domain in such a way that the starting point in time is common for all the impulses. Finally since the proposed method utilizes MLS or ESS signals, it should also satisfy the additional requirements from ISO 18233-2006 [48].