# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 2 Acoustic Source Alternatives to a Dodecahedron Speaker
### Rotation of a Directional Speaker  
A sound source utilizing a common directional speaker through rotation and measurements in different placements was proposed and evaluated [43; 121]. The source mimics the sound field emitted by a dodecahedron speaker by breaking it down in twelve different sound fields created from a single speaker for twelve different placements of the speaker (similar to the twelve positions of the faces of a dodecahedron speaker) [121]. Measurements were performed for every placement of the speaker and as a final step, the measurements were superimposed (the impulse responses) creating a single impulse response. In the following study [43], different common directional loudspeakers were used for creating an omnidirectional sound field for impulse response measurements and different placements of the loudspeakers were performed (different rotations of the loudspeakers for a total sum of twelve, twenty-six and fourteen positions). Considering omnidirectionality, evidence from the research [43] indicated that utilization of different directional speakers results in different directional characteristics of the created sound field. Two-way design speakers that were used in the study employing a tweeter driver have a higher directivity in the high-frequency range compared with typical driver speakers. Hence, differences in the sound field created were detected which resulted in variations in the measurement of acoustic parameters. Another aspect of the results showed that a higher number of speaker placements possibly resulted in better omnidirectionality in the high-frequency range. The results indicated that the design of an appropriate directional speaker can potentially form a sound field with omnidirectional characteristics that fulfill the ISO 3382-1 sound source requirements. Concerning repeatability, since a conventional speaker is utilized, it can be assumed that the same sound field can be generated for every set of measurements if the exact same placements of the speaker are performed. Therefore, there will be no differences for measured impulse responses of a space, common for measurements conducted with an impulsive sound source. Regarding sound pressure levels, the suggested source can reach high levels that depend on the directional speaker that can be used. Also, since signals such as MLS and ESS can be utilized for acoustic measurements with the rotation of directional speakers, synchronous averaging can be applied thus enhancing the signal-to-noise ratio. Relating to frequency response, signal energy tends to be evenly distributed across a wide range of frequencies in the case of a directional speaker of good quality. Hence, if a common directional speaker with a flat frequency response is utilized for the proposed method, then the emitted sound could presumably also approach a flat frequency response. In regard to the measurement of acoustic parameters, measurements with a dodecahedron speaker and the proposed source were assessed in order to quantify the results. Evidence from these studies [43; 121] point toward the idea that RT and EDT can be measured with the proposed source with excellent accuracy compared with measurements with a dodecahedron speaker. RT measurements showed a mean absolute error of less than 0.08 s, compared with measurements with a dodecahedron speaker. Results for C\({}_{80}\) and definition (D\({}_{50}\)) showed also a satisfactory accuracy but not as much as the results for the RT and EDT. A possible explanation might be that the differences in the sound fields created between the dodecahedron speaker and the proposed method are more profound in the early stages of the measured impulse responses for each case. Hence, since C\({}_{80}\) and D\({}_{50}\) parameters are an indication of early-to-late arriving sound energy, these differences affect the measurement of C\({}_{80}\) and D\({}_{50}\) more than the measurement of RT and EDT.