# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 2 Acoustic Source Alternatives to a Dodecahedron Speaker
### Ring Radiator  
An omnidirectional sound source consisting of two electrodynamic drivers, positioned face-to-face at a short distance (10 to 40 mm) has been proposed by [PERSON] et al. [125; 126]. The drivers are equipped with smooth conical cabinets which are filled with damping material. Sound is emitted from the edge of the cavity which has a ring-shaped form. Different cabinet design, driver size and driver arrangements have been considered for the sound source. Results from the aforementioned studies are presented in the following paragraphs.  
Considering omnidirectionality, measurements of directivity (250 to 12000 Hz) were performed for the proposed sound source in an anechoic chamber [125]. The directivity patterns were determined by playing back white noise and calculating the 3 rd-octave spectra, recorded at 3 m distance with 2\({}^{\circ}\) resolution. Results are presented for a source with two 5 cm drivers and a source with two 12 cm drivers both at a distance of 20 mm. As the authors state \"a closer inspection of the data reveals a maximum variation of \(\pm\)4 dB up to 8 kHz for the 5 cm drivers\" [125]. Greater variation is observed for the 12 cm drivers. Results for directivity can also be found in [126] for 5 cm drivers. The aforementioned results for the ring radiator seem very promising. However, it is not clear if the source fulfills the ISO 3382-1 omnidirectionality requirements. A study should be conducted according to the specifications of ISO 3382-1 directivity measurements and also in more planes.  
Concerning repeatability, since the sound source is utilizing conventional electrodynamic drivers, it can be assumed that the same sound field is generated for each measurement. 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, a sound pressure of 107 dB at 1 m distance was measured [126] when driving the loudspeakers at their maximal specified power with pink noise between 150 Hz and 6 kHz. Self-built sources by the authors were able to generate 100 dB (small source) and 106 dB (medium source) [125].  
Relating to frequency response, figures presented at [125; 126] demonstrate strong peaks at 2.5 kHz. As authors state: \"At about 2.5 kHz, a resonance can be observed associated with the diameter of the cavity enclosed by the loudspeakers\". Experimentation showed that this resonance can be reduced by placing a sheet of damping material in the cavity. Authors state that inverted filtering could be applied to smooth the frequency response. Also a roll of below 150 Hz is observed, which may be justified by the frequency response of the drivers.  
In regard to measurements of acoustic parameters, there are no results displayed in the research. However a problem concerning the impulse response that could possibly affect the measurement of acoustic parameters was identified by the authors [125]: \"A problem was the impulse response, which in case of the medium (size) source indicated decay times in excess of 30 ms at 10 kHz. At high frequencies, the distance between the two drivers is no longer small compared to the wavelength, and waves emitted from one chassis will be reflected at the other on\". The authors provided a solution by inserting a small piece of acoustic foam into the space between the two drivers, thus dampening axial wave propagation and lowering the decay times to a maximum of 12 ms at 2.5 kHz.  
In conclusion, the ring radiator seems to have omnidirectional characteristics that can potentially fulfill the ISO 3382-1 requirements. However, that still remains to be verified. Also, the source can produce adequate sound pressure levels for acoustic measurements.