# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 1 Introduction
### Requirements for Omnidirectional Sources  
Requirements for an omnidirectional sound source aimed for acoustic measurements are provided in ISO 3382-1 [8], ISO 16283-1 [14], ISO 10140-5 [20] and ISO 140-3 [21]. However, the most referred ones are the requirements on ISO 3382-1. Those are:  
\"The sound source shall be as close to omnidirectional as possible. A maximum deviation of directivity of source in decibels for excitation with octave bands of pink noise and measured in free field is expected (Table 1)\".  
\"The sound source shall produce a sound pressure level sufficient to provide decay curves with the required minimum dynamic range, without contamination by background noise. In the case of measurements of impulse responses using pseudo-random sequences (e.g., maximum-length sequence (MLS) [22]), the required sound pressure level might be quite low because a strong improvement of the signal-to-noise ratio by means of synchronous averaging is possible. In the case of measurementswhich do not use a synchronous averaging (or other) technique to augment the decay range, a source level will be required that gives at least 45 dB above the background level in the corresponding frequency band\({}^{\prime\prime}\).  
Less stringent requirements for a maximum deviation of directivity are described in ISO 16283-1 [14] and ISO 10140-5 [20] (Table 2). Specifications are also described in ISO 140-3 [21] which is now withdrawn. ISO 10140-5 states that: \"Uniform omnidirectional radiation can be assumed if the directivity index (DI) values are within the limits of \(\pm\)2 dB in the frequency range of 100 Hz to 630 Hz. In the range of 630 Hz to 1000 Hz, the limits increase linearly from \(\pm\)2 dB to \(\pm\)8 dB. They are 8 dB for frequencies of 1000 Hz to 5 000 Hz\". It should be noted that the specifications are exactly the same for ISO 16283-1 beside a small difference. The ISO 16283-1 requires \(\pm\)5 dB for 800 Hz a difference which is indistinguishable between the standards.  
Figure 1 presents the limits for the maximum deviation of directivity for all the standards. ISO 140-3 is also presented since it is still sometimes referred in some dodecahedron speaker specifications. Directions for the measurement of directivity can be found in some standards such as ISO 3382-1, ISO 10140-5, ISO 16283-1 and ISO 16283-2. The maximum acceptable deviations from omnidirectionality are measured when averaged over 'gliding' 30\({}^{\circ}\) arcs in a free sound field. In case a turntable cannot be used, measurements per 5\({}^{\circ}\) should be performed, followed by 'gliding' averages, each covering six neighboring points. The reference value shall be determined from a 360\({}^{\circ}\) energetic average in the measurement plane. The minimum distance between source and microphone shall be 1.5 m during these measurements. However some concerns have been expressed about measuring the directivity according to those standards [23; 24; 25]. Also, a new descriptor for measuring the directivity of dodecahedron speakers and omnidirectional sound sources has been proposed [26].  
\begin{table}
\begin{tabular}{c c c c c c} \hline Frequency (Hz) & 125 & 250 & 500 & 1000 & 2000 & 4000 \\ \hline Maximum deviation (dB) & \(\pm\)1 & \(\pm\)1 & \(\pm\)1 & \(\pm\)3 & \(\pm\)5 & \(\pm\)6 \\ \hline \end{tabular}
\end{table}
Table 1: The maximum deviation of directivity of source in decibels for excitation with octave bands of pink noise and measured in free field (ISO 3382-1).  
Figure 1: The maximum deviation of directivity of source in decibels for excitation with octave bands of pink noise and measured in free field for ISO 3382-1, ISO 16283-1, ISO 10140-5 and ISO 140-3 (withdrawn).