# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 3 Discussion  
Fifteen sources have been identified in the literature which can be used as alternative sources to a dodecahedron speaker for acoustic measurements. The majority of the sound sources (9) are of the impulsive type: balloons, guns, firecrackers, handclaps, wooden clappers, shotshell primers, an explosive mixture of acetylene gas with air, laser-induced air breakdowns and electric spark sources. Four acoustic sources utilize an electrodynamic loudspeaker: Inverse horn design, rotation of directional loudspeakers, ring radiators and directional loudspeakers. One study utilizes piezoelectric transducers through spherical distribution. Finally, one study used a compressor nozzle hiss. A categorization of the sources according to whether they are impulsive or continuous is presented in Table 4.  
According to ISO 354 [11] and ISO 3382-1 [8], the methods for measuring RT and acoustic parameters can be classified to the interrupted noise method and the integrated impulse response method. A further classification presented in ISO 354 separates the integrated impulse response method into the direct and the indirect method. An extensive description of the methods can be found in the aforementioned standards. All the impulsive sources can be utilized in the direct integrated impulse response method. All the sources which employ an electrodynamic loudspeaker and the source of piezoelectric transducers through spherical distribution can be utilized in the indirect integrated impulse response method. Those sources can also be utilized in the interrupted noise method. The compressor nozzle hiss can only be employed in the interrupted noise method. A summary of the features examined for the sources (omnidirectionality, repeatability, adequate sound pressure levels, even frequency response, accuracy in measurement of acoustic parameters and fulfillment of ISO 3382-1 sound source requirements) is presented in Table 5. A blank in the table implies that this feature has not been studied or that there is insufficient data to draw conclusions. The table has been created to present a general overview of the characteristics of the sources. However, the data in the table should be evaluated with caution and in conjunction with the corresponding research. Determining whether a source can be utilized and the expected accuracy that can be achieved in the measurements requires deeper knowledge. For example, in the case of the balloon as a sound source, we note that RT can be measured (except in the low-frequency range). This holds true but the accuracy of the measurements will be affected by the background noise in each frequency band as well as the shape and characteristics of the measuring space as it is stated in the presented research. \begin{table}
\begin{tabular}{c p{142.3 pt}} \hline \hline \multicolumn{1}{c}{**Source Categories**} & \multicolumn{1}{c}{**Sources**} \\ \hline Impulsive & Balloon, gun, firecracker, handclap, wooden clapper, shotshell primer, an explosive mixture of acetylene gas with air, laser-induced air breakdown, electric spark source \\ \hline Continuous & Inverse horn design, rotation of directional loudspeaker, ring radiator, directional loudspeaker, piezoelectric transducers through spherical distribution, compressor nozzle hiss \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Categorization of the sources.