# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 2 Acoustic Source Alternatives to a Dodecahedron Speaker
### Electric Spark Source  
An electric spark discharge may be used as an impulse sound source for acoustic measurements [142; 143]. The principle is based on the generation of an electric discharge by applying a high voltage between two electrodes. On the electrical breakdown of the air gap, heat is generated in the spark, causing a rapid expansion of the core gas [142]. This expansion results in the propagation of a shock wave which is the primary source of sound [144]. Following the initial shock, the air in the region of the core is raised in temperature above the ambient and the cooling of this air results in a secondary wave of lower frequency and intensity. Steel electrodes with variable radius can be used combined with a variable gap. Electric spark sources have been mainly used as a source for scale models for the simulation of acoustic phenomena [145]. The most common scale model application being auditorium acoustics [146] while it has also been used for the study of sound propagation in urban areas [147]. Electric spark sources have also been used for the acoustical spectrometry of sound propagation through air-filled porous materials [148]. In addition, the acoustic properties of an absorbent material have been measured with a high voltage spark discharge as an impulse source [149]. Finally, high energy spark discharges have been used in studies for the measurement of impulse responses and acoustic parameters in spaces [150; 151]. Considering omnidirectionality, a study by [PERSON] et al. [145] showed that the behavior of spark discharges and their acoustic radiation depends greatly on the electrodes gap. Polar plots measured in an anechoic chamber for different electrode gaps showed that the overpressure (maximum pressure of the impulse) is reduced 2.5 dB at 90\({}^{\circ}\) (compared to 0\({}^{\circ}\)) and 4.4 dB for electrode gaps of 5 mm and 20 mm respectively. The authors did not present polar plots for smaller electrode gaps. In a similar study, polar plots presented by [PERSON] et al. [152] showed a sufficient omnidirectionality. However, the authors showed results only for 5, 10 and 20 kHz without presenting details how the measurements were performed. It has to be noted that the research was implemented for scale models which require higher frequencies. Finally, a study by [PERSON] et al. [153] presented high correlation between theoretical and experimental directivity patters of a spark discharge. However, results are presented for frequencies above 20 kHz. The study also presents high correlation between the estimated sound pulse waveforms and the measured ones for different directions. Concerning repeatability, in a study by [PERSON] et al. [152] the sound pressure waveforms of the spark discharge sound source (superposed 64 times) were presented with fairly good results. However, in the study, it is stated that \"a waveform with high repeatability and big sound energy are incompatible conditions\". In a study by [PERSON] et al. [145], in order to establish repeatability, measurements were performed 40 times. The standard deviation of the pressure measurements was found not to decrease significantly with a greater measurements number. [PERSON] et al. [150; 151] in his studies for the measurement of impulse responses in rooms, states that \"an electric spark discharge provides the best combination of intensity, repeatability, reset time and portability\". In a study by [PERSON] and [PERSON] [147], a good reproducibility was obtained as small deviations were found in the spectrum for different discharges. However [PERSON] and [PERSON] [136] state about the variation of electric spark discharges that: \"first, there can be significant variations in the spark waveforms from spark to spark under the same testing conditions. This is the result of temporal variations in the breakdown potential, randomness of the air breakdown channel between the electrodes, and vibration of the electrodes\".