# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 2 Acoustic Source Alternatives to a Dodecahedron Speaker
### Laser-Induced Air Breakdown  
One of the many uses of pulsed lasers is to generate acoustic pulses in the air beside solid and liquid media [136; 137] which can be used as a source for impulse response measurements within the audible bandwidth [138; 139]. An acoustic point source can be generated by focusing a pulsed laser beam to rapidly heat the air at a focal point which produces a small expanding plasma ball. Plasma is formatted \"through the cascade process caused by electrons emitted from atoms and molecules that have absorbed multiple photons through a multi-photon process when a laser beam is focused in a gas. A portion of this plasma energy is used to create a shock wave, which is the sound source generated by laser-induced breakdown\" [139]. Referring to research, in a study by [PERSON] et al. [138], the source was used for the measurement of the impulse response of a room. The accuracy of the results was assessed by comparison with impulse responses measured with a custom-made spherical loudspeaker and a directional loudspeaker. In a study by [PERSON] et al. [139], the source was validated by measuring the resonant frequencies (from the impulse response) of a very small space and comparing them with those computed by a theoretical model. The source is also proposed for scale model work [140; 141]. Considering omnidirectionality, measurements were performed in an anechoic chamber for directivity in octave bands at 1 m (10\({}^{\circ}\) resolution) [140]. The central frequencies were 75 kHz, 37.5 kHz, 18.75 kHz, 9.375 kHz, 4.6875 kHz and frequencies below 3.125 kHz. The results present an omnidirectional pattern for all frequency bands. The maximum difference in energy level is less than 1 dB within the range from 0\({}^{\circ}\) up to 90\({}^{\circ}\) for all frequency bands. In a subsequent study by the author [138], the magnitude response of the pressure pulse measured along 0\({}^{\circ}\), 45\({}^{\circ}\) and 90\({}^{\circ}\) directions deviates by less than 0.5 dB at frequencies above 10 kHz, and no noticeable differences were found at frequencies less than 10 kHz. In a study by [PERSON] et al. [139], the point sound source was generated in an anechoic box were twelve microphones (30\({}^{\circ}\) differences) were placed 80 mm from the sound source. The average power spectra measured at each point was presented showing small differences. Concerning repeatability, a study by [PERSON] et al. [140] measured twenty consecutive pulses at a distance of 0.8767 m. The waveform shape was conserved between emissions and only small sound level differences were evident. This was also true for other source-receiver distances. The standard deviation of the peak values of 20 repetitions was less than 0.3 dB for all measurement distances. In subsequent research [138], the standard deviation for the magnitude response of the pressure pulse at 1 m averaged over 100 measurements was less than 0.8 dB for all frequencies. The authors state: \"The repeatability of laser-induced breakdown (LIB) depends mainly on the laser configuration, but other parameters, e.g., dust, may affect the generation of LIB and consequently produce deviations in the waveform from pulse to pulse\". However, in another study [139] it is stated that: \"as the laser pulse energy increases, the sound pressure of the point sound source generated by LIB increases, also the reproducibility decreases and the fluctuations become apparent\". Regarding sound pressure levels, the peak pressure value of the LIB was measured in a study [140] at 0.8 m and was approximately 360 Pa (145 dB). In another study [139] three levels of laser pulse energy were used: 335.9 mJ, 798.2 mJ and 990.9 mJ. As the laser pulse energy increased, the sound pressure of the point sound source generated by LIB increased. Different time responses of sound pressure generated by LIB ranged from to 480.3 Pa (147.6 dB) to 698.9 Pa (150.87 dB). It was also found that the amplitude of the sound pressure varies depending on the spot radius.