# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 2 Acoustic Source Alternatives to a Dodecahedron Speaker
### Gunshot  
A gunshot is produced by a firearm which can be characterized as a \"heat engine that converts stored chemical energy into kinetic energy\" [79]. The sound from a firearm discharge consists of multiple acoustic events: the ballistic shockwave, internal gas leaks or ejections, the muzzle blast and reflections. The primary sound is the muzzle blast which is an explosive shockwave in air produced by propellant gasses under extremely high pressure that expands rapidly once the bullet exits the muzzle. The impulse response and the waveforms from each acoustic events which it is composed are presented in a study by [PERSON] [80]. Impulse responses from different firearms can be found in the literature [81]. Traditionally, acousticians have used a firearm with blank cartridges as a sound source for acoustic measurements since it is an impulsive source that is lightweight and small enough to be easily transported. A gunshot is explicitly specified in the standard ISO 354 [11] as a possible alternative sound source. As the ISO states: \"It is impossible in practice to create and radiate true Dirac delta functions, but short transient sounds (e.g., from shots) may offer close enough approximations for practical measurements\". It is important to notice that a lot of the research for firearms has been conducted for forensic analysis or gunshot detection systems [81; 82; 83; 84]. According to research presented in the introduction, the justifications that prompted its use as a sound source were emission of high sound levels [54] and ease of use [53]. Referring to the research: \"Use of a starter pistol as an excitation source is a potentially useful impulse response recording method, especially in situations where use of the equipment required for sine sweep measurement is impractical or inappropriate\" (Measurements in urban environments [53]), \"A main advantage of such a device is the emission of high sound levels which makes a shot easily identifiable even at locations with high background noise levels\" (Green roofs absorption measurements [54]). Considering omnidirectionality, the gunshot appears to have directional characteristics. As presented in ISO 17201 [85], the muzzle blast is directional with sound levels on-axis ahead of the muzzle higher than levels directly behind the muzzle by up to 20 dB. [PERSON] et al. [79] state that \"one explanation for the directivity of muzzle blasts is that the sound source is rapidly accelerating at muzzle discharge\". [PERSON] [71] states that the pistol appears to be directional at low frequencies, where there is a rise in energy from the side. Similar results about the directional characteristics of the handgun and for large-caliber weapons can be found in [86; 87]. Recordings of different handguns as a function of azimuth can be found in [81]. In a study by [PERSON] et al. [88] utilizing shadowgraphy, shockwaves of gunshots are depicted and variation in directionality is evident among different guns. However, a study of different handguns by [PERSON] and [PERSON] [89] indicated that a 0.38 caliber gun has the best omnidirectional characteristics. Concerning the question of repeatability, [PERSON] et al. [90] state that differences in the noise characteristics between individual cartridges for the same gun are usually small, so the impulsive source can be replicated to a high degree. However, a study by [PERSON] and [PERSON] [81] comparing the on-axis peak pressure levels for ten shots for different handguns revealed that variability was observed among different guns. [PERSON] [71] states that handguns have poor repeatability. Regarding frequency response, several studies have shown that energy falls rapidly in the low-frequency range for handguns [79; 89; 90; 71; 71]. The frequency spectrum of a gun typically has a peak energy output in the 1 kHz to 2 kHz region [71]. Below this frequency the energy falls off rapidly (~14 dB/octave on a 1/3 octave analyzer). However the 0.38 caliber gun exhibits a significantly flatter frequency response [89].