# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 2 Acoustic Source Alternatives to a Dodecahedron Speaker
### Inverse Horn Design  
An inverse horn design sound source is formed by fitting the diaphragm of a conventional loudspeaker to an aperture hole through an inverted horn for concentrating the acoustical energy. The design was proposed by [PERSON] et al. [105] and is based on the idea that a source of small dimensions is a good approximation of a point source with omnidirectional characteristics. The performance of such a point source depends on the interaction between the loudspeaker and the inverted horn. The careful design makes it possible to construct a very compact omnidirectional sound source satisfying the international standards. The original design has a very irregular frequency response due to the resonances of air generated inside the inverted horn. [PERSON] et al. [106] proposed applying inverse filtering in order to improve the frequency response of the source. Impulse responses of the source with and without inverse filtering are also presented in this research. It is worth noting that there are commercial implementations of the inverse horn design that fulfill the ISO 3382-1. Also construction of an inverse horn design source through 3-d printing is possible [107]. A similar approach to the inverse horn design is the loudspeaker-pipe sound source [108]. A point source can be formed if a loudspeaker is connected with a waveguide whose exit is small compared with the wavelength. The source is included in this section of the review since it is based on the same idea as the inverse horn design, even if it is not used as much as an alternative to a dodecahedron speaker. The major difficulties that hinder its use are its non-flat frequency response and the reflections between the pipe exit and its interior, but a digitally synthesized input signal can be used to overcome these drawbacks [109]. It has been used for the study of nozzle transmission characteristics [110] and to control internal noise propagation from aircraft engines [108]. It is stated in the ANSI S1.18 standard [111] for determining the acoustic impedance of ground surfaces, that a loudspeaker-pipe source can be utilized. Application for in situ ground impedance measurements can be found in the literature [112]. Considering omnidirectionality, various research and implementations showed that the inverse horn design can fulfill ISO 3382-1 directivity requirements [105; 106; 107; 113]. In some of the research [106; 107; 113] inverse filtering was applied for flattening the frequency response but it also had a positive effect on the omnidirectionality of the sound source. Excellent results are presented in research by [PERSON] et al. [106], where the source, when equalized by inverse filtering, deviates less than \(\pm 1\) dB in various frequencies. However, it has to be noted that the omnidirectionality of the source depends on the implementation. Concerning repeatability, since the sound source is utilizing a conventional loudspeaker, it can be assumed that the same sound field is generated for each measurement. Therefore there will be no differences for measured impulse responses of a space, common for measurements conducted with an impulsive sound source. Regarding frequency response, strong resonances can be seen in figures from various publications [105; 106; 113]. The amplitude of the resonances deviates more than 20 dB from the flat part of the frequency response [106]. As [PERSON] et al. [113] states \"it is governed (the frequency response) by the strong influence created by the Helmholtz resonance due to volume compliance and the mass of air in the aperture opening that affords regularly spaced peaks\". The frequency of the resonant peaks and harmonics is determined by the volume of the cone. However, to equalize such irregular frequency response arising from resonances of air inside the inverted cone, as mentionedbefore, [PERSON] et al. [106] proposed applying inverse filtering. The technique pre-emphasizes the MLS signal driving the sound source so that zero phase or minimum-phase cosine magnitudes are radiated.