# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 2 Acoustic Source Alternatives to a Dodecahedron Speaker
### Ultrasound Piezoelectric Transducer (Spherical Distribution)  
An omnidirectional acoustic source can be realized consisting of a spherical distribution of hundreds of small ultrasound transducers [23; 122]. The transducers emit audible sound thanks to the parametric acoustic array phenomenon which was first proposed by [PERSON] [123]. In the parametric acoustic array phenomenon, an ultrasonic wave is emitted from a piezoelectric transducer, consisting of an ultrasonic carrier modulated with the desired audible signal. Thanks to nonlinear propagation effects in air, the primary field gets naturally demodulated, resulting in a strongly focused beam of audible sound.  
Most utilizations of the parametric acoustic array phenomenon have been devoted to exploiting the highly focused beams that can be generated through planar arrays of piezoelectric transducer such as the audio spotlight applications [124]. However, it was not until the work of [PERSON] et al. [122] that it was used for the generation of omnidirectional sound fields instead of focused ones. A continuation and improvement of the work were provided by [PERSON] et al. [23]. Results from these two studies and implementations are going to be presented in the following paragraphs.  
Considering omnidirectionality, measurements in an anechoic chamber with the ultrasound piezoelectric transducer (spherical distribution) [23] fulfilled the ISO 3382-1 requirements above 1 kHz and performed better than a dodecahedron speaker in the high-frequency range above 2 kHz. However, the directivity index deviations revealed that the source below 800 Hz does not fulfill the ISO 3382-1 sound source requirements. Similar results for omnidirectionality were reported for the prototype ultrasound piezoelectric transducer (spherical distribution) [122].  
Concerning repeatability, since the source is using ultrasound piezoelectric transducers, it can be assumed that the same sound field can be generated for every measurement. Therefore there will be no differences for measured impulse responses of a space, common for measurements conducted with an impulsive sound source.  
Regarding sound pressure levels, the source prototype [122] reached 90 dB SPL. Measurements presented for generation of pure tones [23] showed levels ranging from 46 to 59 dB for the ultrasound piezoelectric transducer for different octave bands. The sound source clearly has lower SPLs than the dodecahedron for all the frequencies. However, the authors state that in future works, broadband signals such as sine sweeps will be evaluated to get higher sound pressure levels and to characterize the source according to current regulations.  
Regarding frequency response, measurements performed in octave bands [23] and 1/3 octave bands [122] indicate that the source experiences difficulties to generate strong sound pressure levels for the low-frequency range.  
In regard to measurements of acoustic parameters, there are no results displayed, however the authors state [23] that in the future \"the behavior of the source when performing acoustic tests in buildings, such as the RT of a room or the airborne sound insulation of a partition, will also be examined\".  
In conclusion, the ultrasound piezoelectric transducer (spherical distribution) does not meet the ISO 3382-1 omnidirectionally requirements in the low-frequency range. Also, considerations should be taken in order to avoid contamination of background noise in acoustic measurements, especially in the low-frequency area. However, the omnidirectionality of the source in the high-frequency range can possibly be utilized in conjunction with other acoustic sources.