# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 2 Acoustic Source Alternatives to a Dodecahedron Speaker
### Directional Speaker  
Directional, electrodynamic (direct-radiator type) speakers have been used as a source for acoustic measurements [75; 127; 128; 129]. An electrodynamic or moving-coil loudspeaker is an electromagnetic transducer for converting electrical signals into sounds [130]. There are two principal types of loudspeakers: Those in which the vibrating surface (called the diaphragm) radiates sound directly into the air (direct-radiator type), and those in which a horn is interposed between the diaphragm and the air (horn type). The direct-radiator type is the most common one and it is used in home and car entertainment, mobile devices and in public-address systems. Directional speakers are not alternative sources to a dodecahedron speaker since it is well documented that they are not omnidirectional [131]. However, they have been used as a sound source for acoustic measurements due to low cost, convenience and also for speech intelligibility measurements. Therefore they have been included in this review. Referring to the research that utilized the source due to low cost and convenience: \"simplification in the measurements due to technical limitations\" (Acoustics of Orthodox Churches [127]) and \"this paper objective assessment of the room's acoustics, using simple low-cost equipment and available software\" (Experimental methods [75]). Directional speakers have also been used for the determination of speech intelligibility. The IEC 60268-16 [65] (International Electrotechnical Commission) specifies objective methods for rating the transmission quality of speech with respect to intelligibility and requires a mouth simulator having similar directivity characteristics to those of the human head/mouth. Commercial implementations are available. A study by [PERSON] et al. [128] utilized an electrodynamic speaker in order to investigate the effects of the style of the liturgy on acoustic parameters. In the study, it was stated \"a directional sound source might be a better approximation than an omnidirectional sound source for the purpose of the present research\". The same approach was followed by [PERSON] [129] for the measurement of intelligibility and clarity of speech in Romanesque churches and by [PERSON] et al. [132] for an Orthodox church. Considering omnidirectionality, directional speakers do not fulfill ISO 3382-1 directivity requirements for the sound source. Directivity patterns can be found in [PERSON] and [PERSON] [131] or in specifications of commercial implementation. As [PERSON] and [PERSON] state: \"above the frequency where \(ka=2\) (\(k\) is the wavenumber and \(a\) is the radius of the diaphragm of the speaker, usually between 800 and 2000 Hz), a direct-radiator speaker can be expected to radiate less and less power\". The rate at which the radiated power would decrease, if the cone were a rigid piston, is between 6 and 12 dB for each doubling of frequency. This decrease in power output is not as apparent directly in front of the loudspeaker as at the sides because of directivity. That is to say, at high frequencies, the cone directs a larger proportion of the power along the axis than in other directions. Concerning repeatability, it can be assumed that the same sound field can be generated for every set of measurements. 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 suggested source can reach high levels that depend on the speaker that will be used. Also, since signals such as MLS and ESS can be utilized for acoustic measurements, synchronous averaging can be applied thus enhancing the signal-to-noise ratio. Relating to frequency response, signal energy tends to be evenly distributed across a wide range of frequencies in the case of a directional speaker of good quality. Hence, if a common directional speaker with a flat frequency response is utilized for the proposed method, then the emitted sound could presumably also approach a flat frequency response.