# Review of Acoustic Sources Alternatives to a Dodecahedron Speaker
## 2 Acoustic Source Alternatives to a Dodecahedron Speaker
### Wooden Clapper  
An impulsive sound can be created if two plates of wood are struck against each other. A study by [PERSON] et al. [116] created and measured a specially designed wooden clapper intended to operate as a source for acoustic measurements. The clapper consists of two identical parts connected by a hinge, allowing the two parts to revolve around the same axis. However, the shape of the waveform of the wooden clapper does not have the typical N-shape found in other impulsive sounds. It is more complex and represents a short intrinsic reverberation process lasting some few tenths of milliseconds. According to the classifications found in the literature, a clapper impulse can be considered as reverberant impact pulse waveform with B-duration of about 15 ms [117].  
According to a research, the justifications that prompted its use as a sound source was the experimental estimations of impulse responses of old wood churches located at remote places (no road, no electricity) where none of the standard sound sources could be used [118]. The source was also used for the measurement of classroom acoustics [60]. Also, a wooden clapper can be used as an acoustic source for survey acoustic measurements through the use of a smartphone app [102].  
Considering omnidirectionality, measurements performed in an anechoic environment showed that the proposed source does not fulfill the requirements of the ISO 3382-1. The polar diagrams presented in the study indicate that the deviations from omnidirectionality at the lower octave bands at central frequencies of 125, 250 and 500 Hz in both planes are within \(\pm\)3 dB, above the ISO 3382-1 limits. However, the radiation of the wooden clapper is within the standard limits at higher frequencies.  
Concerning repeatability, the variations of the impulse levels are relatively small if the clapper is operated by a trained person. They are within the limits of \(\pm\)1-2 dB in all octave bands.  
Regarding sound pressure levels, octave spectrum levels of the clapper impulse level at 1 m distance are ranging from (about) 72 dB (low-frequency range) to 100 dB. As the authors' state \"measurements performed in rooms located in urban environments, where the ambient noise level was higher, showed that the dynamic range in the octave band at 125 Hz might be insufficient and that the measurement results have to be considered valid for the octave band at 250 Hz and higher\".  
Relating to frequency response, the authors present only the octave spectrum levels, in which the source seems to have a roll-off below 250 Hz. Similar roll-off was found for a wooden clapper in another study [102].  
In regard to measurements of acoustic parameters, research where the wooden clapper was utilized as an alternative source and results obtained were compared with measurements with a dodecahedron speaker or another source could not be found in the literature.  
In conclusion, the wooden clapper as a sound source does not meet the ISO 3382-1 sound source requirements since it cannot be considered an omnidirectional source and also does not have sufficient sound power in the low-frequency range.