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The acoustic property analysis system consisted of 
a function generator (SFG 2110, GW INSTEK, Taipei, Taiwan), 
a microphone set (Type 46AE, Brüel and Kjær, Copenhagen, Denmark), 
a data collection device (Apollo Box, SINUS, Leipzig, Germany), 
and analysis software (Amadeus 2ch SLM/Multi Analyzer, SINUS, Leipzig, Germany).