This paper deals with the onset mechanism of primary acoustic instability of downward-propagating flames in a combustion tube. We focus on the effects of a coupling constant, beta M, where beta and M represent the Zel'dovich and Mach numbers, respectively, and the variation in the flame surface area. To change the coupling constant, various gas compositions for lean ethylene flames diluted with carbon dioxide or nitrogen are used. We obtain a linear relationship between the coupling constant and the average acoustic intensity, and the critical values of the coupling constants are acquired through linear approximation regarding the onset of the primary acoustic instability. Furthermore, we adopt the CO2 laser irradiation method to alter the shape of the flame front, and experimental results show that the variation in the flame surface area does not always cause spontaneous generation of the primary acoustic instability in initially non-vibrating flames. Furthermore, even in initially vibrating flat flames, the growth rate of the primary acoustic instability is not associated with the increase or decrease in the flame surface area in the present experiments. Finally, we also estimate the effects of acoustic losses on acoustic instability, and experimental results show that larger total acoustic losses tend to suppress acoustic vibration even at the same coupling constant. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.