For practical applications of a thermoacoustic system using factory exhaust heat or solar heat, it is necessary to decrease the onset temperature ratio. In a previous study, a cascade thermoacoustic system with a number of prime movers connected in series was examined. This system can be driven at a lower onset temperature ratio than a conventional thermoacoustic system with a single prime mover. However, there are some problems with this system: it cannot be driven continuously and its thermal efficiency is low. Therefore, a parallel thermoacoustic engine was proposed. In this study, we compared the onset temperature ratios and thermal efficiencies of three systems: a normal thermoacoustic engine with a single prime mover, a cascade thermoacoustic engine, and a parallel thermoacoustic engine with two prime movers. The results indicated the parallel thermoacoustic engine to be advantageous in terms of the onset temperature ratio and thermal efficiency.