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Silicon micromachining is a promising approach to realize low frequency capacitive ultrasonic transducers and condenser microphones. The construction of micromachined low frequency capacitive ultrasonic transducers or microphones, respectively, differs considerably from that used for high frequency cMUTs. This paper presents an analytical model used for the simulation. The effects of different airgap geometry and backplate perforation on frequency response and sensitivity for transducers in the audible and ultrasonic frequency range will be shown. Finally optimization criteria are discussed considering the maximum receiving sensitivity and the resonance frequency shifting of the membrane due to the effect of the backvolume.