Acoustic transducers that are designed for airborne sound suffer from impedance mismatches when used in different media. These mismatches result in lowered energy density at the transducer and increased corruption by external noises. Coupling layers can mitigate mismatches, but restrict real-world applicability through limited bandwidth, thickness- dependent sensitivity, and bonding issues. We introduce a self-powered, flexible, electrostatic transducer with a highly tunable acoustic impedance and wide frequency bandwidth, without matching layers. The transducer can be precisely tuned to impedance values in the range of 1 to 2.5 MRayls, which includes skin, fresh and saltwater, and most plastics. Metal-coated microstructures on the surface of an elastomer create small gaps with the charged polymer electret film that are compressed by mechanical vibrations in the elastomer and induce a change in the voltage output. The transducer exhibits a sensitivity greater than 2V/N and an SNR improvement of 35 dB in highly noisy environments. Major factors that affect the output of the transducer are analyzed, including the dimensions of the microstructures, elastomer thickness, and properties of the electret polymer. The acoustic properties, such as sensitivity and improved rejection of environmental noise, are experimentally evaluated through a body phantom in simulated noise environments in a semi-anechoic chamber.