Many of our recent Piano Miking Suggestions recommended use of PZM microphones. PZM (Pressure Zone Microphone), or more correctly boundary mics (PZM is a trademarked term) use a small electret capsule mounted close to a backing plate. The idea is that the mic capsule/plate is mounted to a large flat surface (or boundary). This increases the sensitivity of the mic by 6 dB (due to pressure doubling from reflected soundwaves), and gives it a hemispherical pickup pattern. The practical frequency response of the mic will depend on the size of the flat surface it is mounted to. If the surface is too small, low frequencies will not be reflected resulting in an apparent high frequency (treble) boost.