A microphone changes sound waves (variations in air pressure) into electrical waveforms (variations in voltage). A mic’s capsule contains the mechanism that does this. The capsule has either a diaphragm or ribbon in it- both are flexible membranes that move in response to the sound waves to which they are exposed. Mics are the critical first stage of the recording chain. The sound quality of a recording is limited by the weakest link in the recording chain. Wide-cardioid mics are less directional than cardioids, but more directional than omnidirectional mics. Bidirectional mics favor sound sources in front of and behind the mic capsule. Electret microphones are similar to condenser mics, but instead of requiring phantom power, they use a permanently charged back-plate, diaphragm, or capsule surface. Original classic vintage tube mics, built using very high quality materials and tubes, were expensive. Stereo microphones have two capsules housed in a single microphone body.