DC Bias is the direct current electrical voltage used to "charge" the capsule of a traditional condenser microphone.

For a condenser microphone capsule to function, it needs to have a static electrical charge.

DC biasing is the classic method for creating this charge: an external voltage (usually between 40V and 60V) is applied directly to the capsule (formed by the diaphragm and the backplate). This voltage is different and separate from the phantom power (+48V), although the phantom power is the power source that the microphone's internal circuitry uses to generate this high bias voltage. By varying the bias voltage, it is possible to change the microphone's sensitivity. This is how microphones with multiple polar patterns work: they apply different voltages to the front and back of a dual capsule to create cardioid, omni, figure-8, etc. patterns.

Alternatives to this method are electret microphones (which use a pre-charged material) and RF polarization microphones (which use a radio frequency oscillator).