Cardioid
The cardioid pattern doesn't amplify sound from the front; it cancels sound from the rear. The front pickup is the same as an omni. It's the back that's different.
Not a Spotlight
The cardioid pattern doesn't amplify sound from the front. It cancels sound from the rear. The front pickup is the same as an omni; it's the back that's different. The name comes from the heart-shaped pickup pattern.
The Rear Port
A small port behind the diaphragm lets sound reach the back. Sound from behind arrives at both sides of the diaphragm simultaneously, creating equal pressure that cancels out. Sound from the front reaches the diaphragm first, creating the pressure difference that drives it.
Proximity Effect
When close to a cardioid mic, bass frequencies are boosted. This happens because the pressure gradient (which drives directional mics) is proportionally larger at low frequencies when the source is close. Radio announcers use this for a warm, intimate sound. More on this in Part 111.
Try it: drag the source behind the mic and hear the signal disappear. Then bring it close and hear the bass boost.