You might be looking at the title above and thinking that somebody accidentally combined two different articles. Cardioid is a word we usually use to talk about microphones, not speakers.
Both speakers and microphones are transducers (devices that convert between electricity and air pressure) that just happen to be at the far ends of the audio chain. They are really doing the same thing in opposite directions. So while we talk about cardioid when we discuss microphone pickup patterns and show that pretty heart-shaped diagram, we can apply the same idea to speakers and talk about dispersion patterns.
Almost all speakers are directional to some degree, so we don’t bother to call them cardioid, but subwoofers are a different story. They are all