“I’m getting ready to purchase a lavalier microphone for some live sound work I’m about to do. I see they make them in cardioid and omni directional patterns. I’m not sure why anyone would want an omni directional lavalier – it seems like it’s hard enough already for them to pick up the desired source amidst all the ambient noise. What am I missing?”
Several things (in no particular order). First, lavaliers are often used in video/film recording situations where live sound reinforcement and gain before feedback are not a concern. Omnidirectional mics do not have much (if any) proximity effect. This means that as you move them away from the source you don’t lose all the low frequencies. Additionally it means you don’t get a massive build up of low frequencies by being close to the source. In lavalier applications the mic is some distance from the desired source (the voice), but very close to a source of undesired noise in the clothing worn. So omni’s can sound quite a bit more natural, which is usually helpful in recording applications. Omni’s also tend to be less susceptible to plosives.
A little publicized fact about cardioid lavaliers is that once these mics are placed directly up against clothing their polar patterns become much more sphere-like, and not too different from how an omni behaves in the same position. With a little separation through proper use of a mic clip, a cardioid can still provide some benefit in terms of gain before feedback in a live scenario, but usually at some sacrifice in overall sound quality.