The main distinction and advantage of a lav microphone is the smaller, more discreet size over other microphone types. Their compact size allows them to be attached to an actor's clothing or body without being visible in the shot.
The most common place to attach a lavalier lapel microphone is near the neckline either on the collar or the knot of a tie if applicable. If a lapel microphone is attached too far away from the actor’s mouth, then the audio quality will suffer.
For a lav mic to be truly invisible, it must be underneath the actor’s clothes, either attached to their skin, or to the inside of the clothing. When the equipped actor moves, their clothing may produce a rustling sound on the mic, which will harm the integrity of the audio recording. Luckily there are methods to avoid this drawback.
There are a number of ways to prevent and combat clothing rustle. One common solution is to shield the mic with a material that will not produce a rustling sound. Another frequent solution is to package the microphone within a pillow of body tape, preventing the head of the mic from making direct contact with cloth.
When shaped correctly, a tape-pillow can also help to hide the shape of the microphone if the outline is visible through the clothing.