I frequently hear quarterbacks say something like "53 has the mic" when pointing out the defensive formation. Usually the player number that he points out is a linebacker.
I believe the defensive player that the quarterback points out has the microphone that connects to the defensive coordinator, but what does that signal to the rest of the offense? Is that player covering, or is there some other job that comes with "having the mic"?
In most cases he is actually saying "53 is the Mike".
There are four types of linebackers that are used in typical 3-4 and 4-3 defenses, respectively:
While the Mike Linebacker typically carries a microphone, the quarterback is not referring to the microphone that he is wearing, instead he is talking to his blockers and informing them who should block the "Mike". The term Mike linebacker has been used since well before any electronic communication was worn in the helmets. The Mike is considered the "quarterback of the defense" and is typically one of the biggest and most athletic players on the field and deserves much of the blockers' and running backs' attention. So, when a quarterback says "53 has the Mike" he is not saying "53 is in possession of the microphone" but instead he means "53 is very important and deserves your attention".
Now, it is important to note that when the quarterback calls out the Mike, it is not necessarily what the defense considers the Mike, nor is it necessarily even the middle linebacker.
In running plays, the offensive tackles and guards are typically responsible for blocking the defensive linemen, which leaves the center and fullback left looking for someone leftover to block. When the quarterback calls out the "Mike", it is typical that the guy who is being singled out will get a lot of attention on that play (maybe they will run right at him, or right away from him). If a fullback's defensive assignment is to "block the Mike" but not necessarily a particular player, the quarterback is basically saying "[for this particular play, and for your blocking assignment purposes] 53 is the Mike [so block him!]"
In passing plays, the Mike is called out similarly for pass blocking reasons. Often when a team is in a "max-protect" scheme (where running backs stay in the backfield to help block for the the quarterback), the Mike is called out to the running backs to let them know that they need to block him if he blitzes. The reason that this is important is that if the Mike Linebacker does not blitz, then the running back is often free to running a short passing route into the flat in support of the quarterback.
According to this article, up to two defensive players can have helmets with radios/microphones to communicate with their defensive coaches, starting in the 2008 NFL season. However, only one player can have a radio on the field at any given play. Each of these radio-equipped helmets is marked with a green dot.
Usage:
As with the offensive transmitters, the defensive devices will go live immediately after the play clock begins and will remain active until 15 seconds are left on the play clock or the snap of the ball, whichever comes first.
and
Two defensive players from each team will be identified and will be authorized to have receivers in their helmets, one as the primary and the other as the backup.
The primary player will have one "live" helmet on the field and a second "live" helmet stored in a secured trunk or container as a backup in case of a malfunction.
The backup player will wear his regular helmet on the field and will have a "live" helmet stored in the secured trunk or container in the event of an injury to the primary player.
The advantage of having these helmets is that the defensive coordinator can make changes to defensive calls, based on what the defensive staff sees from the offensive. This change provided defenses a counter to the quarterbacks' communications advantage. From the offense's perspective, knowing which player is making defensive audibles is probably of limited value, since the radio turns off with 15 seconds left on the play clock. The defensive player with the radio would be quite obvious - the two designated players are chosen before the game, and the active one would be wearing a helmet with the green dot and would be passing along updated defensive plays.
However, there is another possibility, as Peter King notes in a recent Sports Illustrated column. He relates an Eli Manning anecdote describing the importance of knowing who is "the mike":
When quarterbacks go to the line of scrimmage, they most often point to the foe they're using as the middle linebacker, in order for the offensive line to know which man they're going to block. The first man to the right of the "mike'' linebacker, for instance, will be blocked by the right guard, etc. And so when Manning would see Ray Lewis, number 52 on the Ravens, across the line and bark out, "52's the mike,'' Lewis would scurry to the outside of the formation and yell, "I'm the mike!'' And Ed Reed or another defender would slip into Lewis' spot and yell, "I'm the mike!'' They were taunting Manning, and it shook him up.
So according to this column, knowing which defensive player is "the mike" sets up the offensive blocking assignments, which is likely to be of more importance to the offense than knowing the identify of the defensive player with the radio-equipped helmet.
EDIT: Fixed typo in last paragraph.
Each team can have one player on the field with an active radio headset.
For what it's worth, the defense has been doing this to the quarterback for years. Now that the defense has an announced coordinator with the bench, it allows the offense to start picking up on this too. Unfortunately, only 3 people often have an opportunity to see what the defense is doing, and two of them are usually so far away from everyone they can't communicate with them anyway.
When I was in high school decades ago, the mike man the line man covering the center -- the nose guard or tackle. He was the point where counting began when numbering blocking assignments. This was long before any radios went inside helments.
The offense "declares" the mike (or middle) linebacker to set almost all run and pass blocking schemes. Most blocking schemes are set up against a basic 4-3 defense with the mike right in the middle. If the defense doesn't have anyone aligned there the offense will designate a linebacker near the middle to be the mike so the linemen can set the rest of the scheme. In zone blocking and most pass blocking schemes the center will be working with one of the guards in a combination to block a lineman and the mike. The guard he works with will usually be to the side of the designated mike LB. In the pros, almost all the QBs are ultimately responsible for declaring the mike because they can see the overall structure of the defense and make the right call - the safeties moving around can alter which player is best designated as the mike...the QBs are very motivated to get right in no small part due to it's importance in pass protection and, ultimately, their own health and welfare :)
