Why most of the microphones are placed on the left of headsets / headphones?
I've noticed that the mayority of the well-known brands (Sennheiser, Logitech, Plantronics, Razer, SteelSeries, Audio-Technica, Phillips, etc) tend to make headsets with the microphone on the left, is there any UX related reason for that tendency? If not, what's the reason of that choice?
The position of the mic would be equidistant to the the mouth when placed on the right side, so there's no difference when recording.
My improbable theories:
1) Maybe it could be useful for people who eat (or make something using with their right hand and mouth, like smoking) while wearing their headphones, supposing that the person is right-handed(what it's true in most cases) and the mic is neither in front nor too close to their mouth.
2) I think we usually align our body more to the left side of the keyboard (centered between 'g' and 'h' keys or so, and probably to center of monitor too, to avoid neck tension) for writing comfortably, and right-handed ones place the mouse on the right, so the mic on the left could go along with the "left side weight" and avoiding the possible sensation of the microphone "being in the middle". This theory is enhanced in the case that the cable is not divided in two to reach each side and just reach the side where the microphone is placed, so the cord will hang closer to that side and not centered to your body.
Apple earbuds, one of the most-worn headphones  with mic have it on the right.
rewobs, as worded right now this question is likely to generate a lot of discussion and not many useful answers. Is there a specific problem you are facing regarding the placement of microphones on headsets?
It is an industry standard to feed the cables from the left side, maybe that includes the microphone? I mean, given that the mic wire would have to run extra distance if it were placed on the right, it would make sense for it to be on the same side as the rest of the wires.
I've been wearing my Plantronics headset with the mic on the right. I just now noticed I can wear it either way. Maybe some of your data points are ambidextrous headsets that people have chosen to wear to the left? (I'll switch to left - I keep bumping the mic when I go to adjust my glasses with my right hand.)
@msparer Beyond how many people use them I was asking if the decisions that manufacturers do have any UX related foundation. I edited it to make it clearer. Here apple products are not common, anyway I thought mics on apple earbuds were attached on the cord single cord part and not on any left or right division.
Theory #1 sounds fairly solid. Wires get in the way. Most people are right handed. Put the wires on the left to keep them out of the way.
@JoshuaBarron As an engineering student and UX lover this question just came up to my mind and although it is not on my plans to start making headphones, I wasn't going to wait until that to get a possible answer. I've researched it for a while on the web without success, and because of possibly being related with UX design, I thought it was okay to ask it here and not just in a technical forum. I'm aware that it can bring up some discussions but I don't see why it couldn't have an objective and non-opinion based answer. I edited the question and added some text to make it a bit more precise.
With every headset I've ever had, the mic can be rotated through somewhere around 300' so that you can wear it with the mic on whatever side you like. Personally I wear mine to the right and am left handed if that makes any difference.
My thinking is along your theory #1.
As some people have commented, the microphone can also be on the right side, which indicates to me that this isn't an absolute design standard. It seems to be more convenience based then. So what is convenient about it?
As stated in your first theory, it frees up space, allowing for uninterrupted interaction with the face. People naturally eat using their dominate hand, related. With the mic on the non-dominate side, it doesn't impede eating/drinking. I see this being one of the largest factors.
From personal experience, I find that when using a headset, it's dependent on the situation I am in whether or not it being on one side or the other is better. If I am at my computer, I use my right hand for the mouse, and typically rest my head on my left hand. With a microphone that is close to the face, this can be restrictive. However, when eating, it is beneficial.
In the end, it's up to personal preference, which is why a lot of companies make headphones that allow for the mic to turn 180 degrees, to be usable for both kinds of preferences.
There does not seem to be a UX answer as far as I can tell. With a little searching, other people seem to believe it is for right hand dominant users where their right hand is always on the mouse and busy so the left hand is available to mess with the headphones. I would also have to agree with this. It seems there isn't a scientific reason for companies doing this though.
Most people have their PCs sitting to the right of their monitor which means the cord has to go across ones body/lap to reach the jack, often rubbing against ones chin along the way. Also, every laptop I've ever owned has had its headphone jack situated to the right, creating the same messy problem with left oriented cords.
Now, of course, someone is going to say 'I always have my PC to the left' or 'my laptop isn't configured like that'. But I believe I am speaking on behalf of the majority of people who live in a right-handed world.