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Mid side miking

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  • Mid side miking

    Here's Is a question about mid side miking. The left and right are opposite phase with each other and cancel out in mono, but when they are mixed with the mid the right side is reduced and the left is summed with the mid and vice versa. How do the sides still cancel in mono with the mid added if the they become different levels? Two signals out of phase have to be the same level to completely cancel right? I'm seeing it like this- left equals mid plus side and right equals mid minus side, so wouldn't it seem like that would collapse to mid plus side on the left plus just mid on the right And total 2 Mids and 1 side?

  • #2

    I'm not sure what you're question is asking, and its likely the same issues many have trying to understand how it works.

    They key you seem to be missing is the right side of the mic has a phase reversal added so all three mice go positive at nearly the same time when generating a signal.

    You figure 8 mic recieves sound from both sides. When the diaphram is pushing in on one side its pushing out on the other. This will cause the sounds on thr right to be going in a negative direction in comparison to the sounds on the left to be going in a positive direction when the diaphram is pushed in from those sides.

    You can make a mental picture of this. Think of a pure sine wave AC wave form going from Zero to positive then back down to zero to negative. If you had a loud transient occur like a gun shot, and say its closer to the left side of the mic, the diaphram pushes inward. As it does, it makes the AC wave go positive from the air pressure pushing in. When the diaphram retracts and gets sucked outward, the right side is now pushing inward creating a negative peak.

    Thats a normal operation of any mic of course. The key with a figure 8 is both sides are equally exposed and other mics like an hand held dynamic mis only has the front exposed (Kind of like a speaker in a box. You only hear the front side)

    When you turn the figure 8 so the front positive side faces the left of the room and the back negative side faces the right, neither side if facing the sound sourse directly.

    When sound comes directly at the mic from the center and reaches both sides of the diaphram at the same time, the diaphram barely moves. When you think of it, it makes sence. If you have a knife blade in the water with the edge of the blade facing the wave coming directly at it, the blade cuts through the wave with very little disturbance to the wave.

    If the wave comes at the blade from one side or the other the blade will appear wider, the pressure of the wave will push on the blade, build up higher on that side and get deflected. The other side of the mic will have a troth created and less pressure applied to that side.

    Same thing happens with sound. If the sound is dead center hitting the diaphram it cuts through the air wave like a knife causing minimal diaphram movement.

    The sound that gets reflected off of the side walls of a room bounce back into the left and right sides of the mic. The key here is, the reflected sound is delayed and those sounds dont make the diaphram move at the same time. Its these differences in transient times that make up the coding from one mic. 

    Say you have a sound source left stage. The reflected sound should make it to the left positive side of the figure 8 diaphram first. The reflection off a wall on the right side is a longer distance. It will reach the diaphram later and also be colored with different frequeny content (another key aspect)

    Whatever hits both sides at the same time will be phase canceled and whats left is the differences between the two.

    This makes the center very weak so you add a mic that receives 100% center positive signals.

    When you sum the two mic signals together the positive left of the figure 8 adds with the center mic and you have a string center and left side when the figure 8 is panned hard left.

    But how do you get the right side to happen?  You had/have two choices here. You could have split the figure 8 cable and recorded a second channel from it, or since its so easy to copy digital tracks, you make a copy of the figure 8 output and pan it to the right.

    This wont sound right because you have the left signal on both sides, so you reverse the phase of the right side. This makes everything that was going negative on the mics diaphram to be going north now.

    With both channels panned center,  anything that was simular to both sides gets nulled out and anything different between the two sides of the figure 8 diaphram, like the delayed reflections off the walls and different frequency content striking the diaphram remains.

    This will sound very hollow by itself because you hear no direct sound from the instrument or sound source.

    This is where the center mic comes into play. The center mic adds the direct sound back into the mix (what was nulled out by the two sides) 

    Now if you pan the two sides of the figure 8 out hard left and right, and the sound souce is to the left, the sum of the center and left side of the figure 8 will be stronger then the sum of the center and right so when the three channels are played back through an amplifier, the left speaker will reproduce a stronger signal on the left.

    If the source of sound is to the right, the sum of the center mic and back side of the figuer 8 (which was reversed to go positive) adds together to make the right side stronger, and produces a stronger sound in the right speaker.

    This isnt any different than when I said a knife in the water with a wave coming at it from an angle is going to push against that side harder on the one side than it does on the other.

    They key to your question was likely you missed the negative side of the diaphram is made positive using reverse phasing. you can split the mics signal and reverse one side recording it to two tracks or you can copy the one track and reverse it.

    The cool part is you only need to record two tracks with two mics. The decoding is super simple, you simply revers one tracks phase so all three are going positive at the same time.  Then its just a matter ow whats the same or different between the three tracks that reproduces the stereo imagery.

    You can mono all three and colapse the stereo width to any size you want. The front and back of the figure 8 cancels out any like signals and the center mic produces all the center information, what ever is left from the figure 8 channels canceling out is going to be the positive going side information of the two diaphram sides and it adds what the center mic doesnt hear, side reflections. 


    • #3
      I understand how the decoding works and that all makes perfect sense to me but let's say for example you're miking a drum kit in mid side and the drummer hits a cymbal all the way to his left. It will add to the left and subtract to the right leaving a loud signal on the left of the L/R bus and very little on the right correct? That being said, what signal is remaining on the right of the bus that would cancel out the side portion of the left of the bus if they are drastically different in level?


      • WRGKMC
        WRGKMC commented
        Editing a comment

        Hum, I get what you're saying but I think you are missing whats happening.

        I'll try to make it as simple as I can so you'll catch whats going on.

        Say you're in a room with zero reflections, with the figure 8 turned sideways left and right. Your sound source is dead center between the two sides.

        The diaphram moves very little either positive or negative.

        If you move the sound source left, the sound pressure on the left side of the mic will cause the diaphram to move in and generate a positive signal before it gets sucked back out and generates a negative signal.

        This is a single mic remember with only a single cable and it generates an AC waveform going positive and negative. The sound pressure is an alternating wave too. Its increases and decreases in air pressure that we call a sound wave, and the mic diaphram resonates with the air being moved, positive and negative.

        With the sound source on the left of the figure 8 the AC wave created by the mic is positive going.

        If you move the sound sourse to the right,  the right side of the diaphram moves first, and it goes negative from the sound pressure. The output is still AC because the diaphram moves in and out but the signal sounds make on the right side of the mic are negative going waves in comparison to the left which are positive going.

        Now we split the signal into two, and reverse phase one so both signals go positive.

        This may be where you are missing whats happening.

        When you reverse two identical waveforms they completely null each other out. Its not that you erased them and they dont exhist its just equal and opposite forces working against each other and your speaker doesnt move because neither wave is bigger than the other.

        When you add the third mic pointing straight center, that tug of war changes. If the sound sourse was on the left, you add the center to the left side and the left speaker moves more than the right.

        If the sound sourse was on the right, (the waves that were going negative before we reversed them), the center adds up with the right channel from the figure 8 and the two combined makes the right speaker move.

        The key here is the direction the diaphram from the figure 8 moves first. The right side moves before the diaphram moves back out and that will be combined with the center mic because its in phase with the center mic. .

        This would be easire to see watching it on an oscilloscope with the mic connected up and split. You can actually see the waveforms and how thay add together to make a larger combined waveform. .

        You're really not worried about the other side of the mic if the room has no reflections. The sound isnt hitting that side of the diaphram because nothings being bounced back into it. The diaphram is being moved from the one side only, just like a dymamic stage mic is when you sing directly into it. If you move your voice and sing across the stage mic the sound level drops dramatically. Same deal with the figure 8 except we cant get to the back side of a stage mics diaphram.

        When we sing across the mic diaphram its like pointing the side of a figure 8 towards the sourse of sound. Very little sound is produced.

        As we move over to one side, that diaphram begins to move either positive going or negative going depending on which side we move the sound source. Both sides produce an AC waveform.

        In a reflective room you're going to have different reflections hitting the sides of the figure 8 diaphram. Loud sounds on one side will dominate the polarity the mic produces during that transient, but the mic diaphram can vibrate with say a low and high frequency all at the same time, or staggerd times.

        So whatever is hitting the right side goes more negative than whats hitting the left side and vice versa.

        You belnd this wuth the center mic that cannot tell left from right. It can only go louder or softer or pick up frequency content. The center mic picks up direct sound, and the sides of the figure 8 pick up more reflected sound with gradually more direct as you move to the right or left. This is precentages of direct, not complete this or that.

        Its the percenteges of change left and right thet produce a larger direct sound from either side of the figure 8, but mostly its going to be the reflected side thats going to remain when you flip the polarity on one side and cause the same sounds to cancel leaving only the side materail being added with the center mic.

        I dont know if I can explain it any easier than that as long winded as it is.

        Best thing you can do is just try it and let your ears tell you whats happening. You can also create the three tracks, then enlarge them in the track view so they are full screen and you'' be able to see how the waves align with each other. When you have a positive and negative wave occuring at the same, and you set those tracks for mono, the two will have practically no output. Then if you flip the polarity of one, the two become louder than either one alone. If one wave is partially off center, you get a partial null, if one wave is larger than the other and they are 180% out of phase then you hear whats left of the larger wave.

        Like I said I could show you on a scole in two seconds. Doing it with words can be fifficult if you dont have a good mental image of how waves work against anbd with each other. 



    • #4
      Thanks, I think I got it. I think I wasn't condsidering the fact that the two mics are picking up different sounds and they cancel different than two exact sounds of opposite phase.


      • WRGKMC
        WRGKMC commented
        Editing a comment

        Yup thats the key, when you reverse phase onme channel, the frequencies that occur "at the same time" and are picked up in a straight line to the mic without reflecting off a wall of something will cancel so long as the signals are of equal amplitude.

        When the sound source is to the right or left, that side receives a signal greater in amplitude then what is being canceled with the phase reversal so that part of the direct signal gets through and mixes with the center mic.

        Then you have the sounds reflected into the mic. Like a ball going around a pool table, sound would only get reflected into both sides of tyhe mic if the source is dead center and the mic is too. It would be like shooting a pool ball of the far bumper and having it come right back to you. Those reflected sounds may get phase canceled if the wals reflected the same and it was a perfect bounce only. You'd want that because the center mic is doing the direct center work.

        Shooting the sound around the room off center again can be like a 10 bank pool shot before it gets to the side hall and bounce into other objects in the room and have different wall absorbtion. all those echos and resonant trails that get picked up by the figure 8 and sound natural to the ears because thats pretty much how our own ears work.