How to Mic a medium/low-traffic area?
 

I may be doing some outside interviews, and am unable to control traffic noise. I just got to wondering if I could do something during shooting to help. My thought was that if I took the interview on one channel (probably lapel mic), and had a second directional mic that captured the occasional car or other "noise" without dialog, there might be some way in post I could use the noise track to subtract from the interview track and end up with a moderately clean interview. It does not have to be perfect.

Is this just totally stupid?

Unfortunately it's impossible to do this with ambient sources that are moving.
If you use a good lav that's mounted correctly and the person speaks up, you should be able to use a low-cut filter in editing to get reasonably good sound. You should test this with your lav in that location ahead of time and see for yourself if it will work satisfactorily.
Hand-held interview mics usually provide much better isolation, but must be held close enough to be seen in the shot, usually right below the chin.
A good shotgun mic, when used correctly and placed just out of frame can also get a clean enough track to use. This requires a boom operator though, because mounting it on the camera puts it too far away.
Again, test your location under realistic conditions and see what you get. When recording for real, you must have good isolating headphones to tell what you're really getting.

Also, if you can have your subjects FACE the traffic, (with their backs to a wall or -ooo better yet- a hedge row) you'd be doing yourself a favor. Especially if your using a boom or camera mounted mic. Not so much help with a with a lav though. The mic being blocked from the traffic by a thick peice of human torso will benefit you should use choose to use a lav.

Get good levels, as close to 0db as you can. That way, you won't have to turn any traffic up in post in order to 'hear' your subjects over cars. It's best to have a high sound to noise ratio, noise being cars in this case and sound being your subject.

Jay- is it even concievable to use a 2 mic setup as John described and place the "straight traffic" track OUT OF PHASE with the dialog track?

I've heard about this being used someplace. Not sure where, but the Grateful Dead spring to mind.

Also, remember, you're directing the doc so if, say, a humongous 18 wheeler drives by, wait until it's passed and ask the question again, perhaps in a way that the interviewee won't feel compelled to amateurly re-enact the exact phrase that was gobbled by the peterbilt.

Good luck, John.

The Grateful Dead used pairs of crossed Senheiser MD 421 as vocal mics in front of the Wall Of Sound PA system they used for a short while. The 421s were (in pairs) connected with one 180 degrees out of phase with the other. The object was to reduce feedback for vocals while standing directy in front of the system. It worked, mostly. The singer had to be within a pretty critical distance.

My SD 442 has a polarity switch on channel 2. I have found using it to reverse the polarity while using other mics in CH 1, 3, 4 an effective method for reducing HVAC noise during internal shoots ....sometimes. CAUTION: IF YOU CHOOSE TO TRY THIS, MONITOR IN MONO TO MAKE SURE THE MICS YOU ARE USING AREN'T COMB FILTERING THE AUDIO YOU WANT SO IT SOUNDS LIKE CRAP! I WAS USING LAVS AND THE PEOPLE WEREN'T MOVING MUCH. THIS TRICK WOULD PROBABLY NOT WORK ON BOOMED MICS AND LAVS unles tthe boom wass locked down and the people weren't moving.

I'd go with an EV RE50 handheld if the noise is too bad. You see that all the time. It's an omni. you shove it into your face and then the person whom you are interviewing.

The other suggestions are quite acceptable as well and BTW, if you boom, you likely won't be as successful with a 416 as you will a cmc641.

Smiles,

Ty Ford