what would one make a microphone rig for a car to record dialogues ? Which microphones are preferable for these situations and why ?
My favorite way of doing this is to sit in the opposite side behind the one being filmed, using an as short a boom I can get it, with either a Sennheiser MKH-40 or Oktava 012 (which capsule also depends), depending on the voice and car, facing a best sweet-spot I can find and staying there. Of course that means you have to follow the movements of both the actor and the car as well as keep out of shot, but used in the right way it's well worth the extra work on your behalf!
It's much easier if you just use lavs, but especially in cars with the proximity to pretty much everything reflective in mind, in my opinion the much better sound of a cardioid or super aimed in perfect focus makes it well worth despite the fact that it's an everything but comfortable position to boom in.
Early in my career, before I could afford more good mikes, I used my 416 to everything. Including car interior. Doesn't miss that time.
There's all kinds of ways to tackle this problem and on top of that, there's a bunch of factors in terms of how the scene is being shot and what kind of car. Here's my 2 cents...
-If you've got some a lav mics, plant them in the visors above each front seat or on the dashboard, if it's a two person scene.
-If you have a boom operator with a shotgun mic and you're able to hide them in the back seat, have them have the mic in the middle to split the difference.
-One of the best ways to handle a situation like this is with a modular mic set, like the Schoeps CMC series or an Oktava MK-012. Another easy route is any sort of Lavalier mic that can be easily hidden in order to get a good clear signal.
Those are just a few options to keep in mind and I hope this helps.
I know sometimes you will have to use the audio recorded from within a car's interior, though getting great sounding results will be tough regardless of the mics used and setup approach that you choose.
Also, by trying to record cleaner audio from the inside of the car, you may begin to impose on the actor's ability to do their best job acting (even if this is reality TV or a documentary, they are still actors). Funny, that reminds me of a saying I heard (or read once)... "What is the difference between actors in scripted fiction and in documentaries?" "You have to pay the actors for a scripted film, but that is the only difference".
Though let me get back to the point of the question. Go out and experiment with any and all of these methods mentioned here, and just experiment in general with different mics (what you have available to you) and mic placement and hope for the best (though make sure you experiment before the actual shoot). And when deciding on mic placement, try your best to stay out of the way of the actors (and obviously out of the view of the shot) as you do not want to disrupt the scene or their performance by making them uncomfortable and in-turn, negatively impact the sene, or make it unusable because there is a mic on the shot.
My advice, get the actors to re-record the lines from the car interior during ADR and then you can process them (along with adding a car interior ambience, even recording the ambience of that specific car on set). This give you a lot more control over the audio fidelity and the final result of the dialogue for the scene.
The best mics for these jobs I found so far were the DPA 4081 miniature supercardiod mics. You can hide them easily in the front part of the car and they sound superb. The directional principle of these mics make sure you have the least amount of engine-noise in your dialogue The sunshields and the ventilation shafts are my favorite spots. If you have time, try and find a way to open up the dashboard as well. A 24dB/oct or 36dB/oct lowpass round 70 Hz (for female voices 100 Hz) will also take some of the engine humm away, without bothering the human voice spectrum. Good luck!
Yo! Another effective rig which I've used several times is placing the transmitter pack between the sunroof and sunroof cover, with the tram/cos etc being taped (Gaffa/durapore tape) just above head height on the roof of the car. This is a very handy place to rig if the camera guys are seeing the sun visors......
It's quite dangerous to rig the transmitter packs under the sun visors while the actor's driving - just in case the tape doesn't hold and it falls out when in motion! What I have done to combat this (but still having the mic in the visor) is to run the cable down the rubber seal of the car and place the back either in the glove box or in the door compartment of drivers side.
Hope this gives extra options to you!