How far away must you go to get a clean recording? I'm planning on recording some nature ambiences and want to know what people's experiences are.
The only real answer is to use your ears - before recording. If you hear traffic, don't set up because yet you're just going to record it.
In my experience, getting about 2 miles away from the highway on a perpendicular angle is about right. It also helps if you actually lose sight of the highway, which is more difficult than one would think out on the west Texas plains where I've done most of my ambiances.
the other obstacle you'll run into is air traffic. Even if you're way off of a normal flight path, some yokel in his prop plane is likely to be cruising overhead no matter how far off the highway you get. This can be overcome somewhat by waking up insanely early and doing your recording before they get gassed up and on the tarmac.
also, don't bring your dog. :)
once your in place and rolling, I'd be prepared to roll at least 2x what you intend to keep. Inevitably some audio annoyance will come along and you'll have to edit around it after the fact. just keep rolling and you'll be fine.
don't forget to take pictures. the locations of ambiances can be difficult to discern by ear, so photographs that can be embedded into the files via soundminer (if you use that) are incredibly useful when digging through them later on.
I usually try to look at a map in the general area I want to record in and look for country roads that will take me as far from highways and busier thoroughfares as possible. If I have the opportunity beforehand, I'll usually scout the area out to look for good places to set up.
It's much easier if you're trying to get night ambiances, because you can also plan those for the middle of the night, during the earlier part of the week, to reduce the chances of people driving by on even the more rarely travelled roads. (Which, it becomes frustrating clear once you begin recording, happens more than you'd ever think, even in the middle of nowhere.)
I think @Rene was right about rolling more than you need. Probably end up needing quite a bit more than 2x for daytime stuff, depending on how far out you can get and how much you're willing to fudge in editing.
What microphone are you using? That will be an important factor ie off axis rejection.... Be aware the rear of most shotgun microphones is more sensitive than the sides, so if eg you point the front of the mic at some birdy trees, make sure the rear of the mic is not pointed in the direction of the traffic source....
Just got back from the mountains myself and contending, as usual, with the same issue.
- If you can see traffic, you're too close. No cheating with fog or darkness, now. :-)
- If you know where the road(s) are, do a 360° with your mic (as Tim said, more directional is often good, but sometimes not if you're doing stereo) and optimize placement based on rejection of the traffic source.
- I've recorded FIVE MILES away from train tracks and still hear the horn and rumble of freight trains clearly.
- Air traffic increases on weekends due to recreational flyers; if you're going to a remote area, that increases the likelihood of hearing air traffic since only small informal airstrips that service civilian planes are found in remote areas.
- Recording prior to dawn or after dark helps immensely, but complicates logistics and scheduling.
- You can replace distance with intervening geography. If it's flat, I don't find 2 miles to be enough. If it's wooded and hilly, 2 miles can be good enough...if you're in a valley. The higher your elevation, the farther you must be.
I just did a hike of 3,000' in elevation to an altitude of 8500' over 4 miles of hiking and 6 miles from the nearest paved road to get things quiet enough that I could record the sound of wind in pine needles without birds or even insects. A rare opportunity, but it took half a day to get to that location. Conversely, I've gone into a forested canyon a 0.5-mile drive from my house and within 4 miles of the US's busiest highway and got clean recordings of frog choruses, using the landscape as a giant sound blanket.
These efforts often pays off, and you must be patient all those times it doesn't.
How far is far enough? All the way. As Rene said above, use your ears. Do a test run of some recording, then take it back to the lab and really analyze it. Sometimes Headphones on location just don't give you all the details. It also matters if you are recording production audio, or looking for isolated SFX.
It is a tough one. I was in western South Africa recently and we wanted some real quiet, authentic ambiences minus the traffic and it proved to be really tough. Out there, the tires on a long stretch of highway just travel forever. I found that choosing your time of day was important- early morning, just when things are waking up and the traffic is at it's absolute minimum. It is great to try and get away from a road and to travel inland for a few miles but sometimes that is not possible. As some of you said, use your ears, record too much so it can be edited down a bit and be patient.
It seems like in L.A. the answer is far, as far as you can get. I found a park that looked far away from everything on a map... Even though I was three miles from any road or highway, I could still hear the highway. I am waiting for my portable recording rig to arrive in a few more weeks so I wasn't able to record. I'm not giving up though, I was mostly walking on ridges that had a clearer line to the highway. I figure if I can get a few mountains between me and the highway (and possibly walk down into the valley of one), I will get much cleaner recordings.