The seven (well...eight actually) steps below are important keys to achieving great field production audio.
Control the Sound Source
This is the most important step to achieving good audio. If the sound source is not acceptable, no manner of post processing can solve original acquisition problems. It is far easier to have the talent speak up, not talk-over guests, and speak into the microphone than solving them in post.
Control the Location
Getting more signal (wanted sound) than noise (unwanted sound) involves controlling the location of audio acquisition. Eliminate or move away from noise sources. Sound levels reduce by 6 dB per doubling of distance for a point source, 3 dB per doubling for line source radiators (a highway is a great example of a line radiator). Noise is often not apparent until mics are up because our brains are excellent signal processors and can discriminate, microphones can’t.
Control Subject-to-Microphone Distance
Keep to an absolute minimum to reduce reflected and indirect/unwanted sound. Give microphones “air” if ambience and sense of location is needed. Remember the inverse square law, doubling distance reduces levels (and requires additional gain) by 6 dB.
Choose the Right Microphone
Select microphones based on electrical type, polar response, and physical characteristics that match the production situation. Lav mics tend to sound tubby and unnatural unless lots of EQ is used. If matching shot-to-shot, use the same type of mic so transitions are smooth. Booming a cardioid or hypercardioid will sound far more natural at the expense of increased production complexity. Dynamic mics tend to be less prone to wind noise at the expense of lower sensitivity and reduced high frequency transparency.
Set a Proper Gain Structure
Match levels from outputs to inputs to minimize noise and maximize voltage transfer. All modern audio circuits transfer voltage, not power—no more 600 Ω terminations. Know an input’s maximum level before clipping and run the the source just below it for maximum dynamic range. Modern circuits and digital converters tend to hard clip. Remember mic-level versus line level!
Keep levels as consistent as possible. The ear notices change more than noise. Use frequency-dependent level control, specifically high-pass filters, to reduce unwanted sounds in specific frequencies. Use limiters in the field to prevent excessive levels and clipping.
“VU” means virtually useless for acquisition to digital recorders. Peak metering is essential for digital recording devices. Keeping just below max input before clipping will yield a digital signal with the most significant bits (most dynamic range). Peak-reading meters show you the absolute maximum signal levels so that sufficient margin can be maintained for headroom.
Ears are the best judge of audio quality. Decoupling from the local acoustic environment is essential. Use closed-ear headphones or the new class of in-ear monitors which provide 15 dB or more isolation from the local environment.