How can you create a natural sound for a live orchestra performance?

It goes beyond the microphones to start .. the venue needs to have a well place , well defined speaker system in zones . The idea is go get the feel of the orchestra and its frequencies to be balanced through out . Anytime I have done some theatre it’s about placement and sitting in the those locations . Microphone enhanced the instrument and should be in the frequency range of the desired instruments . Large sections call for some strategic placement

One of the keys to success is knowing what an orchestra sounds like in the first place. From there, work with the conductor towards the "vision" of his or her sound preferences and what they expect from the reinforcement. They may want specific things like "more presence in the strings" or certain solo voices to be brought out. There is also a difference between a "pure acoustic sound" (i.e. an orchestra in a hall) and more of a "movie sound" and then also a "pop orchestra sound" with the latter two being related.

This is where it is truly “sound reinforcement”. You are reinforcing the orchestra. In pop/rock you are creating a larger than life sound that doesn’t exist without the sound system. With an orchestra you are helping the natural sound of the instruments reach a larger area. It’s a different technical and mixing mindset. The end result should be localizing to the orchestra and not even realizing there is a sound system involved. Keeping this in mind during your decision making process will go a long way towards achieving satisfactory results.

For a live orchestra program, Ivan is correct as the goal is to have it sound so natural the PA system disappears and just makes the acoustic audio available to a larger area than would happen naturally. It’s all about mitigating physics and variables.

It will also depend on the music being played; is it baroque? Or something from the romantic period, or 20th century? Each of these will have its own unique needs in the way of the acoustics of the room which will influence mic selection and positioning to capture the ‘natural’ sound

When I think "natural", I try to remember that less is more. The more microphones, the more effects used, the more you are altering the natural sound. Unless the venue is a place not acoustically treated to accomodate orchestras, ambient mics should be fine enough. At UT Austin, we used a decca tree setup above the 15th row (estimated). A mid-side in the middle and a spaced pair in figure 8 mode. Listen during rehearsal, speak with conductor/assistant, then tweak gain/eq accordingly. Other considerations: the recordings we did were primarily for archival purposes. We added close mics for jazz orchestras. In smaller halls we used a single ribbon mic.