Sound effects with orchestra

Maybe send the orchestra to its own bus (if it isn't already) and notch out a frequency or two to accommodate the sound effects as they happen. You could even automate it so that the frequencies are only notched out when the effects are sounding. If you make the cuts small enough I guess it wouldn't affect the sound of the orchestra too much. I'm just trying to imagine how a movie sound engineer would fit music behind the dialog - I'm pretty sure there's some notching going on there. I guess it all depends if you want those sound effects to take center stage as they happen, or whether you want them to "gel" with the rest of the orchestra. 

The best way by far is to look at what the orchestra is doing at the time of the sound effects. I have scored music for films for example and sometimes in cues that were effect laden. They used to give me a version of the video edit with the effects in. The trick is thin out what the orchestra is doing at the time of the effects.  If it was a massive explosion then you had to drop things down to almost nothing behind it.  In the sound dubs the effects always got the first priority so if the orchestra was full on at that point then they would drop the music right down. Better to drop the orchestra right down to only a few things going on instead and still have the music turned up.
 
Same with dialogue. If I was doing a cue that was under dialogue its dumb to have complex oboe lines and melodies going on. They will only mix that right down and have the dialogue up. So what you do is very underscored like with string sections and softer brass things not stating too much. Then they can keep the music up louder. Sounds better.
 
Listen to the effect too for eq and stuff. If the effect has lots of mids going on then keep the mid range things in the orchestra out of the way just in those seconds. If the effect is bass heavy then keep the low end of the orchestra out of the way. Think of the effect itself as being a melodic statement. 
 
What you DONT do is try to make the orchestra really busy at the time of the effects and then try to carve out EQ and stuff to make room for them or panning either. You will never get it right, only chasing your tail. By all means create the music first but then you have to alter the music to allow room for the effects.  Just to add, even half way through the effect as it is dying away you can start bringing orchestral stuff back in, that way it won't be obvious that sections of the orchestra have dropped out while the effects are present and suddenly when the effect is over the whole orchestra is back in. 

It is not hard. You have got an effects track obviously and then you place the effects in the correct position. Now you look at all the tracks above the effects track (or below depending how you arrange things) and see what you can silence, mute or drop out at those points. Actually you can take things out bit by bit under the effects. What you will find is you don't have to take that many things out before the effect starts becoming very clear. 

When composers insist on having the whole orchestra going at the time of the effects they do often tend to duck things down but if you think about it then by only taking a few things out here and there under the effects that will actually sound better in the long run and much less ducking is required.  What do you think sounds better, the whole orchestra ducking under effects and back up again or just a few things within the orchestra not being present under effects and staying pretty well at the same volume. Less is more remember.
 
It is interesting how people still favour the idea of putting too much in and trying in a round about way to make room for still something else instead of thinning out what is happening under.  It is the same when many are producing songs and music as well. 90% of the time there is too much going on.  (same goes for music under vocals)
Once you start taking things out then clarity goes up.
 
Any good producer would be questioning every sound that is present in mix and making you justify it. (and taking them out usually!)
 
But good point about though whether you want the effects more on top of everything or blended in.  For blending in, being very careful about what is there and only taking a few things out as well will sound better.  Some of you may be thinking you have to drop out half or more of the orchestra in order for an effect to be heard.  As per my post before this it only takes one or two things to be down or out under an effect for it to be much clearer.
 
I suggest try both way and see what works better.

Would it be silly to use side-chaining for this?

Sounds like a perfect job for Melda Production MSpectralDynamics.  Put MSpectralDymanics on a group track of the full mix of the orchestra and use the effect sound as the sidechain.

jamesg1213
As soon as I read the thread title, Scott Bradley's brilliant music for the Tom & Jerry cartoons came to mind.
 

 
Now THAT was fun!!  especially so for me being a trumpeter/trombonist, GREAT orchestra!