Hi - I don't think I have a plug in that is actually an expander - I would like to experiment with one if I could be steered to some, to check out..I have read this well written description of what they do, and advantages over a noise gate: " Like a faint starlight in the night sky, noise is most noticeable when it's by itself. Mix in a little signal (or sunlight in this analogy), and you're less likely to notice the faint background stuff. An expander works on the same principal as a noise gate, but an expander is a bit more subtle; it's not as obvious to the ear when it's doing it's thing."

     An expander actually works the opposite way of a compressor when used aggresively. I have used the ProChannel Gate/Expander set to expander and taken audio tracks that had no dynamics at all and set the expander to a rather high setting. This results in the louder passages being reduced in volume while raising the volume of the lower. VIOLA! I have dynamics back in an overly compressed track. This has saved my #$%@ many times.

     An expander actually works the opposite way of a compressor when used aggresively. I have used the ProChannel Gate/Expander set to expander and taken audio tracks that had no dynamics at all and set the expander to a rather high setting. This results in the louder passages being reduced in volume while raising the volume of the lower. VIOLA! I have dynamics back in an overly compressed track. This has saved my #$%@ many times.

Sorry, bout the double post. My mouse is still asleep!
 

There are two types of expander. Downward expander and upward expander. Upward is less common. Here is a SOS article to introduce the concepts.
 
http://www.soundonsound.c...advancedgating.pt2.asp
 
What Sidroe has described is more akin to a compressor not an expander. (ie louder parts being made softer and softer parts made louder) I think he might be confused. A downward expander does not touch loud passages at all but attenuates further softer passages that fall under a threshold. Hence an apparent increase in dynamic range. An upward expander actually increases the volume of louder parts that go above a threshold.
 
A true upward expander might be able to return some of the dynamic range that may have been squashed by over compression. Downward expansion can sort of do it too. So Sidroe would have improved the dynamics for sure even using the downward expander.
 
A downward expander is better at attenuating background noise eg room tone during a vocal recording. It is better than a gate because even if sound falls below the threshold it can still be heard unlike a gate that shuts off completely. They are also great for cleaning up noise from noisy synthesisers that are not making any musical sounds just the output noise of the instrument. (Old Kurzweils are notorious) Even a soft sound will still get through ie the downward expander will still open up even for the softest sound. (depending on threshold setting of course)
 
They can be setup for very transparent operation if you take the time to fiddle with them. They work best though if the signal is well above the noise they are attenuating though. If the noise is high then, every time the expander opens up you will still hear the noise come in with the instrument or voice etc and go away when the signal goes away too. So it's not so great in that situation. But if you are recording a singer and they are quite close to the mic then a lot of room tone or background sound can be very effectively and almost totally removed with one. 
 
They won't be so effective in removing noise while trying to record an acoustic guitar two feet away in a slightly noisy environment though. In that situation the signal to noise ratio is not so good then and it has problems dealing with that.
 
 
 
 
 

Thanks for these write backs...I need to check into it more. If anyone knows of some other expander plug-ins for me to look up - will appreciate it. I will have to look more closely - I guess I did not notice that a ProChannel plug in has a dual mode like that.

Does an expander plug in need to be labeled / designated as a "downward" one, or is that just how you set parameters?

Most expanders are downward only, just like almost all compressors are downward only (an upward compressor increases the level of a signal below its threshold).
 
For noise reduction, I've found that a low expansion ratio (something like 1.4:1 or less) and a somewhat higher threshold can work better than a higher ratio with a lower threshold. But it's a balancing act to try to make it not sound unnatural while still getting rid of as much noise as possible. And it depends on the signal - in some cases a little noise may be the lesser evil.

Check out FabFilter Pro-G (gate/expander) or Flux Pure Expander II. Both are excellent, but I only own the Pro-G.