How can I do this?

[mbuster]

"This means that you can get away with using a mastering plug-in or processor to sum all the frequencies below about 120Hz to mono, without disturbing the stereo image --"
This is a quote from an article I was reading. What will do this? I use SONAR.

 

[ambi]

That's a little confusing, and we don't know the context in which it was said. But it sounds like they are talking about a specific plug in or effect, or technique or something that does that. So first you might start by telling us what the article is called, what it's about, and some info about what they are doing.