I don't mean actual silence, I mean mental silence. A quiet space in which your imagination can breathe. I mean the focus that comes when your gaze is turned thoroughly inward and the real world is but a shadow.
As a novelist you have to chase that silence and, when you find it, guard it fiercely. It's no good trying to write your magnum opus (or even your crappy first draft) while your muse is competing with Twitter, Facebook, your to-do list and that argument you had with your sister/boyfriend/postman.
Some writers, like Virginia Woolf, require a room of their own, others just a pair of headphones. Some people like loud music or the television on, some people need complete quiet. There are writers who like being surrounded by humanity and find the gentle hubbub of a coffee shop perfect for concentration.
Experiment until you find your optimum working conditions, your personal route to that blissful quiet. Notice when you're finding it easy to maintain focus and when you're not, and adjust your habits to suit.
If you can't focus for long periods of time, then try breaking up your writing session with short breaks every twenty minutes. Or, perhaps you need longer blocks of time and you're only giving yourself twenty minute snippets. Schedule a few hours in your diary, take the phone off the hook and see what happens...
Without wishing to sound precious about it, you also have to protect your silence from mental clutter. It's difficult to write when you're worried about life stuff and sometimes, of course, that is unavoidable. However, if there are distractions or stresses in your life that you can do something about, then feel free to kick them to the kerb in the name of your writing.
Finally, I've also heard that some novelists like to begin each writing session with some yoga or meditation. Personally, I've never had much success with quietening my mind in this way (my process involves playing loud music and forcing myself to type until I hit 'the zone'), but I'm sure it's worth a try.