|How fast you draw the bow across the string is a key in producing a beautiful sound. Assuming that the placement and weight (or pressure of the bow on the string) stay the same, the faster the bow the louder the sound.
This is not always the case because when the bow is close to the bridge, you can afford to spend less bow (a slower bow speed) and still make a crisp, clean sound. But the closer to the fingerboard you move the bow, the faster the bow has to move to produce the same sound quality. You also have to decrease the weight or pressure of the bow to keep that sound clean.
Practice bowing in various places on the string at various speeds. Experiment to see what kind of sound you get. And remember to keep the bow moving in a nice straight line perpendicular to the string!