When the ear arrives first, the eye keeps its bearings
Imagine a character closing a door, then standing on a subway platform in the next shot. If the door shuts into complete silence and train noise begins afterward, the scenes divide with precision. Bring in a distant rail squeal just before the door closes, while the room is still on screen, and the audience starts moving before the image does. This does not require a large effect. It needs a cue distinct enough to suggest the next place, yet quiet enough not to push the current scene aside. A sound bridge is not a trick for hiding a cut. It opens the direction in which attention is about to travel.
Continue the action, not just the location
When choosing transition sound, it is tempting to search for ambience shared by both spaces. A firmer connection usually comes from action. Steam escaping a kettle can hand its motion to a bus air brake; the friction of a turning page can become small waves at the shore. The locations differ, but the movement continues. Pitch does not need to match exactly. If attack shape, duration, or density carries across, the ear can bind two events into one sentence. Match every quality too smoothly, however, and the intention becomes obvious. One resemblance and one change leave the scene with more aftertaste.
The length of the bridge is the distance in the story
How early the next sound enters changes the relationship between scenes. An overlap of only a few frames smooths physical movement. A lead of one or two seconds makes the next event pull on the present; longer can feel like a premonition or a memory. In the session, begin with the two sounds cleanly divided at the picture cut, then move the incoming sound left in small steps. Do not erase the boundary automatically with a crossfade. Find the instant at which the audience recognizes the next space. That point defines the bridge and the psychological distance between the scenes.