Categories of Sound Effects

Sound that is organic to a scene is often called diegetic sound.  These sound effects can be realistic or altered for effect.  External sound effects, those not logically heard in the scene, can also be added for dramatic value.  These external sound effects, that is, those not part of the story world, are called non-diegetic.
For our purposes we can divide sound effects into four categories as follows:

Realistic: This is any sound effect that one could naturally hear if situated in the filmed scene.  The source of the sound might be on screen or off screen.  Adding the most common sound effects like a car honking, a metronome, or a buzzing mosquito can greatly change the feeling of a scene.

Expressive: For our purposes an expressive sound is one that is realistic, but has been altered.  That might mean a phone's ring starts out normally and suddenly gets louder and louder.  The sound comes from the scene but has been manipulated for effect.

Surreal: Sound effects are often enlisted to externalize a character's inner thoughts, nightmares, hallucinations, dreams, or wishes.  We might hear, for example, the laughter of a child as a woman picks up a doll from childhood.  This gives the scene a surreal feeling.  This effect is often called meta-diegetic.

External: This is a sound effect that clearly does not come from the scene. It is an effect that is not heard or responded to by the characters.  For example, if a character is making his last walk down death row, and slowly, the audience hears a church-bell, and we know there is no church for miles, we consider this sound external to the story world.  The purpose of the sound effect is to signal to the audience the meaning of the scene. This kind of effect is known as non-diegetic.