For example, a film-maker might opt to include the background sound of an ambulance's siren while the foreground sound and image portrays an arguing couple.
Another example is the word siren, familiar to us as the mechanical device that makes such an alarming sound when police cars, ambulances, or fire engines approach.
"I cried for help but nobody was there to help me," said Saeed. Eventually he was assisted by a passing ambulance crew and the pair were taken to hospital.
The pope had been wounded in his stomach, right elbow and index finger of his left hand. He was transferred to an ambulance, which sped him to a hospital.
The proposed budget includes more than $1 billion in cuts that will shrink the police department through attrition, have halted some ambulance shifts, and have suspended plastic and glass recycling.