A vacuum sound typically refers to the sound made by a vacuum cleaner when it's in operation. This sound is caused by the motor inside the vacuum creating suction to pick up dirt and debris, which generates a characteristic noise. The sound can vary based on the design and power of the vacuum, but it’s usually a combination of a low-pitched hum from the motor and a higher-pitched whine or whoosh from the airflow.

In a vacuum (an environment with no air or matter), sound cannot propagate because sound waves require a medium (like air, water, or a solid) to travel. In this case, "vacuum sound" could mean the absence of sound. In automotive terms, a "vacuum sound" could describe the hissing noise produced by a vacuum leak in the engine's air intake system.