Subsonic (adjective)

1. Less than or equal to the speed of sound in air or other medium.
2. Designated for or capable of operating at speeds below that of sound.

Origin:

From sub- 'below' + sonic, from greek sonikos 'pertaining to sound', from sōnē 'sound'. the word dates back to the mid 20th century.

Examples:

1. The subsonic flight was much quieter than the supersonic one.
2. The subsonic missiles are used for precise targeting.
3. The subsonic aircraft are more fuel-efficient than the supersonic ones.
4. The subsonic bullet was designed to reduce the noise of the shot.
5. The subsonic trains have been proposed as a solution to reduce noise pollution.