The definition of a ramjet by the Merriam Webster online dictionary states that:
a jet engine that consists essentially of a hollow tube without mechanical components and depends on the aircraft's speed of flight for the air compression necessary to produce the thrust obtained from the expansion of gases caused by the combustion of fuel.
If the engine needs this compression at the speed of sound it would need to slow down the air, as it is supersonic. If the engine took in supersonic airflow, it would become a supersonic combustion ramjet or scramjet. The ramjet needs intake airflow to be moving subsonically.
If, and only if, the intake and combustion coexisted then can a ramjet operate at subsonic speeds as the intake airflow is fast enough and doesn't need to be slowed?
If so, how slow can they be started, 300mph?