How did Barker know about the Douglas murder?

Cecil Barker was in the Douglas home and heard the "report" of the gun.

“It was just half-past eleven. I had not begun to undress, and I was sitting by the fire in my bedroom when I heard the report. It was not very
loud—it seemed to be muffled. I rushed down—I don’t suppose it was thirty seconds before I was in the room.”