Dear Friends, it seems to me that the above expression, when referring to a sound, means "very loud". In context, a young lad has got caught stealing some money and the police are phoned. "La police est arrivée presque tout de suite avec la sirène à fond." So we might translate "à fond" here as "wailing" or "blaring". Can anyone confirm this, or do you have other suggestions? Many thanks for your help.