For what primary communication mode were speaking tubes highly effective?

Point-to-point communication where the speaker and listener were separated by distance or physical barriers.

The speaking tube was engineered specifically as a focused transmission device for dedicated lines of communication. Its effectiveness lay in bypassing intervening obstacles, such as walls or floors in large buildings, or simply bridging a moderate distance where one-to-one clarity was paramount. This made them common in large homes, offices, and naval vessels for private conversations without the need to shout. They were inherently limited in their utility for mass address because the sound energy was confined to a narrow channel, sacrificing broadcast potential for transmission efficiency along a pre-set path, making them unsuitable for addressing a general audience.