Scientific American has a super-cool article detailing audio technology from days long past. Speaking tubes, phonographs and and even a more recent “tree turn-table” are discussed. Incoming blurb:
In the early 18oos, Jean-Baptiste Bio had experimented with how sound travels through long tubes, using the water pipes of Paris, and found that the confines of the piping served to keep speech intelligible over a good 1040 yards, compared to how well sound carried in free space. Increase the diameter of those pipes, however, and there would be a corresponding decrease in intelligibility.
So Elliott had some solid science to draw upon when he conducted his own measurements of the speaking tube system in the Cambridge home, part of an electric system for giving household staff the heads-up.
Each end of the tube — one just outside the kitchen pantry, the other in the second floor corridor — is covered by a whistle valve. If someone wanted to communicate, he or she would open the valve and blow through the tube, producing a sound like a whistling tea kettle. Then whoever was on the other end would know to open their valve as well, and the two parties could have a spoken conversation through the tube. Oh, and both ends had flared openings, the better to hear the other party. It is, after all, a waveguide, one that in this case guides sound waves.