Speaking tubes, once standard issue on naval vessels as a guaranteed way to send messages without relying on power, were important enough that in 1926 the US Navy’s Bureau of Construction and Repair commissioned “Transmission of Sound Through Voice Tubes”. This was a meticulous study that drew on science experiments, calculus, and a ridiculous canvas helmet with tubes for earpieces.
According to Matt Woods of the Intrepid Sea, Air & Space Museum, speaking tubes could carry sound up to about 100 metres. He helped us design one for the backyard with modern touches, including a slot for a smartphone (for tunes), and a swivel to aim the sound.
Ships used metal to avoid corrosion, but 110-mm PVC is cheaper and easier to work with. Find a part of your yard to bury the tube, then pick locations for the ends and start digging.
- 110 mm PVC pipe, length based on path of the tube
- four 110 mm elbow joints
- four 110 mm couplings
- additional joints for path
- two 110 x 150 reducing couplings
- PVC primer and glue
- scrap wood
- dry graphite lubricant
- appropriate glue
- chicken wire
- mitre saw
How to make speaking tubes
Dig a route for the underground run that is large enough to accommodate your PVC. Straighter routes are better, as joints can degrade the integrity of the tube and allow some sound to escape.
At each end of the run, use an elbow to direct the pipe vertically out of the ground. Cut to an appropriate height and glue a coupling on top. If necessary – maybe you decided to run the tube into a tree fort — support it by bracketing it to a solid surface.
Cut a 75-mm piece of pipe with a mitre saw. Glue an elbow to one end, and lubricate the other end with graphite. Press the lubricated end into the coupling and ensure it swivels easily.
To add the phone slot, cut a 250-mm piece of PVC with a mitre saw. Trace the bottom of your phone on the middle of the pipe and cut 6 mm around that line with a jigsaw. Inside the PVC, glue a small piece of scrap wood under the hole to keep the phone propped up, so the sound won’t be muffled. Glue the PVC piece to the elbow on top of the swivel.
To close off the system when a phone is not being used, lubricate a coupling and put it on the 250-mm piece of PVC (cut it in half with a mitre saw if it has a ridge inside that keeps it from sliding into place). Ensure it rotates freely on the pipe. Remove the coupling from the pipe and trace your phone on it, cutting a hole to match the one on the PVC pipe. When the holes are aligned, the phone can be dropped in. When the phone is not in use, rotate the coupling to close the hole.
Form a horn on the end of the tube by gluing on a reducing coupling. Cut a circular piece of chicken wire and use glue to affix it a few centimetres back in the opening to block rodents and wayward toys.
Repeat this process on the other end of the tube.