When sailing masters aboard tall ships needed to call out commands to the crew, they used speaking-trumpets, a type of megaphone, to aim and amplify their voices so they could be heard the full length of the ship and up to the topmasts. On a small boat, it’s not likely you’ll have trouble making yourself heard by anyone on board, but your voice may not carry well to people on shore, lockmasters, or other boaters in the area, especially if the wind and waves are making a lot of noise.
Cupping your hands around your mouth is instinctive and will help a bit, but megaphones are much more effective. For short distances, they can be small, like the pre-electronic type coxswains wore strapped to their heads to reach all members of the rowing crew; for more range they can be larger, like the traffic-cone-sized megaphones coaches once used to communicate between their launches and their crews.
As you’d guess, megaphones work by focusing sound in a particular direction, but there’s another factor at work that accounts for the perceived amplification of sound: acoustic impedance. The sound energy produced in the small space of your vocal tract can’t effectively make the transition to the wide-open space around you. I have to admit I don’t fully understand how this works, but I think of it this way: If you poke your finger into still water, it won’t make much of a splash or generate anything more than a small ripple. If you hit the water at the same speed with the flat of your hand you’ll make a splash and a wave. The megaphone effects the transition from a small area of impact at one end to a larger area at the other, setting more air in the open space in motion. A writer on one web site put it more accurately: a megaphone “acts as an impedance-matching acoustic transformer to efficiently couple the sound from your mouth to the open space.” And the effect isn’t restricted by the direction of the megaphone. I’ve noticed that rowing coaches using non-electronic megaphones are very easy to hear even when their megaphones aren’t aimed at me.
The most familiar acoustic megaphones have conical shapes (properly called frustums, as they are cones with their pointed peaks lopped off). Bullhorns and electronic megaphones flare like the horn of a trumpet. A conical megaphone may not be as efficient, but it is much easier to make, and I’ve made a several from plywood, aluminum, leather, and PVC drainpipe.
My largest megaphone, and the loudest, is made of 1/8″ mahogany plywood. I cut eight staves 22″ long and 4-3/4″ wide at the bottom, 7/8″ at the top. It’s a bit much for my smaller boats.
I had intended to join the staves with copper wire, stitch-and-glue fashion, but getting them to line up edge-to-edge wasn’t working. I went back to the table-saw sled that I set up to cut the staves and beveled the edges at 22.5 degrees.
Many common megaphones have been made from sheet material. To make one in that manner, you need to create a frustum pattern.
All of the megaphones here were made from scraps I had around the shop. I wanted to make one from brass, but I didn’t have any suitable material, so I worked with aluminum.
Stiff leather was once used to make megaphones, typically those used by cheerleaders. Metal rings were usually fit to the ends, but they aren’t necessary if the leather is stiff. A scrap of leather I had—2.5mm vegetable tanned full-grained cowhide—was just big enough for the pattern I made for the aluminum megaphone.
PVC is a versatile and remarkably tough material that can be coaxed into new shapes by heating it in a kitchen oven to 170 degrees F. I’ve used some 4″ drainpipe—with walls about 1/16″ thick (Standard ASTM D 2729)—for other projects and it’s well suited to making megaphones.
While the conical megaphones work well enough, I wanted to see if a more sophisticated shape would make a difference. Exponential curves shape the horns for hi-fi speakers and P.A. systems. I couldn’t make sense of the exponential formulas I found online, so I just copied a section of a graph that I found.
With everything sanded smooth, I sheathed the exterior with epoxy and 2-oz fiberglass. I shaped the small opening for a good fit at the mouth. Although I don’t have a similarly-sized megaphone with straight sides as a comparison, this curved megaphone did seem to project my voice louder than I had expected.
Reader John Bishop, in the comments below, thought that a megaphone would be useful, but rightly guessed it would be bulky aboard a small boat. He wondered about making a leather megaphone that could be stowed flat or rolled up to take up less space in a small boat. A century ago, several inventors filed patents for easily stowed megaphones. Here are a few:
Zellers’ 1905 patent, the last of the four above was for “a foldable trumpet or megaphone consisting wholly of a sheet or blank of flexible material, like heavy paper or cardboard….” It could work for the leather megaphone John had in mind. I didn’t have any more scraps of leather large enough to make the Zellers megaphone, so I bought a $9 flexible plastic roll-up snow sled. It wasn’t as thick as I thought it would be, but rigid enough to hold a cone shape.
I came up with my own pattern for a square folding megaphone that could be stowed flat. I had in mind to make it with corrugated plastic (Coroplast) and the only piece I had was a neighborhood sign I’d put in my front yard to remind passing cars to go slow. I painted it gray.
Christopher Cunningham is the editor of Small Boats Magazine.
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