As I was working on our review of sound signaling devices in our August 2016 issue, I took a look on the web for homemade foghorns. I found quite a number of websites and videos that showed how to make foghorns and train horns out of common plastic pipe and fittings. A trip to the hardware store for some 1/2″ PVC pipe and fittings, and 30 or 40 minutes of puttering in the shop was all it took to come up with a few configurations that made a sound that I liked.
The core of the horn is a 1″ x 1″ x 1/2″ T fitting and two 1″ to 1/2″ reducers. One reducer gets shortened up with a hacksaw. (Don’t try using a bandsaw—cylinders like plastic pipe do dangerous things in one.) The cut then gets sanded flat. The other reducer needs to have the stop inside removed so that 1/2″ pipe will slip through. The short reducer is inserted into one end of the T with a piece of freezer bag. The other reducer goes into the other end of the T, and a piece of 1/2″ pipe slips through until it comes in contact with the freezer-bag membrane. You blow through the small hole in the side of the T and adjust the 1/2″ pipe to get a tone. You can change the mouthpiece angle with pipe and angled joints. The length of the pipe slipped through the reducer will determine the horn’s pitch. You can use a long pipe or a short length of pipe—about 6″ so you can still get a grip on it to adjust it—and, if you like, connect a funnel to the pipe. The sound I liked best came from a 24″ length of pipe. It had a deep, resonant, rattling tone.
If I were on the water and heard the deep sound of that 24″ horn coming through the fog, I’d start making tracks fast, thinking I was about to get run down by a freighter. And there’s the problem. You can’t give other mariners the impression that you’re something you’re not. The USCG Colregs specify the frequencies that vessels can use: 70–200 Hz for a vessel 200 meters or more in length, 130–350 Hz for a vessel 75 meters but less than 200 meters in length, and 250–700 Hz for a vessel less than 75 meters in length. (If you’d like to hear what those ranges sound like, check out this online tone generator.) I sometimes have trouble distinguishing octaves coming from different instruments and couldn’t compare the sound of the tone generator and the horn with any certainty, so I loaded a free tuning app (Pano Tuner for Android) on my phone and found my 24″ pipe was 130 Hz, two octaves below middle C on the piano. If I were to use that horn aboard the biggest of my boats in the fog, I’d be masquerading as a vessel between 250′ and 650′ long. Too bad, I rather like the sound. The lowest tone I can use is 250 HZ, or just a bit below middle C. That’s a 12″ pipe.
This homemade horn may look like it belongs under the galley sink but it is loud and measured up well against all of the lung-powered horns we reviewed. It is practically indestructible and the parts cost less than $5.
I found it easy to get carried away with making horns. I thought a dual horn might be worth investing a couple of dollars more, and indeed a two-tone horn has a sound that’s hard to ignore.