Here's a big fog horn on Flat Holm Island off the English coast. This photo shows the enormous horn that allows air to escape in the form of low-frequency vibrations (air waves).
What isn't shown are the air tanks and the compressor which generate the sound waves. The two tanks for this horn are each about the size of a car!
Here's the Portland Bill in Dorset, England. My wife and I visited a few years ago. I found this nice recording on YouTube, so you can hear what it sounds like. Technically this is called a diaphone.
Ships also need to have fog horns, as they are moving through the water amidst other ships and all need to know where each other are located.
Finally, this photo shows a train horn. A Nathan Airchime. This baby produces enough noise to awaken sleeping drunks near train tracks (and everyone else sleeping within miles).
Even though I live several miles from the tracks (as the crow flies), we can hear trains at night.
If you are of a horn-blowing, noise-loving nature, check out air horns here.
If you don't like noise and are getting too much of it, check out occupational hazards of noise here.
Tomorrow we'll get to the math of noise and the instruments used to measure it (sound level meters and noise dosimeters).