It's so strange and frustrating to me when I think of how random it is to find a really neat noise source. There is usually so much time in between these discoveries that it's quite depressing. Since it happens fairly rarely, when I do find something really cool-sounding, it is an event. Unfortunately, to capture this particular sound, I am going to have to find a good contact microphone. (which brings me to another subject: Putting together a list of what I consider a really complete field kit.)
For the first time in ages, I had a few hours to myself. In this case, I was able to have my way with a 400 foot tall radio tower. And man, it was definitely good for me.
Today I had to drive about 100 miles to small town deep in the rolling farmlands of eastern Ohio to perform some basic maintenance at a tower site. At this rather altitudinous location, my employer maintains a tower, currently supporting only a cell phone site. (we almost never use open-air RF for our own use anymore - it's all fiber optic.) As you probably know, a typical tower of this height requires a few very serious guy wires to keep it vertical. Being the strange-noise-loving kind of guy I am, as I waited for someone to let me in the little shack at the bottom of the tower, I wondered what I would hear if I stuck my ear to the guy wires. Since they have a very good mechanical connection to the tower, I thought that I would certainly hear wind noise. In this case, the tower is 400 feet high. I estimate that the 6 guys each on three sides are between 300 feet at the shortest (from the low part of the tower to the ground anchor to perhaps 500 feet or more from the top of the tower to the ground anchor.)
The first thing I heard was the sound of the flashing beacons. They are slow strobe lights. They use so much power, that when they flash, they 'tic' very strongly. That alone was a pretty cool sound, since the small sound was traveling down the tall tower, and then down the guy wire. Then I learned something stupefyingly amazing (well, to me, anyway) about the acoustic properties of cable used for guy wires. My personal discovery is this: stranded guy wires are a dispersive medium. Not only that, but it appears there are some properties that cause the 'input' waveform to have its frequency lowered as it travels.
A dispersive medium is a medium in which the propagation velocity (of sound, in this case) is dependent upon the wave's frequency. Higher frequencies typically travel faster than low. reference: http://www.ndt-ed.org/GeneralResources/Glossary/letter/d.htm
The sonic result of the dispersion of sound waves can be described as a high-to-low 'swooshing' sound. It's VERY cool. If a burst of noise enters the medium, this tiny burst of noise travels through it. As it does, the higher frequency noises travel faster than the lower frequencies. They travel to the end of the guy wire, in this case, at the connection to the tower, and travel back to the ground end, where I am standing. What I hear is the most amazing swooshing sound. (I applied this small burst of noise by simply plucking the guy wire, like a guitar string. A 300 foot long, 3/4 inch thick guitar string.) Even the /lightest/ pluck with the edge of my fingernail will cause this effect. The other effect I hear is that the pitch of the sound continues to fall to the point where I could detect a pitch as low as 5 Hz, by feeling the vibration only of course. And even more interesting, is that the strength of the vibration increased as the pitch fell. A very strong pluck would eventually produce a low frequency vibration so extreme that it was difficult to keep my head from being rattled.
SO now all I need is a contact mic and I can record it! And then, a contact 'speaker' - something to introduce audio directly into the guy wires. I could apply the sound on one guy wire, let it travel, and pick it up 600 feet away. Maybe just a large open coil, since the wire is ferrous.
So that's my really cool experience today. Stand by!
