Responses to a reader's question on tower noise started arriving at my e-mail box within minutes after last week's RF Report was sent out. There was no clear-cut answer to the question "which towers make the most noise?"
Some readers said they noticed guyed towers made more noise than self-supporting towers. Others said the opposite. There was general agreement that noise was determined more by the way the tower was constructed than whether it was a guyed or self supporting tower. Jeff Marousek, Chief Engineer at KHAS-TV in Hastings, Neb. said that the noisiest site he was ever at had a 625-foot structure, but the most wind-generated noise came from a guy wire supporting a utility pole! Paul Alciatore, Chief Engineer at hawthorne direct, inc. in Fairfield, Iowa said that at the sites he had worked at, he doubted you could separate the tower noise from the other, louder sources enough to measure it. That matches my observation -- pumps, fans and other hardware attracted my attention more than tower noise.
Readers that said guyed towers made more noise usually pointed to noise from the guy wires, including guy wire "singing." Readers said self-supporting towers made more noise due to a greater surface area of steel. Mike De Roo, an engineer with KCTV in Kansas City, said his 1,000-foot freestanding tower made a rushing sound, which he found rather pleasant, like ocean surf!
David K. Davies, Engineering Manager at the Structural Division of Electronics Research, Inc. (ERI), said he did not know of any research or independent data concerning which type of tower produces more wind noise, but that did not stop him from looking at it from a theoretical viewpoint. He believes the issue is dependent "on the extent of the disruption to the laminar wind flow." He explained, "This disruption is a function of the total effective wind area of tower members and the members' shape. Let's discuss shape first. As the wind passes a member an eddy or partial vacuum is formed. This causes the wind current to shift in an attempt to fill the vacuum. The result is similar to a resonator where varying pressure nodes cause a standing wave to form in the air column resulting in pressure oscillations and sound -- the 'whistling tower.' The shape of the tower member determines the degree of air current displacement; flat surface members cause more disruption than round members. So, it seems reasonable towers constructed of flat angle or flat bar would generate more wind noise. And, the more members in the tower the more sources of noise."
If this was all that had to be considered, it would appear guyed towers would create less noise. "But guy towers, while primarily constructed of round members also have guy wires," David notes. "The friction of the wind currents passing around these cables can create a vibration resulting in noise just as a column of air passing our vocal cords creates sound. This vibration can be amplified by the tower which acts similar to a sounding board in a violin. The results are pressure oscillations in the air that we hear. Hence the term, "Singing Guy Wires."
After going through this analysis, he said that a self supporting tower composed of round members should be the quietest, but without a good A to B comparison, he aptly concluded "In the absence of reliable data I am just whistling in the wind."
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