Why do railings whistle in the wind?

The whistling fence

I was recently emailed by Matthew W. Shepherd about a recording he made of wind whistling through railings. Unfortunately, Matthew didn’t have the best recording equipment available, but you can still hear the tones above the general rustling wind noise. So how is the sound made?

With a railing one probable cause of a breathy note is an Aeolian tone, which is what causes the soughing sound as wind passes around the needles of pine trees. But I doubt that Aeolian tones caused the sound Matthew heard. The frequency of an Aeolian tone is proportional to the wind speed, so the pitch goes up when the wind blows stronger, and the pitch goes down when the wind decreases. So you expect to hear the tone’s frequency slding up and down as the wind speed varies, causing ghostly glissandi. While the frequency of the railing sound does vary in the recording, it does it in distinct steps. This makes me think that the railing is acting like a flute.

Looking at the picture, a small hole can be seen in the pipe just below the top railing. I think this acts like a flute’s mouthpiece. As the wind rushes past, it causes the air within the railing pipe to vibrate. The air column in the pipe will have particular frequencies it naturally vibrates at, the natural resonant frequencies. Like a flute, the railing can jump between these different natural resonant frequencies if the wind blows softer or harder. Matthew handily measured the frequencies on his recording: 379, 511, 654 and 782 Hz, and in his blog correctly identifies this as a harmonic series based on 129 Hz (379 = 126*3, 511=128*4, 654=131*5, 782=130*6).

If Matthew wanted to confirm his hypothesis, he could start by measuring the dimensions of the railing. The tone at 129Hz should be produced by a pipe (roughly) 132 cm long. So if the hole I can see is acting like a flute mouthpiece, then the railing pipe should be 132 cm high. Another way to test this theory would be to tape over all the holes in the railings and see if the tones disappear.

Composer Pierre Sauvageot uses a similar mechanism to make tones in some of his giant sculptures in his work Harmonic Fields:

http://www.youtube.com/watch?v=FcoaPlx5ML8?start=48s&end=55s

Have you heard any other wind-generated sounds I could investigate? Please comment below.