06.24.2009

Leafblowers

Posted by AJ Harbison at 4:38 pm

The office I work in is about a block down the street from my company’s main office, and I usually walk back and forth between them once or twice a day. I was walking back from the main office this afternoon, and passed a couple of guys with leafblowers cleaning up a parking lot. Each of them had one, and the hum of the two leafblowers was the interval of a fifth apart–in almost-perfect tuning. (A fifth is the distance of, for example, C up to G, or A up to E). They were different sizes, which explained the difference in pitch: the higher one was completely handheld and narrower, while the lower one had a backpack and a bigger tube. Bigger and longer always equals lower in acoustics, whether it’s a string or a column of air–consider how much bigger a cello or double bass is compared to a violin, or a bassoon compared to an oboe.

My first thought was that the “bore” size of the second one must be bigger than the first by a third–because a perfect fifth is created by the ratio of 3:2 to the original note (for example, the rate a string vibrates to produce the tone G is in the ratio of 3:2 to the rate the same string vibrates to produce a C). But I quickly corrected myself when I realized that the hum was created not by the air moving through the tube, of course, but by the motor. (Less interesting that way, but has the virtue of being true). So the fifth was created by some 3:2 ratio between the motors; since I know nothing about mechanics, though, I’m not sure exactly what property it was that created the ratio. But I still found it interesting. Just goes to show that you should always be listening–music (or at least a semblance of it) can be found in the most unexpected places!

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  1. Gravatar

    Mark Harbison on 06.24.2009

    I think that the idea of music coming from the most unexpected places was the concept behind an extremely popular Broadway show . . . you should take a vacation to New York some time and see Stomp, and then write about it.

  2. Gravatar

    Ryan Fleming on 06.25.2009

    Great Post! As you know I am quite fond of both the musical and mechanical realms.

    One possible cause of the sound could be the vibrations from the motor. If this is the case then the difference in pitch between the two motors would be due to a difference in motor speed. As you know the units for pitch are Herz (or cycles per second). This can be likened to the speed of the motor in units of RPM (revolutions per minute). Thus, if the pitches you heard were a fifth apart then the ratio of motor speeds would have to be 3:2.

    Another cause could be from the resonance in the leaf blower tube. This would be likened to the way a pipe organ operates. However, most leaf blowers are plastic which does not resonate very easily.

    My guess would be that the noise was generated due to an imbalanced motor creating rigorous vibrations generating sound waves. It would be interesting to modify a leaf blower by extending the length of the tube and observing the difference in pitch (if any).

  3. Gravatar

    ajharbison on 06.25.2009

    Mark: I’d love to see Stomp and write about it (and I’m sure my lovely wife would too–she’s probably already seen it).

    Ryan: You’re probably right about the vibrations of the motor being the source of the pitch. The resonance of the tube was my first thought, but I doubt that it actually makes any difference in the pitch. But if you feel like trying an experiment along those lines, let me know how it turns out!

    AJ Harbison

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