LETTERS TO THE EDITORS
To the Editors:
In response to the article about Huygens's resonant clock pendulums (Science Observer, July–August), I'd like to note that this phenomenon has been known for at least 1,000 years in pipe organs. If two or more pipes are tuned to slightly different frequencies—say a semitone apart or 6 percent difference—and they are placed close enough together with their mouths facing each other, then they tend to sound together at exactly the same frequency. If the pipes are more than a semitone different, then they will have trouble resonating together and you will hear two notes. The resonance of closely tuned organ pipes is an example of frequency coupling where the air surrounding the pipes acts as the coupling medium, much like the beam on which Huygens mounted his two clocks.
To avoid the undesirable resonance of notes that ought to sound a semitone apart, organ builders orient the mouths of adjacent pipes as distantly from each other as possible in the space provided, for example staggering them so that the pipe mouths face, alternatively, front and back.