Wall Vibrations in Organ Pipes and Their Effect on Tone
John Backus · The Journal of the Acoustical Society of America · 1965
The wall vibrations of seven different flue organ pipes were measured while the pipes were sounded normally, using an accelerometer of very small mass constructed from a phonograph pickup. It was found that the wall-vibration pattern obtained for a given harmonic of the pipe tone could be matched quite precisely by mechanically driving the pipe at the harmonic frequency with a vibrated rod in contact with the lip of the pipe. Sound levels at a selected point were measured both for the pipe sounding normally and for the pipe artificially vibrated with the same acceleration level. Results for the pipes tested showed that the sound levels produced by the wall vibrations are a negligible part of the pipe tone. The effect of the wall vibrations on the internal standing wave was also investigated. A larger tube was placed around the organ pipe, sealed to it at one end, and the intervening space filled with water. The amplitude and frequency of a given harmonic produced by blowing the pipe were then observed as the water was drained off. In no case could any change be detected. The acceleration levels obtained were compared to those obtained on a number of pipes of a small church organ and found to be comparable. It is concluded that the wall vibrations of an organ pipe do not contribute to its tone either by radiating sound or by affecting the internal standing wave.