June 2003
Deep Bass	Show Menu

Adrian Somerfield, St Thomas’s, Mount Merrion, Co. Dublin

In his suggestion of "Easter Bells" in Soundboard #4, Donald Maxwell refers to a bar consisting of a solitary 32 foot pedalnote! A real 32 foot stop, in the form of an Open Diapason, must be very rare, and even a bourdon of physical length sixteen feet, or about five metres, which would be at that pitch, would be unusual. (A stopped pipe is roughly half the length of an open pipe of the same pitch.) Yet I have known two fairly small pipe organs which have stops labelled 32. One stop was called a Harmonic Bass and the other (more accurately, I think) an Acoustic Bass, but they both work in the same fudgy way.

Organists should know that if two pipes are slightly out-of-tune, they will produce a regular rise and fall in loudness. The pipes are said to "beat" and the frequency of the beats is the difference in frequency of the two notes. Organ tuners, and other musicians, use this, and two pipes are adjusted so that the beat frequency becomes less and finally becomes zero when unison is achieved. The Voix Celeste also uses beats; in this case each pipe is tuned to produce beats of a few hertz with the rest of the organ, or sometimes with a unison rank of its own. The warbling sound is imagined to sound like the voices of angels. (I heard of a case, perhaps apocryphal, where an organ tuner had been called out because the Voix Celestes sounded out-of-tune!)

Now let us suppose we have a 16 ft bourdon sounding its lowest C at, for simplicity, 64 Hz. We now sound with it the pipe a fifth higher; the interval of a fifth is 3/2 so this pipe sounds at 96 Hz. But these pipes produce beats at 96 – 64 = 32 Hz, which is one octave below the original 64 Hz. So we get the impression that we have a stop one octave below our 16 ft pitch and we label it 32. All the stop key does is to couple each note of the Bourdon to the note a fifth above it and beats do the rest. And we economically get 32 foot pitch from a pipe only eight feet long or so. You can try the effect by using your two feet.

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